passive solar space heating

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					         passive solar space heating
In designing for passive solar energy
 use in Alaska, four major design ele-
ments can be considered:
                                              energy occurs. Windows of east or west
                                              orientation should either be shuttered or
                                              have at least triple-pane glazing. North
                                                                                           First, the south-facing glazing was varied
                                                                                           and plotted as a ratio of the glazed area
                                                                                           to the floor area. For instance, if the
1. South-facing windows.                      windows should be avoided if possible,       floor area is approximately 966 ft2 (does
                                              because of their net loss for six months     not include second story), then 96 ft2 of
2. Thermal mass.
                                              of the year (with or without shutters). If   window area would be plotted as a ratio
3. Thermally insulating shutters (night       they are present, they should be shut-       of 0.1 (10 percent) on the resulting figures
   insulation).                               tered.                                       (Figures 61 and 62).
4. Building insulation (thermal perfor-          The usefulness of thermal storage in
   mance of the structure).                   the far North has long been controver-       Night Insulation (Shutters)
   Passive design implies that these build-   sial. The changes in solar gain are rapid    The first result of interest is shown in
ing elements enable the building itself to    and dramatic throughout the year, so         Figure 61. This figure indicates clearly
function as a solar collector, instead of     that the amount of storage cannot be ap-     that any increase in shuttered or un-
adding solar collectors to it. The thermal    propriately sized for more than a small      shuttered window area for the home
energy is transferred by natural energy       portion of the year. However, because        is always going to result in worse ther-
flows (conduction, convection, and radia-     of the ever-changing, dynamic nature of      mal performance of the building in
tion), rather than being pumped to a point    solar energy and the effects it has on a     December. December is, of course, the
of use. Passive design techniques, involv-    building, we cannot easily separate out      worst solar month at high latitudes. Since
ing the four elements mentioned, were         elements of the design to analyze them       glazing (even if shuttered) is a poorer
described briefly in the solar technology     individually.                                insulator than the standard house or
section. The value of double, triple, or        The building characteristics used in the   superinsulated wall sections, the thermal
quadruple south-facing glazing has been       computer study are listed in Table 10A.      performance of the building in December
demonstrated in a study by Aspnes and         These are the building characteristics of    always gets worse with increasing win-
Zarling (1979). They showed that if R-9       a well built, modern home with good          dow area. This demonstrates the worst
shutters (or shutters of a higher R-value)    air leakage control. To determine the        case for an average year. The only way
are used, then south-facing windows in        benefits of the four passive elements,       to overcome such an effect would be to
Anchorage need only be double pane to         their effect on building performance         ensure that the insulating value of the
yield a net energy gain every month of        was tested using the computer model.         shuttering device is equal to that of the
the year. Nearly the same result is true      The simulation was begun with the            walls. This is difficult to do, but it is a
for Fairbanks, except that December is        characteristics of the standard house.       technical problem worth pursuing. It is
the only month during which a net loss of

                                                                                         performance for this house at a window
      TABLE 10A: A LISTING OF PARAMETERS FOR THE TRNSYS COMPUTER                         area to floor area ratio of 0.2, or 20 percent.
              MODELING USED TO FIND AN OPTIMUM PASSIVE                                   The added mass, therefore, enables the
                   SOLAR DESIGN FOR FAIRBANKS, ALASKA                                    performance of this house with south-
                                                                                         facing, unshuttered windows to be
                                                                                         improved by approximately 1 percent
 1. Window-triple pane: U = 0.34 BTU/hr•ft2.0F, Area 0 to 300 ft2, R = 2.94              of the annual heating requirements
 2. Infiltration: 0.15 air changes per hour                                              (from 0.94 to 0.93 of the house’s
 3. Thermal capacitance: C=4000 to 16,000 BTU/0F                                         requirements).
 4. Insulation: As given in Table 7B
                                                                                            Curve “c” dramatically indicates the
 5. Ground reflectance: varied from 0.2 during fall and spring to 0.6 during winter
                                                                                         effect of shutters on a passive solar
 6. Thermal shutters: U = 0.125 BTU/hr•ft2. 0F, operated on an open cycle between
                                                                                         structure. With the shutters, the south-
    7:00 a.m. and 8:00 p.m. R = 8
                                                                                         facing window area of a standard
 7. Shading: Two-foot wing walls and overhang with one-foot perimeter gap
                                                                                         structure can be increased to 30 percent
 8. Allowable temperature swing: 650F to 780F
                                                                                         of the floor area before the heat loss of
 9. Ventilation fan turned on whenever interior temperature exceeded 780F
                                                                                         that shuttered area begins to cancel the
10. Transmittance of windows: Assumes a transmittance of 0.70 at normal incidence
                                                                                         solar gain. Shuttering the windows on
11. ASH RAE response factors for light and medium weight construction
12. Internal generation: 750 watts                                                       a standard home can result in up to a 22
                                                                                         percent reduction in required heating,
                                                                                         as indicated from this modeling process,
                                                                                         and depending on the south-facing
also worth noting that thermal mass pro-     September through May. The first curve,     window area as well as other window
vided no benefits during this simulation.    labeled “a,” traces the performance         orientations and shuttering cycles. A
Varying the amount of thermal mass by        of a typical house (as defined in Table     shuttering cycle is the daily pattern of
a factor of 4 did not affect the December    10B) as unshuttered window area             opening and closing the shutters on a
heating load.                                is increased. Thermal performance           structure. For example, open at 7 a.m.,
                                             increases somewhat until the ratio of       closed at 8 p.m., open at sunrise, closed
Analysis Of Parameters                       window area to floor area reaches 0.1;      at sunset, etc.
Figure 62 shows the results of varying all   then the performance declines as the heat
                                                                                            Curve “d” shows the combined effect
the parameters in cumulative fashion to      loss from the increasing window area
                                                                                         of shuttering the windows and doubling
arrive at the best possible performance      gradually cancels the benefits from solar
                                                                                         the interior mass. The effect of the ad-
of a passive structure, which combines       gain through those same unshuttered
                                                                                         ditional mass is similar to that of curve
superinsulation, added internal mass,        windows. Curve “b” is a case similar to
                                                                                         “b”; it is a small, additive effect, totaling
and an area of south-glazing with            curve “a” except that the internal mass
                                                                                         4 percent of the total standard house heat
shutters, for a heating season from          is doubled. This results in an optimum

                                    HOUSE TO A SUPERINSULATED HOUSE
                        Standard House                                         Superinsulated House
  Wall Sections          U-Value         R-Value          Wall Sections          U-Value              R-Value
Inside air film                                         Inside air film
5/8 in. gyp board                                       5/8 in. gyp board
5-1/2 in. fiberglass                                    11 in. fiberglass
1/2 in. plywood           0.047           21.3          1/2 in. plywood            0.025               40
7/8 in. cedar siding                                    7/8 in. cedar siding
Outside air film                                        Outside air film
Inside air film                                         Inside air film
5/8 in. gyp board                                       5/8 in. gyp board
5-1/2 in. Douglas Fir                                   7 in. Douglas Fir
1/2 in. plywood           0.100           10            4 in. fiberglass           0.040               25
7/8 in. cedar siding                                    1/2 in. plywood
Outside air film                                        7/8 in. cedar siding
                                                        Outside air film
 Roof Sections           U-Value         R-Value         Roof Sections           U-Value              R-Value
Inside air film
5/8 in. gyp board
11-1/4 in. fiberglass                                   Same as for standard
5/8 in. plywood           0.025           40            house                      0.025               40
Felt paper
Asphalt shingles
Outside air film
Inside air film
7/8 in. gyp board
1-1/4 in. Douglas Fir                                   Same as for standard
5/8 in plywood            0.062           16.13         house                      0.062               16.13
Felt paper
Asphalt shingles
Outside air film

Figure 61. Increasing the window area of a structure to          Figure 62. Annual heating requirements for houses with and
           improve solar gain always results in increased                    without thermal shutters and various amounts
           heat loss for the month of December.                              of south-facing windows.

load at a window area to floor area ratio
of 0.30 (30 percent).                         COMPUTER SIMULATION
                                              (Editor’s note: these are an update for the second edition).
   Curve “e” shows the performance of
the unshuttered, superinsulated house.           The following list of solar simulation programs is from U.S. Department of Energy
It is identical in shape to curve “a,” but    efforts, and all are limited to PC/Windows platforms. For a Macintosh-compatible
demonstrates the lower energy consump-        simulation tool, see the Canadian RETSCREEN option below.
tion afforded by the additional insulation.
Otherwise, the unshuttered windows            Energy Simulation Software
cause the same increase in heat loss as in    Data from energy simulation software can be very helpful to the design process.
case “a.” Curve “f” is an example of an       It helps architects and building designers quickly identify the most cost-effective,
optimum passive solar design. It shows        energy-saving measure for commercial buildings. A partial list is shown below; a
the results of the computer simulation        comprehensive database of simulation products can be found on the website: www.
of the superinsulated home (see Table software.html
10B) with shutters operated on a 7 a.m.
                                              ENERGYPlus—A new-generation building energy simulation program from the
open—8 p.m. closed daily cycle. The
                                              creators of BLAST and DOE-2.
best-performing structure is a house
with a south-facing glazed area equal to      ENERGY-10—ENERGY-10 is an award-winning PC-based design tool that helps
20 percent of the floor area, lightweight     architects and building designers quickly identify the most cost-effective, energy-
construction (no additional thermal           saving measures for small commercial and residential buildings.
mass), shutters, and superinsulation.         RADIANCE—RADIANCE is UNIX freeware for lighting design and rendering,
   The most crucial insight to be gained      developed by the U.S. Department of Energy and the Swiss federal government.
from this result of the analysis of pas-      DOE-2—An hourly, whole-building energy analysis program that calculates energy
sive solar gain, is the overwhelming          performance and life-cycle cost of operation.
importance of energy efficient, energy-
conserving design of the building. This       Software Overview (for Macs)
is most clearly shown by curves “e” and
                                              Renewable energy technology (RET) projects are not routinely considered by plan-
“f” in Figure 62.
                                              ners and decision-makers at the critically important initial planning stage. The
  These last two curves start with this       RETScreen® Renewable Energy Project Analysis Software has been developed to
energy-conserving house, and clearly          help address this barrier.
show that everything done to the stan-
dard house to improve its passive solar
performance doesn’t get you to the 25%
heat load reduction which you start
with in a superinsulated design!                                                                       (continued on next page)

                                             The lesson couldn’t be more clear from          shutter would perform best if its
   RETScreen International is a re-       this example: build a very well insulated          insulating value were equal to that
newable energy awareness, decision-       structure first, and then add the passive          of the surrounding superinsulated
support, and capacity-building tool       solar features and shutters.                       wall. Size and mobility requirements
developed by the CANMET Energy               Several other instructive conclusions           for shutters preclude this.
Diversification Research Laboratory       for passive solar design of light-construc-    3. Superinsulated construction com-
(CEDRL) with the contribution of over     tion buildings at subarctic latitudes can         bined with direct-gain, passive solar
89 experts from industry, government,     be drawn from the preceding study:                techniques have an additive effect,
and academia. The core of the tool con-                                                     resulting in a 25 to 40 percent reduc-
                                          1. Triple-pane, south-facing windows
sists of a standardised and integrated                                                      tion in the annual heating load.
                                             yield a modest energy savings of 6
renewable energy project analysis
                                             to 8 percent if the window-to-floor-        4. Increased thermal mass in a structure
software that can be used world-wide
                                             area ratio is in the range of 0.1 to 0.2.      can produce energy savings. How-
to evaluate the energy production,
                                             Windows facing any other direction             ever, at high northern latitudes with
life-cycle costs, and greenhouse gas
                                             will experience a net loss of thermal          severe winters and little midwinter
emission reductions for various types
                                             energy over the heating season.                sun, these savings are not dramatic,
of renewable energy technologies
                                          2. Thermal shutters as modeled in this            and are unlikely to warrant the
(RETs). Each RETScreen renewable
                                             study on all south-facing windows              added expense of their inclusion in
energy technology model (e.g., Solar
                                             of the structure can supply up to 22           the structure.
Water Heating Project, etc.) is devel-
oped within an individual Microsoft®         percent of the space heating require-         Highly insulated structures are gaining
Excel spreadsheet “Workbook” file.           ment. Of course, these savings are          popularity as the logic and comfort of
The Workbook file is in turn composed        dependent on the open-close cycle           such homes become obvious, even though
of a series of worksheets. These work-       and the insulating value of the shut-       the extra wall thickness adds to the initial
sheets have a common look and follow         ters. If higher R-value shutters with       cost of the building. Insulation costs are
a standard approach for all RETScreen        a shorter open cycle are used, ad-          directly proportional to thickness and
models. In addition to the software,         ditional savings would be realized.         labor costs for framing increase, as are the
the tool includes product, weather,          One of the most attractive retrofits for    costs of windows and doors with their
and cost databases; an online manual;        existing homes is thermal shutters. A       required jam extensions. However, much
a website; project case studies; and         need exists for a well-designed, low-       progress has been made in integrating
a training course. RETScreen is also         cost, semiautomatic shutter for old         many of these solar, conservation, and
convertible to a Macintosh platform,         as well as new construction. Ideally,       health features into modern housing.
which can be done through instruc-           a shutter system would open only
tions at the website: http://retscreen.      during periods of useful solar energy       The Problem Of Thermal Shutters                      gain, but this is likely to be objection-   Windows are notoriously poor thermal
                                             able on aesthetic grounds. Also, the        insulators and usually are a major source

of heat loss in structures. Insulating        insulation with south-facing glazing for      up due to freezing condensation; they
windows can significantly reduce this         passive solar design. Alaskans should         also conduct valuable heat through the
heat loss. A double-pane window with          continue working to find a better shutter     wall. If these types of mechanisms aren’t
an R-value of 1.84 loses heat at the rate     design for our homes (see Figure 63.)         used, then one must operate the shutters
of 0.54 BTU/hr/ft2/°F. A wall with an            One of the questions often asked about     from outside, an unappealing option at
R-value of 19 loses heat at a rate of 0.05    shutters concerns their position relative     –40°F.
BTU/hr/ft2/°F. Thus the window loses          to the window. Should the shutter be             Placing the shutter on the inside of the
about ten times more heat per unit area       placed outside the window or inside?          window may work, but it has similar
than the wall under the same conditions.      The answer is not simple, because nei-        problems. Interior shutters are conve-
Obviously, when windows are not gain-         ther solution is trouble-free. Placing        nient since they can be operated from
ing useful heat during the dark period        the shutter mechanism outside exposes         inside the building, but this strategy
of the day, they are rapidly losing heat      it to weather and reduces the ease of         causes the inside window surface to be-
to the environment if it is colder outside    operation. The shuttering mechanism           come colder. If the shutter is not sealed
than inside the structure. So windows         can become frozen open or shut from           to exclude the passage of warm moist
need to be insulated at night if they are     ice buildup, especially if it is a track or   interior air to this cold window, one or
to perform optimally in a passive solar       hinged mechanism. Any cranks or lev-          all of the following will happen:
design.                                       els that penetrate the wall can also ice      1. Water will drip down the sills of the
   What kind of shutter (also called mov-                                                      window, along the wall, and onto the
able insulation and night insulation)                                                          floor, discoloring and decaying the
should you use? There are indoor shut-                                                         building materials.
ters, outdoor types, shutters that fit into
                                                                                            2. Water will freeze behind the shutter,
a wall pocket, shutters that fold away
                                                                                               icing over the window and limiting
into a storage area, shutters that open
                                                                                               its usefulness when unshuttered.
and close automatically, and shutters that
                                                                                               When it is unshuttered, the ice will
are controlled by photoperiod. There are
                                                                                               melt and repeat the events described
R-2 shutters and R-15 shutters. But there
are no ideal shutters. Every design has
liabilities. They must open and close, be     Figure 63. The superinsulated, straw-         3. The shutter will freeze in place until
reliable in the most extreme conditions                     bale, solar optimal home of        a thaw comes.
Alaska can offer, and—perhaps most                          Kevin Maxwell, which has          Sealing the shutter from vapor
important of all—they must be used. If                      operable (with steel cable/     problems is possible, but not simple, and
shutters are bothersome, unaesthetic, or                    pulley system) ~R-30 shut-      most commercially available shutters
unreliable in operation, they will be dis-                  ters which open vertically      do not have vapor seals. This discussion
carded or avoided. We do not yet have                       upward on hinges as shown       reflects the situation with window
the technology for ideally coupling night                   in this April 2005 photo.       shutters in the early 1980s, at the first

printing of this manual. The situation       that in March. However, at Bethel, Mata-       These classic designs have been ana-
in 2005 is regrettably little improved       nuska, and Fairbanks, the solar radiation      lyzed for their performance through a
regarding night insulation. What have        in March averages twice as much as that        design project for a rural Alaska school,
improved are window technologies such        in September, on a vertical south-facing       sponsored by the Alaska Department of
as vacuum glazing and heat mirror®           surface. The asymmetry is due to late          Transportation and Public Facilities and
products. These options in no way            summer and autumn cloudiness, the              the U.S. Department of Energy. These
approach an optimum U-value however          presence of high-albedo snow in spring,        Alaska Department of Transportation
(U = 1/R-value.)                             and predominantly clearer weather dur-         and Public Facilities studies include:
                                             ing the period from February through           1. Two Rivers Passive Solar School Analy-
Effects Of Climate                           May. The result is that solar radiation           sis, Interim Report, J.S. Strandberg
The continental Alaska climates are typi-    on a south-facing vertical surface (the           Engineers, Report No. AK-RD-82-18,
cally characterized by long, cold winters    most important consideration for passive          Alaska DOT&PF, December 1981. 23
and short, relatively warm summers. So-      solar design) is out of phase with heating        pp. plus appendices.
lar radiation varies with the seasons, due   degree-days. Solar gain peaks in March
                                                                                            2. Passive Solar Heating in Alaska, by John
to both the seasonal solar elevation angle   and April, when the solar heat is still very
                                                                                               P. Zarling, Report No. AK-RD-81-15,
and day length and seasonally changing       useful. The solar geometry and climate
                                                                                               Alaska DOT&PF, June 1980, 17 pp.
humidity. In Alaska, this can be seen by     provide an unexpected benefit for pas-
                                             sive solar applications in the far North.      3. A Thermal Performance Design Opti-
investigating the average solar radiation                                                      mization Study for Small Alaskan Rural
on a south-facing vertical surface. Figure      As in the case of active solar applica-
                                                                                               Schools, John Zarling and James S.
64 shows the comparison of two related       tions, the presence of snow cover for up
                                                                                               Strandberg, March 1983, Report No.
quantities: the monthly average heating      to six months of the year is a positive
                                                                                               AK-RD-83-2, 118 pp. plus appendi-
index, and the average daily solar radia-    factor, improving the effectiveness of
tion (BTU/ft2) on a south-facing vertical    passive solar energy in Alaska.
surface in Fairbanks. Figure 65 shows the                                                   4. An Analytical Study of Passive Solar
same comparison for the Matanuska Val-       Performance Of “Classic”                          Energy and Mass Storage: Observations
ley of Alaska, and Figure 66 for Bethel.     Passive Designs In Alaska                         from a Test Building in Fairbanks, Alas-
                                                                                               ka, by Richard D. Seifert and George
  In the examples, an important and          As in many fields of design, passive solar        S. Mueller, Report No. AK-RD-85-21,
somewhat unexpected pattern is evident.      technology has “classic” types. There             June 1983, 50 pp. plus appendices.
Intuitively, the average solar radiation     are (1) direct gain systems, primarily
on a south-facing vertical surface (or       using glazing and thermally efficient
any surface) should be symmetrical in        structures; (2) Trombe wall designs; (3)       Direct Gain Passive Solar Design
magnitude about the summer solstice.         greenhouse options; and (4) direct gain        This section reviews the physical fea-
One expects the average solar radiation      with thermal shutters, also referred to        tures of a structure that can influence
in September to be very nearly equal to      as “direct gain with night insulation.”        the performance of a direct gain system.

Figure 64. These graphs illustrate that the Fairbanks heating degree days and average solar radiation (which are an indication of
            a building’s heating requirements) are not in phase with the solar radiation on a south-facing vertical surface. This has
            positive implications for passive solar heating. The solar gain is highest in March and April, when heating is needed.
            Data are from Kusuda and Ishii (1977).

Figure 65. Matanuska heating degree days and average solar radiation. These graphs illustrate that the annual heating degree
            days (which are an indication of a building’s heating requirements) are not in phase with the solar radiation on a south-
            facing vertical surface. This has positive implications for passive solar heating. The solar gain is highest in March and
            April, when heating is needed. Data are from Kusuda and Ishii (1977).

Figure 66. Bethel heating degree days and average solar radiation. These graphs illustrate that the annual heating degree days
            (which are an indication of a building’s heating requirements) are not in phase with the solar radiation on a south-
            facing vertical surface. This has positive implications for passive solar heating. The solar gain is highest in March and
            April, when heating is needed. Data are from Kusuda and Ishii (1977).

It prepares the user for an actual Alaska         rangement encourages absorption of           In order for half of the transmitted
passive solar design calculation.                 sunlight on surfaces where the heat       solar radiation to be transferred rapidly
                                                  can be stored.                            into the room air, it would be necessary
Absorptance                                   2. If dark objects with little thermal        for half of the exposed surface area to
The solar absorptance, α, of internal walls      capacity are placed in a direct gain       be a perfect absorber with no thermal
and furnishings may be a significant             zone, they should be located out of        storage capacity. Or, equivalently, if the
design feature in raising or decreasing          direct sunlight as much as possible.       surfaces lacking thermal storage capac-
the comfort level in a structure. Too                                                       ity have a solar absorptance of 0.5, they
                                                Adherence to these simple rules will
much absorptance, and a building can                                                        must intercept all of the transmitted solar
                                              help eliminate overheating problems in
become too hot. Too much reflectivity,                                                      flux in order to transmit 50 percent of the
                                              properly sized, direct gain structures.
and the building won’t absorb enough                                                        absorbed radiation directly to the air (the
                                                 Lightweight objects with low heat ca-      air heating fraction). These two extreme
heat. Although darker colors are more         pacity (such as furniture) can diminish       cases seem to indicate that a designer
absorptive, they also become very hot         the performance of a direct gain building,    would have to try very hard to design a
when exposed to direct solar radiation        especially if placed in direct sunlight.      structure that would rapidly overheat.
for extended periods of time. The use         However, according to work done by
of a direct gain space must be carefully                                                       However, rapid overheating may still
                                              Balcomb et al. (1980), the penalty for
considered. A dark metal surface with                                                       be a problem in Alaska. Since our com-
                                              absorbing 20 percent of the transmitted
a small amount of mass can reach tem-                                                       puter simulations show that thermal
                                              solar radiation directly on nonmassive
peratures in the range of 120 to 140°F.                                                     mass storage is less useful for structures
                                              surfaces never exceeds 5 percent. This
Substances with absorptivities of 0.5 to                                                    in Alaska, an optimum passive solar
                                              information is useful to an architect or
0.7 will still get very warm when exposed                                                   design for Alaska would more closely
                                              designer who needs to make choices
to the sun, but they reflect more of the                                                    approach the extreme case of a perfect
                                              of furniture and wall coverings in a
incident solar radiation, achieving more                                                    absorber with no thermal storage capac-
                                              building, especially as it affects passive
even heating of the space. Table 11 lists                                                   ity. Thus Alaska designs may require
                                              solar performance. The concern is that a
the absorptances of common materials.                                                       ventilation systems to remove this heat.
                                              large amount of low-mass material in a
  The following suggestions are offered       direct gain sunspace might cause more            Two strategies may help avoid over-
as a means of assuring that absorption        frequent overheating and high levels of       heating problems. First, use interior
levels on nonmassive surfaces be kept         discomfort. An example of the worst-          paints and surface materials with ab-
reasonably low in direct gain zones.          case situation is described in the next       sorptances of 0.5 or less. This would
1. As a general rule, massive surfaces in     paragraph and helps to clarify that the       ensure that the air-heating fraction is 50
   a direct gain zone should be relatively    interior design in passive solar structures   percent or less. Second, avoid using a
   dark in color, and low mass surfaces       is not severely constrained by the type,      surface material that is a good thermal
   should be relatively light. This ar-       amount, and solar absorptance of the          insulator, such as carpeting, especially if
                                              furnishings.                                  its absorptance is greater than 0.5. Thus,

          TABLE 11: SOLAR ABSORPTANCE OF VARIOUS MATERIALS1’2                                        for example, don’t use carpets, or if you
                                                                                                     do, use light-colored carpets.
                 Optical flat black paint                                     .98
                 Flat black paint                                             .95
                 Black lacquer                                                .92                    Wind Speed and Spacing of Glazing
                 Dark gray paint                                              .91                    Wind blowing across (sweeping) a win-
                 Black concrete                                               .91
                 Dark blue lacquer                                            .91
                                                                                                     dow surface removes the insulating air
                 Black oil paint                                              .90                    film on the outside of the glazing. This
                 Stafford blue bricks                                         .89                    can dramatically affect the rate of heat
                 Dark olive drab paint                                        .89                    loss from a window. Most locations in
                 Dark brown paint                                             .88
                                                                                                     continental Alaska have an average wind
                 Dark blue-gray paint                                         .88
                 Azure blue or dark green lacquer                             .88                    speed less than the 15 mph reference
                 Brown concrete                                               .85                    value that the American Society of Heat-
                 Medium brown paint                                           .84                    ing, Refrigerating, and Airconditioning
                 Medium light brown paint                                     .80                    Engineers (ASHRAE) uses as a standard
                 Brown or green lacquer                                       .79
                 Medium rust paint                                            .78                    for reporting film coefficients on exter-
                 Light gray oil paint                                         .75                    nal building surfaces. The actual film
                 Red oil paint                                                .74                    coefficient should be based on one-half
                 Red bricks                                                   .70                    of the actual recorded wind speed at a
                 Uncolored concrete                                           .65
                 Moderately light buff bricks                                 .60
                                                                                                     given location. Using half of the hourly
                 Medium dull green paint                                      .59                    wind speed to compute film coefficients
                 Medium orange paint                                          .58                    on the outside surface of direct gain
                 Medium yellow paint                                          .57                    glazing reduces the calculated amount
                 Medium blue paint                                            .51
                                                                                                     of heat lost from the surface and yields
                 Medium kelly green paint                                     .51
                 Light green paint                                            .47                    higher performance predictions. For
                 White semigloss paint                                        .30                    night-insulated cases the improvement
                 White gloss paint                                            .25                    is small. However, for designs without
                 Silver paint                                                 .25                    night insulation, the fractional decrease
                 White lacquer                                                .21
                 Polished aluminum reflector sheet                            .12                    in effective conductance of the solar wall
                 Aluminized mylar film                                        .10                    or glazing is significant.
                 Laboratory vapor deposited coatings                          .02                      A gap between window panes of 1⁄4
1This table is meant to serve as a guide only. Variations in texture, tone, overcoats, pigments,
                                                                                                     inch has been the traditional standard. It
 binders, etc., can alter these values.                                                              has been established that the air gap thick-
2A perfect absorber has an absorptance of 1.00; i.e., it absorbs 100 percent of the incident solar   ness affects the conductance of double-
 radiation. All common materials absorb less.                                                        glazed windows, but only recently has

the effect on performance of direct gain      of variable wind speed has already been         can be avoided by opening windows or
buildings been studied. Figure 67 (after      accounted for. The direct gain design           venting.
Balcomb et al., 1980) shows that increas-     that was originally a 9 percent loser              Note that the glazing should not extend
ing the air gap from 1⁄4 to 1⁄2 inch raises   now shows a positive solar savings of           to the bottom of the overhang because the
the solar savings by 12 to 15 percent,        11.5 percent. Further increases in air gap      top portion of the window would receive
depending on whether or not the effect        thickness yield very little additional          direct sun only in midwinter but would
                                              improvement in performance because              lose as much heat as any other part of
                                              convection currents between the glaz-           the window.
                                              ings negate the insulating effect of the
                                                                                                 If the overhang is in place during all of
                                              thicker air layer. Using a glazing air gap
                                                                                              the year (fixed overhang) then the design
                                              of at least 1⁄2 inch decreases heat loss from
                                                                                              of the angles becomes a tradeoff between
                                              direct-gain buildings, especially if night
                                                                                              a sacrifice of solar heating during the
                                              insulation is not used.
                                                                                              spring months (when the sun angles are
                                                                                              high but the weather is still cold) and
                                              Effect of Overhangs                             overheating during summer (when the
                                              Overhangs are normally used in most             sun angles are higher and temperatures
                                              passive solar applications to reduce            are warm).
                                              summer overheating. If the overhang                       Important: Murphy’s
                                              is properly designed, there is no block-                   Law Of Overhangs:
                                              age of the sun for most of the heating                “Any overhang which has a
                                              season, but almost entire blockage of
                                                                                                  very significant effect on reducing
                                              the midsummer sun. Figures 68 and 69                the cooling load also has a very
                                              show a simple, convenient scheme for                significant effect on reducing the
                                              determining the sun angles at noon on               solar heating contribution.”
                                              the summer solstice, winter solstice, and
                                              equinoxes. Overhangs are in some ways              An alternative to fixed shading is mov-
Figure 67. The effect of different            more important in Alaska than they              able shading (such as awnings). This is
              values of air gaps between      are elsewhere because our lower solar           awkward and not much favored by de-
              the double glazing layers       angles require exaggerated overhangs to         signers, but it is quite effective. The shade
              and the effect of different     achieve the desired amount of shading.          can be left on until late in the fall, thus
              assumptions of wind velo-       Without proper shading, overheating             substantially reducing overheating. The
              city on performance in          can begin in March and April and con-           shade can then be taken off and left off
              Madison, Wisconsin (after       tinue through the summer. Fortunately,          until late in the spring after the heating
              Balcomb et al., 1980).          however, overheating in most of Alaska          season is over.

Figure 68a. The range of solar elevation angles at the latitude         Figure 68b. Unlike the lower latitudes, a small overhang has
             of Anchorage, Alaska (60°30’N). The maximum                            little effect on shading the summer sun in Alaska.
             elevation is 54° on June 21, and the minimum is                        Larger overhangs are required in Alaska because
             7.5° on December 21.                                                   of the lower solar elevation angles.

Figure 69a. The range of solar elevation angles at the latitude         Figure 69b. Like Anchorage (Figure 62b), a small overhang
             of Fairbanks (64°N). The maximum elevation is                          in Fairbanks will not significantly alter summer
             49.5° on June 21, and the minimum is 2.6° on                           solar gain on a window. A larger overhang is
             December 21.                                                           necessary because of the lower solar elevation

   Another option is to use night insula-     duration of snow cover and lower sun               Thus the input information required
tion as shading. It allows a very simple      angles, causing more solar radiation to            for a detailed design load calculation
and effective means of accommodating          be reflected onto solar collection surfaces.       is unknown.
to the weather; it markedly improves          Tables 12 and 13 show the reflectivity         2. Few designers would take the time to
performance during the winter and is          values for fifteen different surface charac-      go through this involved calculation.
especially effective at reducing summer       teristics and twelve representative winter        Design development is an iterative
overheating. Types of night insulation        landscapes, respectively.                         process, and a much faster procedure
that are located outside the window are                                                         is needed if it is to be used.
particularly effective for summer shad-       Estimating The Building
ing. If they are located inside the window,   Load Coefficient                               Quick and Dirty Heating
the designer must be particularly careful     The first step in the process is potentially   Load Estimate
to avoid material damage associated with      difficult: obtaining an estimate of the
buildup of heat between the glazing and                                                      Therefore, there is a need for a “quick
                                              thermal load of the building, even before      and dirty” method for estimating heating
the insulation by using a light-colored       the design is final. Accepted procedures
or reflective outer surface. Thermal                                                         load. The procedure should take into ac-
                                              that predict the heating load of build-        count the important gross characteristics
stress breakage of glazing can also be a      ings are described in the 1977 ASHRAE
problem. Use of tempered glass will help                                                     of the building that have been established
                                              Handbook of Fundamentals. Given detailed       before design development. These char-
reduce the likelihood of this occurring.      knowledge of the building geometry             acteristics are the building gross floor area
                                              and construction, they provide com-            and perimeter; the number of stories; the
Effect of Ground Reflectance                  prehensive estimates of each element of        R-values of the walls and roof; whether
The effect of ground reflectance on the       the heating load. They are customarily         the building is to be built with concrete
performance of solar energy systems was       used during the construction documents         slab on grade (i.e., no basement), over a
mentioned previously in this manual (see      phase of the design, to accompany de-          basement, or over a crawl space; and a
section on active solar water heating).       tailed drawings and specifications.            rough idea of the fraction of the wall area
There is little doubt that the increased         This procedure provides little help to      that will be allocated to windows.
ground reflectance due to snow cover          the designer during the design devel-             The following procedure fills this
contributes significantly to useful solar     opment phase of a project. It has two          need. It will give answers that are usu-
radiation during the winter season at         failings:                                      ally within 10 percent of the detailed
high latitudes. Willcut et al. (1975), in a
                                              1. Detailed specifications of the build-       ASHRAE* heating load calculation, and
study of Canadian locations, found that
                                                 ing are not known. Windows have             it will show the relative contribution of
ground-reflected solar radiation can
                                                 not yet been precisely sized, wall          the various important factors that make
contribute 8 percent of the total annual
                                                 construction details have not yet been      up the heating load.
usable energy. In Alaska, this fraction
                                                 firmed up, and exact wall areas and
may be even higher because of the longer                                                     *American Society of Heating, Refrigerat-
                                                 building volumes are not yet known.         ing and Air Conditioning Engineers

TABLE 12: REFLECTANCE VALUES FOR FIFTEEN CHARACTERISTIC SURFACES                              Calculating the Building
   (INTEGRATED OVER SOLAR SPECTRUM AND ANGLE OF INCIDENCE)                                    Load Coefficient
                                                                                              The procedure consists of calculating
                  Surface                                                 Reflectance         several components of the Building Load
                                                                                              Coefficient. It is based on Lower 48 expe-
 1.    Snow (freshly fallen or with ice film)                                .70              rience and needs verification for Alaska.
 2.    Water surfaces (relatively large incidence angles)                    .07              This coefficient is the additional heating
 3.    Soils (clay, loam, etc.)                                              .14              that would be required to maintain a one
 4.    Earth roads                                                           .04              degree Fahrenheit increase in the build-
 5.    Coniferous forest (winter)                                            .07              ing inside temperature. For example, if
 6.    Forests in autumn, ripe field crops, plants                           .26              the heat required to maintain the build-
 7.    Weathered blacktop                                                    .10              ing at 70°F were determined to be 400,000
 8.    Weathered concrete                                                    .22              BTU/day, and the heat required to main-
 9.    Dead leaves                                                           .30              tain the building at 71°F were determined
 10.   Dry grass                                                             .20              to be 420,000 BTU/day, then the Building
 11.   Green grass                                                           .26              Load Coefficient is equal to the difference
 12.   Bituminous and gravel roof                                            .13              or 20,000 BTU/day for each °F (often
 13.   Crushed rock surface                                                  .20              expressed as 20,000 BTU/day•°F).
 14.   Building surfaces, dark (red brick, dark paints, etc.)                .27
                                                                                                The procedure consists of adding to-
 15.   Building surfaces, light (light brick, light paints, etc.)            .60
                                                                                              gether several estimated contributions
                                                                                              of heat loss.
   In the process of calculating a heating        and convection as opposed to loss              Start by making rough estimates of
load, a Building Load Coefficient (BLC)           dominated by air exchange, like large       the combined area of all floors (ft2) and
is determined. The primary use of the             public buildings). It is not particularly   the perimeter (the combined length in
BLC is for estimating the solar savings           appropriate for large buildings where the   feet of all external walls at floor level).
of buildings heated by passive solar              bulk of the heating energy is contributed   Then, either estimate the combined area
energy.                                           from internal energy generation. It is      of all east, west, and north windows, or
   The procedure is not intended to be            by no means intended to substitute          use: nonsouth window area = (2/3) ×
comprehensive, and it will not handle             for a detailed ASHRAE heating load          (perimeter) × (ceiling) × (nonsouth win-
all situations. For example, it should not        calculation, which should always be         dow fraction). The nonsouth window
be used for underground structures. It is         done during the construction documents      fraction will normally be between 0.05
primarily intended for small buildings            phase. This procedure should only           (for a situation with minimum window
with skin-dominated loads (that is,               be used for rough thermal estimation        area) and 0.10 for a case with standard
dominated by heat loss by conduction              during design development.                  window area.

                 TABLE 13: REFLECTANCE VALUES FOR TWELVE                                Walls:
                   REPRESENTATIVE WINTER LANDSCAPES                                        Lw = 24 ×         wall area
                                                                                                        R - value of walls
                     Rural Areas                                    Reflectance
                                                                                        where wall area = (perimeter) × (ceiling
  Fields with Snow Cover                                                                  height) • (nonsouth window area) •
     1. Field with wooded area in background                             0.66-0.73        (south window area)
     2. Open field (soil and dry grass), new road                        0.61-0.70
     3. Trees dispersed in field                                         0.62           Nonsouth Window:

  Wooded Areas                                                                            Lg = 26 × nonsouth window area
                                                                                                        number of glazings
    1. Conifer forest (with heavy snow cover)                            0.61
    2. Deciduous forest (with heavy snow cover)                          0.72           Perimeter (slab on grade):
                                                                                           Lp = 100 ×
    1. Open water                                                        0.16               length of foundation perimeter
    2. Water covered with ice and snow                                   0.68           (R - value of perimeter insulation) + 5
    3. Partially open waterway (trees and houses in background)          0.43-0.66      Floor (over vented crawl space if
                     Urban Areas                                    Reflectance
                                                                                           Lf = 24 × area of ground floor
     1.   Commercial and institutional areas                             0.16-0.38                       R - value of floor
     2.   Residential areas (dwelling and roadway)                       0.21-0.45      Basement (heated basement or other
     3.   Educational institution                                        0.36-0.42      fully earth-sheltered wall, including
     4.   Recreational area (park)                                       0.49           floor losses):
                                                                                           Lb = 256 ×
  Next, estimate the south (solar) win-     24 × U × A, where U is the U-value of                    length of wall
dow area, being careful to only include     the element (U is equal to 1⁄R) and A is       (R - value of wall insulation) + 8
the net exposed portion of the window.      the area of the element. For glazings,
(The rest doesn’t contribute to solar       the approximation is made that U = 1.1      Note: normally one of Lp, Lf, or Lb will
gain!) The derivation of the following      × (number of glazings). For the perimeter   apply.
formulas is based on a simplified use of    and basement loss terms, the form is an
the ASHRAE-type heat loss approach.         approximation for rectangular slabs. So
All terms contain a factor of 24 to con-    compute the following.                         Lr = 24 ×        roof area
vert from BTU/hr•°F to BTU/day•°F.                                                                      R - value of roof
The terms Lw, Lg, Lf, and Lr are simply

Infiltration:                                 and perimeter are insulated with 2 inches    Infiltration:
Li = (0.432 × (average air changes per        of styrofoam. There are 60 ft2 of nonsouth
    hour) × (air density ratio) × (ceil-      double-glazed windows, and the roof             Li = 0.432 × 0.3 × 8,000
    ing height) × (combined area of all       has 12-inch trusses with 11.0 inches of
    floors)                                   fiberglass. Ceilings are 8 feet high.           Li = 1,037 BTU/°F•day
Add the appropriate components to               With this information, we can apply
                                              the previous equations. R-values are           Combining all these factors, we get
obtain the final BLC estimate, for ex-                                                     the Building Load Coefficient estimate
ample:                                        obtained from Appendix D.
                                                                                           in units of BTUs per °F•day.
                                              Walls:                                          Lw   =    1,120
        BLC = Lw + Lg + Lr + Lp + Li                                                          Lg   =      848
                                                 Lw =                                         Lp   =      875
  Note that the solar glazing is not in-                    24
                                                                                              Lr   =      908
cluded in the calculation of the Building                                                     Li   =    1,037
                                                 Lw = 1,120 BTU/°F
Load Coefficient. This is done for two
reasons:                                      Nonsouth window:                                BLC = 4,788 BTU/°F•day
1. The solar glazing would not be pres-              (26 × 60)
   ent in a nonsolar building, which is         Lg =                                         This is a very good structure from a
   the principal basis of comparison.                                                      heat loss standpoint. Note, however,
2. The solar wall is a net energy gainer         Lg = 848 BTU/°F                           that this calculation neglects losses from
   (with shutters!), not a loser, and to                                                   south glazing. This assumption is critical
                                              Perimeter (slab on grade):                   for Alaska applications because of the
   represent it as part of the load would
                                                                                           need for shuttering the south facade
   be misleading.                                Lp = 100 × 140 = 875
                                                         11.0 + 5                          when heat from the sun is not available.
Example Building Load                                                                      Obviously, there will be some heat loss
                                                 Lp = 875 BTU/°F
Coefficient                                                                                from the south glazing, and experience
                                                                                           will further define its importance.
Here is an example of a heat loss calcu-        (Only the perimeter heat loss applies
lation using this method. The building        since house is slab on grade.)                 The contributions to the Building Load
is 1,000 square feet in floor area, well                                                   Coefficient from conduction through the
built (more insulation and better vapor                                                    walls, nonsouth glazing, and roof are all
barrier than average), 20 × 50 ft, slab on                                                 significant and roughly comparable in
                                                             140                           magnitude. A large contribution is associ-
grade. The infiltration is 0.3 air changes       Lr = 24 ×
                                                             37                            ated with the heating of infiltration air.
per hour, and the walls have 7 inches of
fiberglass in a 2 × 8-frame wall. The floor      Lr = 908 BTU/°F•day                       This deserves special comment.

Infiltration                                     lounge might require 4 ACH during         test house with a total volume of 12,557
During design development, there is not          periods of occupancy, and many            ft3. For example, assume that the house is
enough information available to estimate         other commercial applications might       a total-electric residence (no open flames)
the building infiltration. The minimum           also require high values. Fresh air       and is occupied by four people. This is
value that might be selected will depend         must be provided in some manner.          the simplest example, and virtually all
on one of two considerations:                    Tight structures, in particular, offer    real situations are worse than this!
                                                 the occupant the benefit of minimal         A primary concern is the respiratory
1. The minimum air change rate recom-
                                                 unwanted air infiltration; hence, one     requirement for the occupants of a house.
   mended for small buildings is 1⁄2 air
                                                 may control the amount of exhaust         Generally humans need 20 percent
   change per hour (ACH). Below this,
                                                 and makeup air required by ventila-       oxygen in the air. They can exist with
   the building becomes stuffy, odors
                                                 tion. Ventilation is necessary for the    15 percent oxygen, but combustion will
   build up, and humidity accumulation
                                                 following reasons.                        not occur. Death for humans will result
   due to water use within the building
   may be a problem. Buildings with              a. To supply the proper amount of         with only 5 percent to 7 percent oxygen.
   lower infiltration rates than this (for          oxygen for the health of the oc-       Table 14 indicates human oxygen and air
   example the Saskatchewan House,                  cupants.                               requirements for various activities.
   the Phillips house in Aachen, Ger-            b. To supply the proper amount of           If the four occupants are assumed to
   many, and the Denmark Zero-En-                   oxygen necessary for combustion        engage in activities of the 50 ft3/min level
   ergy House) often employed forced                if open-flame furnaces, fireplaces,    for 16 hours per day and the 0.21 ft3/min
   ventilation with heat recovery units.            etc., are on the premises.             level for 8 hours per day, the minimum
   This approach is routinely used in            c. To dilute or eliminate excessive       ventilation level for the house would be
   large commercial buildings and it is             moisture in the air during the         2,343 ft3/day. This requires a complete
   now considered necessary for smaller             summer.                                air change to the house only once every
   structures (homes) in Alaska (Seifert,                                                  5.5 days.
                                                 d. To dilute or eliminate odors
   et al., 2002).                                                                             Some recommendations require that
                                                    generated in the lavatory, locker-
2. The air exchange rate associated with            room, and kitchen.                     the quantity of outdoor air introduced
   normal building construction is now                                                     into spaces for normal respiratory and
                                                 e. To dilute or remove the heat pro-
   typically 1⁄3 ACH or less. To achieve a                                                 odor-control needs shall be no greater
                                                    duced by internal sources during
   low infiltration rate requires meticu-                                                  than 5 ft3/min per person. With four
                                                    the summer.
   lous attention to sealing all cracks                                                    occupants and 5 ft3/min per occupant,
   where air might leak into or out of the      In order to make energy-use projec-        the ventilation rate for a house is 29,240
   building. Some applications may re-       tions for well-designed buildings, it is      ft3 per day. This results in about 2.3 air
   quire much higher air exchange rates      necessary to establish a reasonable level     changes per day.
   as a matter of building code require-     of ventilation. The level of ventilation
                                                                                             Although there are no absolutes for
   ments. For example, a restaurant or       will be determined for a 33 × 46 × 8.25 ft.
                                                                                           determining correct ventilation levels

                      FOR VARIOUS ACTIVITIES

        Activity               Oxygen Consumed                    Air Required
                                ft3/min                           ft3/min

    Sleeping                       0.0075                           0.188
    Sitting                        0.0094                           0.219
    Standing                       0.0113                           0.251
    Walking - 2 mph                0.0204                           0.439
    Walking - 4 mph                0.0376                           0.815
    Jogging                        0.063                            1.348
    Maximum exertion               0.094-0.125                      2.04-3.13

for odor and humidity control, some              AHFC, Alaska State Thermal Efficiency
observations are useful. Data available          Standards.)
for infiltration through window cracks              Where open flames, including fire-
and door openings indicate a ventilation         places, are present in well-sealed homes,
level in a relatively tight house of             increased makeup air and ventilation
approximately two air changes per day.           must be provided. For purposes of en-
Actual houses that fit these conditions          ergy conservation, combustion air should
show this is the minimal level for               be ducted to furnaces or fireplaces from
elimination of lingering odors, especially       outside. As an alternate solution, de-
pungent cooking odors. The ventilation           livery of heated makeup air (incoming
rate of two air changes per day is just          fresh air) to the proximity of the fireplace
below the code minimum of 2.33. A                may be considered. Ventilation may also
residence should have no less than two           be required for the removal of excess
air changes per day. Until sufficient            internal heat.
experience is gained in the ventilation
of these houses, each should be analyzed
before construction, and provisions
should be made for increasing or
decreasing ventilation as necessary. (See


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