Selecting Windows for Energy Efficiency Windows and Daylighting

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					What’s New                       Selecting Windows
in Building                      for Energy Efficiency
                                 New window technologies have increased energy
 Efficiency                      benefits and comfort, and have provided more practical
                                 options for consumers. This selection guide will help
                                 homeowners, architects, and builders take advantage of
                                 the expanding window market. The guide contains three
                                 sections: an explanation of energy-related window
                                 characteristics, a discussion of window energy
     U.S. Department of Energy
                                 performance ratings, and a convenient checklist for
                                 window selection.
                                        electing the right window for a    unintentional (infiltration). (See the

                                 S      specific home invariably
                                        requires tradeoffs between dif-
                                 ferent energy performance features,
                                                                           Window Energy Glossary for expla-
                                                                           nations of these terms.)

                                 and with other non-energy issues. An      Insulating Value
INSIDE                           understanding of some basic energy        The non-solar heat flow through a
                                 concepts is therefore essential to        window is a result of the temperature
Solar Control              6     choosing appropriate windows and          difference between the indoors and
                                 skylights. As illustrated on the fol-     outdoors. Windows lose heat to the
Window Energy                    lowing page, three major types of         outside during the heating season and
Rating and Labeling       10     energy flow occur through windows:        gain heat from the outside during the
                                 (1) non-solar heat losses and gains in    cooling season, adding to the energy
Window Checklist          13     the form of conduction, convection,       needs in a home. The effects of non-
                                 and radiation; (2) solar heat gains in    solar heat flow are generally greater
Window Energy                    the form of radiation; and (3) airflow,   on heating needs than on cooling
Glossary                  15     both intentional (ventilation) and        needs because indoor-outdoor tem-
                                                                           perature differences are greater dur-
                                                                           ing the heating season than during
                                                                           the cooling season in most regions of
                                                                           the United States. For any window
                                                                           product, the greater the temperature
                                                                           difference from inside to out, the
                                                                           greater the rate of heat flow.
                                                                              A U-factor is a measure of the rate
                                                                           of non-solar heat flow through a win-
                                                                           dow or skylight. (An R-value is a
                                                                           measure of the resistance of a win-
                                                                           dow or skylight to heat flow and is
                                                                           the reciprocal of a U-factor.) Lower
                                                                           U-factors (or higher R values), thus
                                                                           indicate reduced heat flow. U-factors
                                                                           allow consumers to compare the
                                                                           insulating properties of different win-
                                                                           dows and skylights.
                                                                              The insulating value of a single-
                                                                           pane window is due mainly to the
The three major types of
energy flow that occur
through windows: (1) non-
solar heat losses and gains in
the form of conduction, con -
vection, and radiation; (2)
solar heat gains in the form of
radiation; and (3) airflow, both
intentional (ventilation) and
unintentional (infiltration).


                                            and Conduction



                                   thin films of still air on the interior and   nologies aimed at decreasing U-factors.
                                   moving air on the exterior glazing sur-       These technologies include low-emittance
                                   faces. The glazing itself doesn’t offer       (low-E) coatings and gas fills.
                                   much resistance to heat flow. Additional         A low-E coating is a microscopically
                                   panes markedly reduce the U-factor by         thin, virtually invisible, metal or metallic
                                   creating still air spaces, which increase     oxide coating deposited on a glazing sur-
                                   insulating value.                             face. The coating may be applied to one
                                      In addition to conventional double-pane    or more of the glazing surfaces facing an
                                   windows, many manufacturers offer win-        air space in a multiple-pane window, or to
                                   dows that incorporate relatively new tech-    a thin plastic film inserted between panes.
                                                                                 The coating limits radiative heat flow
                                                                                 between panes by reflecting heat back
                                                                                 into the home during cold weather and
                                                                                 back to the outdoors during warm weath-
                                                                                 er. This effect increases the insulating
                                                                                 value of the window. Most window man-
                                                                                 ufacturers now offer windows and sky-
                                                                                 lights with low-E coatings.
                                                                                    The spaces between windowpanes can
                                                                                 be filled with gases that insulate better
                                                                                 than air. Argon, krypton, sulfur hexafluo-
                                                                                 ride, and carbon dioxide are among the

                                                                                 High-performance windows make energy-
                                                                                 efficient homes possible with greater
                                                                                 freedom of design than in the past.
gases used for this purpose. Gas fills add        nounced influence on the U-factors of
only a few dollars to the prices of most          windows and skylights. As a result, frame
windows and skylights. They are most              and spacer options have also multiplied as
effective when used in conjunction with           manufacturers offer improved designs.
low-E coatings. For these reasons, some              Window frames can be made of alu-
manufacturers have made gas fills stan-           minum, steel, wood, vinyl, fiberglass, or
dard in their low-E windows and sky-              composites of these materials. Wood,
lights.                                           fiberglass, and vinyl frames are better
   The insulating value of an entire win-         insulators than metal. Some aluminum
dow can be very different from that of the        frames are designed with internal thermal
glazing alone. The whole-window U-fac-            breaks, non-metal components that reduce
tor includes the effects of the glazing, the      heat flow through the frame. These ther-
frame, and, if present, the insulating glass      mally broken aluminum frames can resist
spacer. (The spacer is the component in a         heat flow considerably better than alu-
window that separates glazing panes. It           minum frames without thermal breaks.
often reduces the insulating value at the         Composite frames may use two or more
glazing edges.)                                   materials (e.g. aluminum-clad wood,
   Since a single-pane window with a              vinyl-clad wood) to optimize their design
metal frame has about the same overall U-         and performance, and typically have insu-
factor as a single glass pane alone, frame        lating values intermediate between those
and glazing edge effects were not of great        of the materials comprising them. Frame
concern before multiple-pane, low-E, and          geometry, as well as material type, also
gas-filled windows and skylights were             strongly influences thermal performance
widely used. With the recent expansion of         properties.
thermally improved glazing options                   Spacers can be made of aluminum,
offered by manufacturers, frame and spac-         steel, fiberglass, foam, or combinations of
er properties now can have a more pro-            these materials. Spacer thermal perfor-

                   Aluminum Frame without thermal break (with conventional spacer)

          Alum. Frame with thermal break (with conventional spacer)

  Wood or Vinyl Frame (with insulated spacer)

  GLAZING TYPE                           U-FACTOR (Btu/hr-ft2-˚F)

  Single glass                            -----                 1.07               1.30

  Double glass,
  1/2-inch air space                     0.48                  0.62                0.81

  Double glass, e = 0.20*,
  1/2-inch air space                     0.39                  0.52               0.70

  Double glass, e = 0.10*,                                                                      Representative Window
  1/2-inch air space                      0.37                 0.49               0.67          U-Factors
                                                                                                *e is the emittance of the
  Double glass, e = 0.10*,                                                                      low-E coated surface.
  1/2-inch argon space                   0.34                  0.46               0.64          Values are for 3-foot-by-5-
                                                                                                foot windows. U-factors vary
  Triple glass, e = 0.10 on two                                                                 somewhat with window size.
  panes*, 1/2-inch argon spaces          0.23                  0.36               0.53
                                                                                                Source: ASHRAE Handbook—
  Quadruple glass,                                                                              Fundamentals, American Society of
  e = 0.10 on two panes*,                                                                       Heating, Refrigerating, and Air-
                                                                                                Conditioning Engineers, Inc., Atlanta,
  1/4-inch krypton spaces                 0.22                  -----              -----        GA, 1993.
Outdoor air temperature and indoor air rela-
tive humidity combinations at which conden-
sation will occur on the center of the glass for
single, double, and triple glazings, some with
low-E coatings and gas fills.
On or above each curve, the conditions are right
for condensation. Below each curve, condensa -
tion will not occur on that glazing type as long
as the glazing is exposed to room air circula -
tion. Values are based on winter conditions:
70˚F indoor air temperature, 15 mph outdoor
air velocity, and no incident solar radiation.

Source: WINDOW 4.1 (a computer program for calculating
the thermal and optical properties of windows), Lawrence
Berkeley National Laboratory, Berkeley, CA, 1994.

                                     mance is as much a function of geometry       Preventing Condensation
                                     as of composition. For example, some          Air can hold varying amounts of water
                                     well-designed metal spacers insulate          vapor or moisture. The warmer the air is,
                                     almost as well as foam.                       the more moisture it can hold. The
                                        The table on page 3 shows representa-      amount of moisture in the air, expressed
                                     tive U-factors for window glazing, frame,     as a percentage of the maximum amount
                                     and spacer combinations under winter          the air could hold at a given temperature,
                                     design conditions. Due to their orientation   is called its relative humidity. For health
                                     and their greater projected surface areas,    and comfort, indoor air should contain
                                     domed and other shaped tilted and hori-       some moisture. The relative humidity
                                     zontal skylights have significantly higher    should generally be between 30% and
                                     U-factors than do vertical windows of         40% at normal room temperature.
                                     similar materials and opening sizes.             The relative humidity of air can be
                                                                                   increased by adding more moisture or by
                                                                                   reducing the temperature. When the rela-
                                                                                   tive humidity reaches 100%, the air can
                                                                                   hold no more moisture, and water begins
                                                                                   to condense from it. The temperature at
                                                                                   which this condensation occurs is called
                                                                                   the dew point temperature of the air.
                                                                                   When moist air comes in contact with a
                                                                                   cold surface in a home, it may be cooled
                                                                                   to its dew point temperature, resulting in
                                                                                   condensation on the surface.

                                                                                   Single-glazed windows characteristical -
                                                                                   ly suffer from water condensation prob -
                                                                                   lems and the formation of frost on the
                                                                                   inside surface of the glass in winter.
    Windows don’t cause condensation, but     Recommendations for Selecting
historically they have been the first and     Window U-Factors
most obvious place it occurs. This is         When shopping for windows and sky-
because windows generally have lower          lights, pay close attention to whether the
thermal resistances than insulated walls,     U-factor listed by the manufacturer
ceilings, and floors. As a result, their      applies to the glazing only or to the entire
inside temperatures are usually lower than    unit. If it is for the glazing only, the over-
those of other surfaces in a home during      all U-factor may be considerably higher
cold weather. If the air in a home is humid   because of the frame and spacer effects.
enough, water will condense from it when      These effects increase with decreasing
it is cooled at a window surface.             total window area. Compare different
Condensation is most often thought of as      window types or makes by their total U-
a cold climate winter problem. However,       factors, which are best obtained from
in hot, humid weather, moisture can con-      NFRC labels. (See Window Energy
dense on the outside surface of a poorly      Rating and Labeling, page 10.) New win-
insulated window in an air-conditioned        dow energy ratings and the RESFEN
building.                                     computer program can be used to estimate
    Left unchecked, condensation can dam-     the relative energy usage associated with a
age window frames, sills, and interior        particular window type and U-factor.
shades. Water can deteriorate the sur-            Avoid aluminum-frame windows with-
rounding paint, wallpaper, plasterboard,      out thermal breaks if possible. Even in
and furnishings. In severe cases, it can      milder climates, these windows tend to
seep into adjoining walls, causing damage     have low inside surface temperatures dur-
to the insulation and framing.                ing the heating season, giving rise to con-
    The indoor air coming in contact with     densation problems. Aluminum-frame
energy-efficient windows is less likely to    windows with properly designed thermal
be cooled to its dew point temperature        breaks can be used in moderate climates.
because the inside surface temperatures       Wood, vinyl, and fiberglass are the best
remain higher during cold weather than do     frame materials for maximum insulating
those of windows with single glazing, tra-    value.
ditional metal spacers, and metal frames.         Single-pane windows are impractical in
    The figure on page 4 illustrates condi-   heating-dominated climates. In these
tions under which condensation will form      regions, multiple-pane, low-E, and gas-
on the center of the glass of five glazing    filled window configurations are advis-
types with widely varied U-factors. The       able. In most climates, glazings with low-
graph shows clearly that the risk of con-     E coatings and gas fills will be a choice
densation at the center of the glass is       that provides significant energy savings in
reduced as the insulating value of the        a cost-effective product. Low-E and gas
glass increases. Even at an outdoor air       fills have now become a common option
temperature of -30˚F, the indoor air rela-    for many manufacturers, which reduces
tive humidity must be nearly 70% before       their added cost. The resultant total win-
condensation will form on the triple glaz-    dow U-factor should be 0.5 or lower and
ing with two low-E coatings and a gas fill.   preferably below 0.4 for maximum energy
On the other hand, at an outdoor tempera-     savings. Consumers should select win-
ture of 10˚F, condensation will form on       dows with long warranty periods, which
the single glazing at an indoor relative      indicate sound window design and con-
humidity of only 18%.                         struction, and a reduced probability of
    Condensation is even more likely to       insulating glass seal failure or gas leak-
occur at window spacers and frames,           age, which would reduce performance.
which are usually less insulating than the    Remember that lower window and sky-
corresponding glazings. With so many          light U-factors mean less energy con-
insulating glazing types available, efforts   sumption, lower utility bills, and greater
to prevent condensation have shifted          comfort in the living space.
toward the development of better insulat-
ing spacers and frames.
Daily total solar heat gain
through 1/8-inch clear sin -
gle glass for various win -
dow orientations on very
clear days at 40˚N lati -
tude (for example,
Columbus, Ohio, and
Boulder, Colorado).

Source: ASHRAE Handbook—
Fundamentals, American Society
of Heating, Refrigerating, and Air-
Conditioning Engineers, Inc.,
Atlanta, GA, 1993.

                                      Solar Control                                  40˚N latitude. South-facing windows
                                      Solar transmission through windows and         allow the greatest and potentially most
                                      skylights can provide free heating during      beneficial solar heat gain during the heat-
                                      the heating season, but it can cause a         ing season, while admitting relatively lit-
                                      home to overheat during the cooling sea-       tle of the solar heat that contributes to
                                      son. Depending upon orientation, shading       cooling requirements during the cooling
                                      and climate, solar-induced cooling costs       season. The reverse is true for skylights
                                      can be greater than heating benefits in        and east- and west-facing windows. North
                                      many regions of the United States. In fact,    exposures transmit only minimal solar
                                      solar transmission through windows and         heat at any time. The ultimate importance
                                      skylights may account for 30% or more of       of these climatic and orientation effects
                                      the cooling requirements in a residence in     will depend on the type of glazing under
                                      some climates.                                 consideration.
                                         Because the sun’s position in the sky          The Solar Heat Gain Coefficient
                                      changes throughout the day and from one        (SHGC) is a measure of the rate of solar
                                      season to another, window orientation has      heat flowing through a window or sky-
                                      a strong bearing on solar heat gain. The       light. (AShading Coefficient (SC) is the
                                      figure above shows the solar heat gain         previous standard indicator of a window’s
                                      through 1/8-inch clear single glass for var-   shading ability and for simple glazings is
                                      ious window orientations on very clear         approximately equal to the solar heat gain
                                      days in the heating and cooling seasons at     coefficient multiplied by 1.15.) Solar heat
                                                                                     gain coefficients allow consumers to com-
                                                                                     pare the solar heat gain properties of dif-
                                                                                     ferent windows and skylights. The solar
                                                                                     heat gain coefficient accounts for both the
                                                                                     transmissive glazing element, as well as
                                                                                     the opaque frame and sash.
                                                                                        Additional glazing layers provide more
                                                                                     barriers to solar radiation, thus reducing
                                                                                     the solar heat gain coefficient of a win-
                                                                                     dow. Tinted glazings, such as bronze and
                                                                                     green, provide lower solar heat gain coef-

                                                                                     The NFRC label provides useful informa -
                                                                                     tion—such as the window’s U-Factor,
                                                                                     Solar Heat Gain Coefficient, Visible
                                                                                     Light Transmittance, and Air Leakage
                                                                                     rating—to help you choose wisely.
ficients than does clear glass. Low-E coat-       or space. Mirror-like reflective glazings
ings can be engineered to reduce window           are commonly used in office buildings,
solar heat gain coefficients by rejecting         but only occasionally chosen for resi-
more of the incident solar radiation.             dences. While they may have very low
Spectrally selective glazings, including          solar heat gain coefficients, they block so
some low-E coated glazings with low               much of the light and view that they are
solar heat gain coefficients and new light        not normally desirable in homes.
blue and light blue-green tinted glazings,           The table below shows representative
block out much of the sun’s heat while            solar heat gain coefficients and visible
maintaining higher visible transmittances         transmittances for glazings with typical
and more neutral colors than more heavily         wood or vinyl frames and aluminum
tinted bronze and grey glazings. High-            spacers. Aluminum-frame windows of
transmittance, low-E coatings, used in            comparable size and glazing type general-
conjunction with a tinted outer glass layer,      ly have slightly higher solar heat gain
also reduce solar heat gain by preventing         coefficients because of their thinner
the absorbed heat from reaching the interi-       frames and greater glazing areas.

                                                           Solar Heat Gain Coefficient

                                             Visible Transmittance

                                                 glass      total      glass      total
   GLAZING TYPE                                   only     window       only     window

   Single glass, clear                            0.90      0.66        0.86       0.66

   Single glass, bronze tint                      0.68      0.50        0.73       0.56

   Single glass, green tint                       0.82      0.60        0.71       0.55

  Single glass, clear, solar control              0.25      0.18        0.40       0.33
  retrofit film

   Double glass, clear, 1/2-inch air space        0.81      0.59        0.76       0.59

   Double glass,bronze tint                       0.62      0.45        0.62       0.49         Representative Window
   outer pane, 1/2-inch air space                                                               Solar Heat Gain
                                                                                                Coefficients and Visible
   Double glass, green tint                       0.75      0.54        0.60       0.47         Transmittances
   outer pane, 1/2-inch air space                                                               Values are for a 3-foot-by-
                                                                                                5-foot horizontal slider
   Double glass, clear,                           0.76      0.55         0.65      0.51         window with wood or vinyl
   e = 0.20*, 1/2-inch air space                                                                frames and aluminum
                                                                                                spacers. Solar heat gain
   Double glass,                                  0.45      0.33        0.35       0.29         coefficients vary somewhat
   “Southern” low-E, e=0.08*,                                                                   with window size.
   on tint, 1/2 inch air space
                                                                                                *e is the emittance of the low-
                                                                                                E coated surface; emittance
   Double glass, spectrally selective,            0.72      0.52        0.40       0.33         will vary slightly with specific
   e = 0.04*, 1/2-inch argon space                                                              products.

   Triple glass, clear,                           0.68      0.50        0.49       0.39         Source: WINDOW 4.1 (a computer
   e = 0.08 on two panes*,                                                                      program for calculating the ther -
                                                                                                mal and optical properties of win -
   3/8 to 1/2-inch air or argon spaces                                                          dows), Lawrence Berkeley National
                                                                                                Laboratory, Berkeley, CA, 1994.
Ultraviolet Protection                        tion levels at that time on south-facing
Ultraviolet radiation is the main compo-      windows, especially those with adequate
nent of sunlight that can fade and damage     roof overhangs.
drapes, carpets, furniture, and paintings        Skylights and east- and west-oriented
when transmitted through windows and          windows may warrant lower solar heat
skylights. Efforts to produce window          gain coefficients since they transmit the
glazings that transmit less ultraviolet       most solar heat during cooling periods. In
energy have met with some success. In         most climates, there is not much point in
general, windows and skylights with plas-     spending more money to obtain lower
tic glazing layers or low-E coatings          solar heat gain coefficients for north-fac-
reduce ultraviolet transmission. Even         ing windows.
without any ultraviolet radiation, sunlight       In hot, sunny climates, select windows
can still cause fading of fabrics and other   with spectrally selective glazings to pro-
furnishings.                                  vide low solar heat gain coefficients with-
                                              out loss of light. Darker tinted glazings
Recommendations for                           also provide lower solar heat gain coeffi-
Solar Control                                 cients, but they will yield somewhat
                                              decreased outdoor visibility, particularly
Consumers should consider two aspects         at night. Where glare is a concern, this
of window selection to control solar          effect may be desired, but under other
gain—the selection of the window itself       conditions it may not. In climates where
and the choice of interior or exterior        cooling loads are large, look for windows
shading devices. Traditional windows          with a SHGC of 0.4 or less. To maintain
with clear glass required the use of shad-    good light transmittance and visibility,
ing devices to obtain adequate perfor-        select windows whose glazings have visi-
mance, especially when the orientation        ble transmittance of 0.6 or higher.
admitted substantial sunlight in summer.         In some hot climates, where winters
However, modern high-performance win-         are mild, it might seem reasonable to
dows can do such a good job of control-       select a single-glazed window with a low
ling sunlight that the importance of these    Solar Heat Gain Coefficient rather than a
shading systems is reduced.                   more typical double glazing. However,
   Window Solar Heat Gain Coefficients        single glazings have a more limited range
should ideally be selected according to       of solar control (even if laminated glass
orientation, but it may not always be         and glue-on plastic films are considered),
practical to do so. If south exposures are    so a double-glazed window with appro-
to admit beneficial solar heat during the     priate glazing choice as noted above, may
heating season, their Solar Heat Gain         be the best overall solution, even in hot
Coefficients should be high. These high       climates.
coefficients will not usually result in          Exterior or interior shading devices—
overheating problems during the cooling       such as awnings, louvered screens, sun-
season because of the lower solar radia-      screens, venetian blinds, roller shades,
                                              and drapes—are essential for shading
                                              clear glass, and can complement and
                                              enhance the performance of windows
                                              with low Solar Heat Gain Coefficients.
                                              One advantage of many shading devices
                                              is that they can be adjusted to vary solar
                                              heat transmission with the time of day
                                              and season. But windows with “built-in”
                                              lower Solar Heat Gain Coefficients pro-

                                              Commercially available windows must
                                              meet numerous standards and build -
                                              ing regulations addressing condensa -
                                              tion resistance, sound control, main -
                                              tenance requirements, and overall
                                              durability of the unit.
vide better visibility and require less man-      and skylight frames, sash, and glazings.
agement and maintenance in today’s busy           This leakage can account for up to 10%
households.                                       of the energy usage in a home. The air-
   Exterior shading devices are more              tightness of a window depends on both
effective than interior devices in reducing       the characteristics of the window—such
solar heat gain because they block radia-         as sash type and overall quality of win-
tion before it passes through a window.           dow construction—and the quality of the
Light-colored shades are preferable to            installation. Operable windows with com-
dark ones because they reflect more, and          pressing seals are generally more airtight
absorb less, radiation. Horizontally orient-      than purely sliding seals, because of the
ed adjustable shading devices are appro-          way the sash element seals against the
priate for south-facing windows, while            framing.
vertically oriented adjustable devices are           An air leakage rating is a standardized
more effective for shading windows on             measure of the rate of infiltration through
east and west orientations.                       a window or skylight under specific envi-
                                                  ronmental conditions. Air leakage ratings
                                                  allow consumers to compare the airtight-
                                                  ness of different windows and skylights
     Sash Type                Effective           as manufactured products; they do not
                              Open Area*          account for leakage between the installed
                                                  product and the wall or roof. Lower air
     Casement                    90%              leakage ratings indicate greater airtight-
     Awning                      75%              ness.
     Hopper                      45%
     Horizontal sliding          45%              Airflow Recommendations
     Single-hung                 45%              In milder climates, or in spring and fall in
     Double-hung                 45%              more severe climates, operable windows
                                                  can provide ventilation, improve comfort,
Representative Window Ventilation Areas           and reduce the need for air conditioning.
*Effects of window screens are not                Operable windows are often specified to
included.                                         meet building code requirements for
                                                  emergency egress. Although operable
Source: R.K. Vieira and K.G. Sheinkopf, Energy-   windows are sometimes useful in house-
Efficient Florida Home Building, FSEC-GP-33-      hold areas with high moisture production,
88, Florida Solar Energy Center, Cape             such as bathrooms, kitchens, and laundry
Canaveral, FL, 1988.
                                                  rooms, exhaust fans provide more reliable
                                                  control throughout the year.
                                                     Select windows with air leakage rat-
Ventilation and Airtightness                      ings that meet or exceed standard industry
Airflow through and around windows                requirements of 0.37 cfm/ft 2 to minimize
occurs by design as ventilation and inad-         discomfort from uncontrolled infiltration.
vertently as infiltration. The use of win-        Select windows with even lower values
dows for natural ventilation is as old as         for particularly windy sites or harsh cli-
architecture itself. Opening windows, par-        mates. Check the seals between window
ticularly on opposite sides of a living           components for airtightness. To minimize
space, can cool a home for free. The sash         infiltration around installed windows, fol-
type of a window influences the ventila-          low the manufacturer’s installation proce-
tion airflow rate through the window rela-        dures carefully and seal and caulk joints
tive to its size. Some common sash types          and cracks.
and their effective open areas for ventila-
tion purposes are shown in the table
above. Casement windows are especially
effective for ventilation because they tend
to direct the greatest airflow into the liv-
ing space when fully open.
   Infiltration is the uncontrolled leakage
of air into a building from the exterior
through joints and cracks around window
Window Energy Rating and Labeling

         any windows, skylights, and
         glazed doors now bear energy rat-
         ings or labels, similar to those
                                             Q       How will designers and homeown -
                                                  ers use these energy labels?

being placed on household appliances, to
assist consumers in selecting energy-effi-
cient products. The labels have been
                                             A      Energy labels will show a variety of
                                                  product performance attributes,
                                                  enabling designers to compare and
developed by a non-profit group, the              select products directly, based on
National Fenestration Rating Council              each project’s specific energy per-
(NFRC). The following interview with              formance needs. Until now, design-
NFRC staff, provides homeowners, archi-           ers have had to spend too much time
tects, and builders with some important           trying to understand a mixed bag of
information on these new window energy            rating techniques, test methods, and
ratings.                                          performance claims. A nationwide
                                                  system for rating whole-product

Q       Why are energy ratings or labels
     important for windows and sky -
                                                  energy performance will not only
                                                  give designers the energy informa-
                                                  tion they seek, but will also permit
                                                  direct product comparisons.

A       Fenestration—windows, skylights,
     and glazed doors—can account for
     over 25% of the heating and cooling
                                                      Homeowners have faced a simi-
                                                  lar dilemma. When selecting fenes-
                                                  tration products for a remodeling
     energy bills in a typical home.              project or new construction, home-
     Designers, builders, and homeown-            owners have had no way to compare
     ers have never had a tool for deter-         the energy performances of two
     mining or comparing the energy per-          products directly. This difficulty has
     formances of fenestration products to        been compounded by the different
     assist them in their purchase deci-          energy rating techniques employed
     sions. Many manufacturers offer a            by the various industry segments.
     variety of energy-efficient products,        Window energy labels will enable
     but have not been able to demon-             consumers to compare products
     strate their superiority through com-        directly, regardless of glazing and
     parable performance ratings.                 frame type.

                                             Q      How will the energy ratings be

                                             A      The energy ratings are determined
                                                  using advanced computer tools
                                                  developed in the United States and
                                                  Canada, combined with standard-
                                                  ized product performance testing.
                                                  The WINDOW 4.1 program, devel-
                                                  oped at Lawrence Berkeley National

                                             In evaluating the cost of new or
                                             replacement window units, the aesthet -
                                             ic, functional, and energy performance
                                             characteristics come together. To
                                             make a better decision, it is useful to
                                             think of the life-cycle cost of the unit.

                                                                       National Fenestration Rating Council

                                                                                       1300 Spring Street, Suite 120

               NFRC Label
                                                                                            Silver Spring, MD 20910

The                                                                                          Telephone: (301) 589-6372
                                                                                                   Fax: (301) 589-0854
When shopping for windows, look for the                                                          Web:
National Fenestration Rating Council
(NFRC) label as your guide to buying energy-efficient windows. The NFRC is a non-profit public/pri-
vate collaboration that provides contractors and homeowners with standardized, unbiased meth-
ods of comparing various brands and types of windows.
Below is an example of an NFRC label. All the parts of the label are described, but the U-factor and
Solar Heat Gain Coefficient, which rate the efficiency of the entire window (glass and frame), are
the most important in helping choose the best window for your purposes. All labels have U-factors;
Solar Heat Gain Coefficients and Visible Light Transmittance are now being added. Air leakage rat-
ings and annual heating and cooling factors will be added in the near future.

                        The NFRC insignia is           This box contains the name of the
                        your assurance that            Independent Certification and
                        this window has been           Inspection Agency (IA) selected
                        independently rated.           by the window manufacturer.

 The U-factor is a
 measure of heat
 transmission due to
 a temperature dif-
 ference. The smaller                                                                              Name of the window
 the U-factor, the                                                                                 manufacturer.
 less heat is trans-
                                                                                                   Description of the
                                                                                                   particular product
 The Solar Heat Gain
                                                                                                   to which this label is
 Coefficient is a
 measure of the rate
 of solar heat flow
 through the window.                                                                               Since ratings can
                                                                                                   vary depending on
 The Visible Light                                                                                 the typical size of
 Transmittance value                                                                               windows in different
 is a measure of the                                                                               building types, values
 fraction of visible                                                                               are given for both
 light that passes                                                                                 residential and com-
 through the window.                                                                               mercial applications.

                              The Air Leakage rating is a measure of the rate of infiltra-
                              tion through this particular window.
    Laboratory, is one of the fundamen-             a result of this effort, consumers
    tal building blocks of the rating sys-          across the country now have energy
    tem. This program and the FRAME                 rating labels on windows, skylights,
    program are used to calculate the U-            and glazed doors analogous to those
    factors and Solar Heat Gain                     on automobiles, appliances, and
    Coefficients of windows. Air leakage            insulation.
    and other energy performance attrib-
    utes are also being rated. Soon,
    homeowners will see two new rat-
    ings, the Fenestration Heating Rating
                                               Q       Are these computer tools available
                                                    to the public?

    (FHR) and Fenestration Cooling
    Rating (FCR), which provide a com-
    parative index of heating and cooling
                                               A       Yes. These computer tools are avail-
                                                    able through the National
                                                    Fenestration Rating Council for use
    season energy use. The RESFEN                   by building energy professionals,
    program, developed at Berkeley Lab,             engineers, architects, and others.
    can also be used to estimate the                NFRC also provides detailed train-
    annual energy consumption and                   ing for manufacturers and design
    costs associated with a particular              professionals in the proper use of
    window type and orientation in a                these window-related computer
    specific geographic location, based             tools. For more information on these
    on local utility costs.                         computer programs, contact:

Q      Who is responsible for implementing
    the window energy performance rat -
    ing and labeling program?
                                                    National Fenestration Rating Council
                                                    1300 Spring Street, Suite 120
                                                    Silver Spring, MD 20910
                                                    Telephone: (301) 589-6372
                                                    Fax: (301) 589-0854
A      The National Fenestration Rating
    Council has developed and is imple-
    menting this rating and labeling sys-

    tem. NFRC is a non-profit coalition
    of manufacturers, builders, state and
    federal energy officials, private and
                                               Q       Where might I see NFRC labels ref -
                                                    erenced or used?
    government laboratories, utilities,
    consumers, and others working
    together to develop a nationwide
                                               A       Several state building codes and
                                                    other organizations with an interest
                                                    in promoting energy efficiency, such
    energy performance rating system
                                                    as utilities, are already referencing
    that is fair, accurate, and credible. As
                                                    NFRC ratings. The ratings are a pre-
                                                    requisite for some special programs,
                                                    such as low-interest financing to pur-
                                                    chase energy-efficient windows.
                                                    Look for labels on products dis-
                                                    played in your local building materi-
                                                    als supply store or window store.
                                                    NFRC ratings are listed in the prod-
                                                    uct literature, available from many
                                                    window manufacturers, architects, or

                                               Choosing a better-performing window
                                               to save on fuel costs will also improve
                                               comfort and performance in other
                                               areas. When shopping for windows, look
                                               for the NFRC Label as your guide.
Window Checklist
For Design, Specification, and Installation

      his checklist guides homeowners, architects, and builders in selecting residential

T     windows and skylights. Selecting the right window can be difficult because of the
      many factors involved and the great variations in climate, utility costs, and occu-
pant needs. Check boxes are provided for marking entries during the selection or design
process. Note that each entry below does not apply to all circumstances and that some
general guidance may appear to be contradictory because all of the detailed conditions
cannot be specified. Users should mark the items that apply to their particular needs.
Other local sources of information for window selection are utilities, state and local
building code officials, design professionals, and building materials suppliers.

Insulating Value and Condensation              If shading devices are to be used to sup -
Resistance                                     plement the use of high-performance win -
t Look for NFRC U-factor ratings and           dows, consider the following points:
  labels to guide window selection.            t Select light-colored shading devices to
t Select double-pane windows in all cli-         minimize solar heat gains.
  mates where heating is needed. Select        t Select exterior shading devices to mini-
  double- or triple-pane windows with            mize the inward flow of absorbed solar
  low-E coatings and gas fills in cold cli-      heat.
  mates to reduce heat losses and con-         t Select interior shading devices to
  densation.                                     reduce solar heat gains while providing
t To reduce frame and edge heat losses           for privacy and aesthetics, or when
  and condensation in all climates where         exterior shading devices cannot be
  heating is needed, select windows with         used.
  wood, vinyl, fiberglass, or properly         t Select horizontally oriented shading
  designed, thermally broken aluminum            devices for south-facing windows and
  frames.                                        vertically oriented shading devices for
t Use heavy drapes, thermal shades, or           east- and west-facing windows.
  thermal shutters to provide additional       t Specify overhangs, exterior awnings, or
  window insulation in cold climates.            the planting of deciduous trees and
                                                 shrubs to shade south-facing windows
Solar Control and Ultraviolet                    during the summer while allowing ben-
Protection                                       eficial solar heat gains during the win-
t Look for NFRC Solar Heat Gain                  ter.
  Coefficient ratings and labels to guide
  window selection.                            Daylight and View
t Select windows with spectrally selec-        t Look for NFRC Visible Light
  tive glazings (special tints or modified       Transmittance ratings and labels to
  low-E coatings) to reduce solar heat           guide window selection.
  gains (SHGC less than 0.4) while             t Select window size, location, and glass
  maintaining high visible transmittance         type to provide adequate daylight lev-
  (glass transmittance greater than 0.6).        els in each space.
t Select tinted windows to reduce solar        t Select windows with high visible trans-
  heat gains and control glare by lower-         mittances (greater than 50%) to maxi-
  ing visible transmittance.                     mize outward visibility.
t Select special glazings (with plastic        t Specify window sizes and positions in
  layers or low-E coatings) to reduce            walls to take advantage of desirable
  ultraviolet transmission in rooms with         views.
  materials subject to fading. (If this is a
  critical concern, consult expert assis-      t Position windows away from bright
  tance.)                                        external surfaces that create glare.
Ventilation and Airtightness                      t Select laminated glass or tempered
t Select operable windows for rooms requir-         glass with screens for skylights and for
  ing substantial ventilation during mild           windows near doors or close to the
  weather and to meet building code egress          floor.
  requirements.                                   t Select windows with locks or latches
t Select casement or awning windows to              that can be easily opened from the inte-
  maximize effective ventilation area.              rior but cannot be opened from the
t Select awning windows to better exclude
  precipitation while ventilating.
                                                  Maintenance, Durability, and
t Position operable windows in opposite           Lifetime
  walls of living spaces to maximize cross-
  ventilation.                                    t Check warranties for indication of
                                                    durability and lifetime before selecting
t Select fixed windows or windows with              windows and skylights.
  compression seals to minimize infiltration.
                                                  t Check the quality of window construc-
t Select windows and skylights with contin-         tion.
  uous edge seals to minimize infiltration.
                                                  t Use protective paints, stains, or
t Seal and caulk around window and sky-             sealants on wood window and skylight
  light frames and sash to reduce infiltration.     frames or select clad wood products.
  Follow the manufacturer’s installation
  instructions.                                   t Follow the manufacturer’s instructions
                                                    to maintain glazing, sash, frame, and
                                                    hardware in good repair.
Sound Control
t Position windows away from external             Installation
  sources of extreme noise.
                                                  t Check all applicable building codes
t Select double- or triple-pane windows             before installing windows and sky-
  with panes of unequal thickness, laminated        lights.
  glass, or gas fills to minimize noise from
  the exterior.                                   t Follow the manufacturer’s installation
                                                    instructions carefully.

Privacy, Safety, and Security
t Select interior shading devices that
  obscure direct view for additional privacy.     t Consider the relative effects on utility
                                                    bills when selecting windows and sky-
t Check building codes on fire, wind-load-          lights. Contact the NFRC (see Window
  ing, and seismic safety before selecting          Energy Rating and Labeling, page 10)
  and positioning windows and skylights.            or consult energy specialists or utility
                                                    representatives for estimates of the
                                                    energy and cost savings provided by
                                                    energy-efficient windows and sky-
                                                  t Consider the effects on the resale value
                                                    of a home when selecting windows and
                                                  t Check local, state, and federal energy
                                                    efficiency programs and utility energy
                                                    conservation programs for economic
                                                    incentives for installing energy-effi-
                                                    cient windows and skylights.

                                                  New materials and improvements to all
                                                  parts of the window assembly have
                                                  contributed to their high performance.
                                                  These double-glazed windows with vinyl
                                                  frames have a high insulating value.
Window Energy Glossary
Air Leakage Rating: A measure of the rate       person’s body can lose heat to a cold win-
of infiltration around a window or skylight     dow or skylight surface in a similar way.
in the presence of a strong wind. It is
expressed in units of cubic feet per minute     R-Value: A measure of the resistance of a
per square foot (cfm/ft 2) of window area       material or assembly to heat flow. It is the
or cubic feet per minute per foot (cfm/ft)      inverse of the U-factor (R = 1/U) and is
of window perimeter length. The lower a         expressed in units of hr-ft 2-˚F/Btu. A high
window’s air leakage rating, the better its     window R-value, has a greater resistance
airtightness.                                   to heat flow and a higher insulating value.

Conduction: The flow of heat through a          Shading Coefficient (SC): A measure of
solid material, such as glass or wood, and      the ability of a window or skylight to
from one material to another in an assem-       transmit solar heat, relative to that ability
bly, such as a window, through direct con-      for 1/8-inch clear, double-strength, single
tact.                                           glass. It is equal to the Solar Heat Gain
                                                Coefficient multiplied by 1.15 and is
Convection: The flow of heat through a          expressed as a number without units
circulating gas or liquid, such as the air in   between 0 and 1. A window with a lower
a room or the air or gas between window-        Shading Coefficient transmits less solar
panes.                                          heat, and provides better shading.

Fenestration: A window or skylight and          Solar Heat Gain Coefficient (SHGC):
its associated interior or exterior elements,   The fraction of solar radiation admitted
such as shades or blinds. The placement of      through a window or skylight, both
window openings in a building wall is one       directly transmitted, and absorbed and
of the important elements in determining        subsequently released inward. The Solar
the exterior appearance of a building.          Heat Gain Coefficient has replaced the
                                                Shading Coefficient as the standard indi-
Gas Fill: A gas other than air placed           cator of a window’s shading ability. It is
between window or skylight glazing panes        expressed as a number without units
to reduce the U-factor by suppressing con-      between 0 and 1. A window with a lower
duction and convection.                         Solar Heat Gain Coefficient transmits less
                                                solar heat, and provides better shading.
Glazing: The glass or plastic panes in a
window or skylight.                             Spectrally Selective Glazing: A specially
                                                engineered low-E coated or tinted glazing
Infiltration: The inadvertent flow of air       that blocks out much of the sun’s heat
into a building through breaks in the exte-     while transmitting substantial daylight.
rior surfaces of the building. It can occur
through joints and cracks around window         U-Factor (U-Value): A measure of the
and skylight frames, sash, and glazings.        rate of heat flow through a material or
                                                assembly. It is expressed in units of
Low-Emittance (Low-E) Coating:                  Btu/hr-ft2-˚F or W/m2-˚C. Window manu-
Microscopically thin, virtually invisible,      facturers and engineers commonly use the
metal or metallic oxide layers deposited on     U-factor to describe the rate of non-solar
a window or skylight glazing surface pri-       heat loss or gain through a window or
marily to reduce the U-factor by suppress-      skylight. Lower window U-factors have
ing radiative heat flow through the win-        greater resistance to heat flow and better
dow or skylight.                                insulating value.

Radiation: The transfer of heat in the form     Visible Transmittance: The percentage or
of electromagnetic waves from one sepa-         fraction of visible light transmitted by a
rate surface to another. Energy from the        window or skylight.
sun reaches the earth by radiation, and a
Selecting Windows for
Energy Efficiency                                                                What’s New
New window technologies have increased                                           in Building
energy benefits and comfort, and have provided
more practical options for consumers. This                                       Energy
selection guide will help homeowners,
architects, and builders take advantage of the
expanding window market. The guide contains
three sections: an explanation of energy-related
window characteristics, a discussion of window
energy performance ratings, and a convenient                                     We would like to acknowledge the many win-
                                                                                 dow and glazing industry reviewers of this doc-
checklist for window selection.                                                  ument. We appreciate the time they took to
                                                                                 assure the usefulness of this document. Special
                                                                                 thanks to V&WPatio Door & Window Co., Inc.
                                                                                 of Berkeley, California, for making one of their
Resources                                                                        window retrofit installations available for photo
                                                                                 illustration of this brochure. This work was
                                                                                 supported by the U.S. Department of Energy,
In seeking information concerning windows and energy efficiency in general,      Assistant Secretary for Energy Efficiency and
                                                                                 Renewable Energy, Office of Building
there are several local resources worth investigating:                           Technology, State and Community Programs,
                                                                                 Office of Building Systems.
• Local utilities
• State or municipal energy agencies                                             This document was prepared as an account of work
                                                                                 sponsored by the United States Government. While this
• Regional universities with architecture, construction, or extension programs   document is believed to contain correct information,
                                                                                 neither the United States Government nor any agency
• Bookstores                                                                     thereof, nor The Regents of the University of
• Product literature at home improvement centers                                 California, nor any of their employees, makes any war-
                                                                                 ranty, express or implied, or assumes any legal respon -
• Local chapters of the American Institute of Architects                         sibility for the accuracy, completeness, or usefulness of
                                                                                 any information, apparatus, product, or process dis -
• Local builder’s associations                                                   closed, or represents that its use would not infringe pri-
                                                                                 vately owned rights. Reference herein to any specific
                                                                                 commercial product, process, or service by its trade
Recommended Web Sites                                                            name, trademark, manufacturer, or otherwise, does not
                                                                                 necessarily constitute or imply its endorsement, recom -
Search for specific window and glazing manufacturers and trade organiza -        mendation, or favoring by the United States
                                                                                 Government or any agency thereof, or The Regents of
tions by name on the World Wide Web.                                             the University of California. The views and opinions of
                                                                                 authors expressed herein do not necessarily state or
American Architectural Manufacturers Association:         reflect those of the United States Government or any
                                                                                 agency thereof, or The Regents of the University of
American Institute of Architects:                             California.

American Solar Energy Society:                         Produced for DOE’s Office of Energy Efficiency and
                                                                                 Renewable Energy by the Lawrence Berkeley National
American Society of Heating, Refrigerating & Air Conditioning Engineers:         Laboratory, a DOE national laboratory.
Home Energy Magazine:
National Association of Home Builders:                                         PUB-788 January 1997 - 5000

National Fenestration Rating Council:
National Wood Window and Door Association:                  For More Information
National Research Council of Canada:                                  Energy Efficiency and
Natural Resources Canada:                                 Renewable Energy Clearinghouse
                                                                                 P.O. Box 3048
Passive Solar Industries Council:                            Merrifield, VA 22116
U.S. Department of Energy:                               (800) DOE-EREC
                                                                                 (800) 363-3732
Recommended Reading                                                              (703) 893-0400 fax
Residential Windows: A Guide to New Technologies and Energy Performance,
by John Carmody, Stephen Selkowitz, and Lisa Heschong, W.W. Norton &
Company, 1996;                                                         Printed on recycled paper
                                                                                                using soy-based inks.

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