Hobby Greenhouse Construction

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					                                    A L A B A M A   A & M    A N D    A U B U R N   U N I V E R S I T I E S

                                    Hobby Greenhouse
                    ANR-1105        Construction
Introduction                        Bedford, MA 01730-2048). Com-          Step 1: Decide what
    A hobby greenhouse can          pile a list of manufacturers, and      plants you will grow,
add a new dimension to the use-     request information on different
                                    models. Compare costs and fea-         what times of year you
fulness and productivity of a
home garden. Tropical plants        tures for the style and size you       will use the green-
can be grown and enjoyed year-      are interested in. Generally, plas-    house, and how a
round, or seasonal plants can be    tic-covered greenhouses are eas-       greenhouse fits into
grown to get a head start on        ier to assemble than glass hous-
                                    es are. However, if any aspect of      your lifestyle.
spring. However, the costs of
                                    the assembly is beyond your                For example, you might
building a hobby greenhouse
                                    skills, many manufacturers will        want to use the greenhouse in
can vary from several hundred
                                    erect their products on-site for       one of the following ways:
dollars to thousands, and operat-
ing costs can also be high if       an additional fee.                         • To start vegetable or
year-round heating and cooling                                             flower seeds or propagate cut-
are required.                       Build Your Own                         tings in the spring to plant in the
                                                                           garden. On a small scale, this
    You may decide to build a           A hobby greenhouse can be          can be accomplished with a
greenhouse or to purchase a         constructed easily and inexpen-        structure as simple as an outdoor
greenhouse kit. Before you de-      sively by anyone able to use           cold frame or hotbed. A free-
cide whether to build or buy,       simple hand tools. Most of the         standing greenhouse can also be
examine as many greenhouse          construction materials can be          used for this purpose but will
styles and equipment options as     purchased at building supply           probably be a simple and inex-
possible. A greenhouse is a         stores. Materials may also be          pensive model.
long-term investment that should    available inexpensively at con-
provide the growing area and                                                   • To grow year-round tropi-
                                    struction sites or salvage yards. If
service desired while blending                                             cal foliage in a conservatory set-
                                    any aspect of the construction is
well with the home and land-                                               ting. A greenhouse for this pur-
                                    beyond your skills, you may be
scape.                                                                     pose will probably be more
                                    able to hire a local carpenter or
                                                                           permanent and formal.
                                    handyman to help. Plans for dif-
                                    ferent styles of hobby green-              • To grow specialty flowers
Greenhouse Kits                                                            or ornamentals. Many green-
                                    houses are provided at the end
    Greenhouse kits are avail-      of this publication.                   houses are constructed because
able in a wide range of costs                                              owners develop an interest in
and with a wide range of fea-           When you do decide to
                                                                           speciality flowers or ornamentals
tures. Prices range from a few      build a greenhouse, choosing
                                                                           that have unique requirements,
hundred dollars to well over        the type of structure, covering,
                                                                           such as orchids, African violets,
$5,000 depending on the size,       and environmental control
                                                                           or bromeliads. These greenhous-
style, accessories, and type of     equipment can be confusing. A
                                                                           es should be designed with the
construction materials. Manufac-    step-by-step approach can help
                                                                           needs of the particular plant in
turers can be found in the yel-     you organize the planning and
low pages of a telephone book,      implementation of your hobby
                                    greenhouse.                                A hobby greenhouse can be
in advertisements in gardening                                             a part of your home in the form
magazines, by searching the                                                of a sun-room or porch. Sun-
World Wide Web, or by contact-                                             rooms or porches usually have
ing the Hobby Greenhouse                                                   clear covering on one or more
Association (8 Glen Terrace,                                               sides but not on the roof. A

Visit our Web site at:
greenhouse can also be attached
to the house, with an entrance
to the living area. Plants, walks,
                                                                                            (b) tri-penta
furniture, a water pond, or a
fountain may also be included
and arranged formally or infor-
mally as an extended living                    (a) Quonset
room. Plants may “spill over”
from the greenhouse through a
sliding glass door into the living
area of the home. This type of
greenhouse can be used for re-
laxing, reading, or family gather-
                                                (c) dome             (d) gothic arch

Step 2: Decide what
style of greenhouse
to build.                                                                     (f) A-frame
     Greenhouse design styles
vary widely and include Quon-
set, tri-penta, dome, gothic arch,
slant-side, A-frame, gable roof,
straight-side lean-to, curved-side
lean-to, and slant-side lean-to           (e) slant-side
(Figures 1a-g and Figures 2a-c).
Some styles are more suited to
flexible coverings like polyethyl-
ene, such as the dome, gothic
arch, Quonset, curved-side lean-               (g) gable roof
to, and tri-penta. Others work
better with rigid coverings like
glass or plastic, including the A-
frame, gable roof, slant-side, and        Figure 1. Freestanding
straight- or slant-side lean-to.          hobby greenhouse styles
Some design styles are more effi-
cient to heat and cool, such as
the gable roof and Quonset.               In addition to deciding what         Attached greenhouses are at-
Others may look unusual and at-      style your greenhouse should          tached to the home and may or
tractive but are difficult to con-    be, you need to decide whether        may not have an entrance to the
struct or heat and cool, such as     it will be freestanding or whet-      home. They can be designed to
the dome and tri-penta. The A-       her it will be attached to your       blend with the architecture and
frame style is easy to construct     home. Freestanding greenhouses        landscape of the home and are
and is inexpensive, but the us-      stand alone in the landscape.         useful where space is limited. An
able growing area is small and       They can be constructed in a          attached greenhouse may also
awkward, and the shape may           wider range of styles, can be         cost less per square foot to build
not blend well with normal sur-      larger, and can offer greater flex-    than a freestanding greenhouse.
roundings. The most commonly         ibility in location than attached     With an inside entrance, you can
used styles are the gable roof,      greenhouses can. These green-         maintain the greenhouse without
gothic arch, Quonset, and slant-     houses can be placed almost           going outside during bad weath-
side lean-to.                        anywhere in the landscape             er. Utilities such as electricity,
                                     where the ground is level and         water, and heat can be shared
                                     adequate light is available. The      with the home if a greenhouse is
                                     most widely used styles are the       planned for during home con-
                                     gable roof, gothic arch, Quonset,     struction. If the greenhouse is at-
                                     and slant-side.                       tached later, you may need an

2 Alabama Cooperative Extension System
                                                                                   to utilities, and a pleasing blend
                                                                                   with the landscape. For many
                                                                                   homeowners, the appearance of
                                                                                   the structure is most important,
                                                                                   so compromises must be made
                                                                                   to meet other location require-
                                                                                       One of the most important
                                                                                   location requirements is sun ex-
                                                                                   posure. Many flowering potted
                                                                                   plants require full sun to per-
                                                                                   form well. A freestanding green-
(a) straight-side lean-to
                                                                                   house for these plants should be
                                        (b) curved-side lean-to                    constructed with the long sides
                                                                                   of the building facing southeast
                                                                                   to southwest (Figure 3). Vege-
                                                                                   table and flower seedlings for
                                                                                   transplanting outdoors in the
                                                                                   spring need maximum sunlight,
                                                                                   so choose a location that re-
                                                                                   ceives full sun. For plants requir-
                                                                                   ing less light, the greenhouse
                                                                                   can face northeast to southeast
                                                                                   or northwest to southwest.
(c) slant-side lean-to                                                             Exposure is particularly impor-
                                                                                   tant for attached greenhouses.
                                                    (d) attached gable             Consider these locations in
Figure 2. Attached hobby greenhouse styles                                         order: first—south or southeast,
                                                                                   second—east, third—southwest,
additional heater because green-        Step 3: Choose a                           fourth—west, and last—north.
houses lose heat five to ten             location for your                          Keep in mind that a western ex-
times faster than an equivalent
area of home. The cost of heat-         greenhouse.                                posure can be too hot in sum-
                                            Often, there may be a limit-           mer, and a northern exposure
ing an attached greenhouse may
                                        ed choice of locations that will           usually does not receive enough
be less than that of heating a
                                        have adequate sunlight, ade-               light for most plants. Also be
freestanding greenhouse of the
                                        quate soil drainage, easy access           aware that tall structures and
same floor area because one
                                        for people and materials, access           trees near the greenhouse may
wall is not exposed; however, at-
                                                                                   block light for parts of the day.
tached greenhouses usually re-
ceive less light for the same rea-                                         Summer Noon             Best location
                                        Figure 3. Location priori-
son. Straight-side lean-to, slant-
                                        ties for a hobby greenhouse                                Good location
side lean-to, and curved-side           based on ambient sunlight
lean-to styles are ideally suited                                                                  Poor location
for small, easy-to-construct at-             Winter Noon
tached greenhouses (Figures 2a-                                   Winter
                                                                                  W             Summer
c), although the Quonset, gothic                                                                Sunset
arch, slant-side, and gable roof
(Figure 2d) freestanding styles
can also be attached. A solid
foundation similar to the house
foundation is often required or
highly desirable for attached
                                       S                                                                             N
greenhouses. Joining and sealing
the greenhouse to the house
needs special attention. Consult
an architect or building contrac-
tor to determine the method of                        Winter Sunrise                          Summer Sunrise
attachment.                                                                   E
                                                                                     Hobby Greenhouse Construction   3
Falling limbs can also be a major     house is that it is too small to       houses were 18 by 16 inches,
problem if the greenhouse is lo-      meet the needs of the owner. In        but larger sizes are more com-
cated too close to trees.             addition, temperatures can fluc-        mon now. Actually, larger panes
    Other location considerations     tuate rapidly in a small green-        are less fragile than smaller
include ensuring that the site is     house, and heat losses can be as       panes. Many greenhouses are
level and that the soil drains        high as they are in a larger           covered with double-strength
well. Many locations may have         greenhouse. Small houses may           float glass (1/8 inch thick) cost-
to be graded to ensure that the       also have limited headroom and         ing $0.85 to $2.00 per square
foundation is level. Slope the        be hard to work in. A taller, larg-    foot. Large glass panes in many
soil away from the greenhouse         er greenhouse obviously has            more expensive greenhouse kits
to drain rainwater away. If           more space, and it heats and           are tempered glass (5/32 inch
drainage is a problem, consider       cools more uniformly than a            thick) costing $3.00 to $7.00 per
installing drainage tile before       shorter, smaller one.                  square foot, depending on the
constructing the greenhouse.              As a starting point, 100           pane size.
    Avoid low areas with poor         square feet would be a mini-
air drainage, especially those        mum size, but 200 square feet is       Polyethylene Film
surrounded by woods or build-         better. A house 9 to 14 feet wide           Polyethylene film (PE) is a
ings. Cold, humid air can stag-       by 20 feet long can be managed         good choice for home-built
nate in these locations and in-       in a few hours per week. A larg-       greenhouses because less struc-
crease heating costs. Conversely,     er greenhouse can also cost less       tural support is required and it
avoid high elevations with direct     to cover per square foot than a        costs much less than other mate-
exposure to strong winter winds.      small greenhouse, so choose a          rials. However, PE film only lasts
Convective heat loss through the      size slightly larger than you          about 2 years. Clear PE is used
covering material can increase        think you need. On the other           for growing most plants, but
heating demand.                       hand, ask yourself, do you have        white PE can be used to reduce
    The greenhouse should be          the time and dedication to main-       light and heat for growing low-
convenient to a driveway to re-       tain a large greenhouse, now           light plants or for propagation.
ceive supplies and to haul away       and in the future?                     PE manufactured for greenhouse
plants or garbage when needed.                                               application comes in widths
Walkways to and from the gar-         Step 5: Choose a                       from 10 to 50 feet, thicknesses
den, house, and storage areas         covering material.                     from 1 to 8 millimeters, and
and access to water, fuel for                                                costs $0.06 to $0.09 per square
                                          There are several covering         foot. Two layers of PE are fre-
heating, and electricity should all
                                      materials to choose from, each         quently applied to greenhouses
be considered.
                                      of which has its own advantages        to reduce heating demand.
    The greenhouse should con-        and disadvantages. Common
tribute to the appearance of the                                             Double-layer PE houses general-
                                      covering materials are glass,          ly cost 30 to 40 percent less to
home and landscape and not be         polyethylene film, fiberglass re-
an eyesore either to the owner                                               heat than single-layered houses
                                      inforced panels, and double-           do. The two layers are kept air-
or neighbors. Consider room for       layer structured panels.
expansion if you think you may                                               inflated by a 100- to 150-cubic-
be so inclined in the future.                                                foot-per-minute squirrel cage
                                      Glass                                  blower mounted to the inside PE
                                           Glass is the traditional green-   layer. Purchase 4-millimeter PE
Step 4: Choose a                      house covering against which all       for the inside and 4- or 6-mil-
greenhouse size                       others are judged. Good-quality        limeter PE for the outside. Use 6-
to suit your needs.                   glass is an attractive, very trans-    millimeter PE for single-layer ap-
                                      parent, and formal (in appear-         plications. PE can be installed on
    Available space and cost          ance) covering material. It is         wood-frame greenhouses by
usually have a large impact on        very strong (tensile strength), but    nailing wood batten strips over
the choice of size for a hobby        it is subject to shattering and can    the film into the foundation
greenhouse. Keep in mind, how-        become brittle with age. Glass is      boards and rafters or arches.
ever, that a greenhouse that is       also very expensive and because        However, because PE must be
too small may cost more to op-        of its weight requires sturdier        replaced frequently, investing in
erate than the initial cost of        framing support than is required       special fasteners makes the job
building a larger one. The obvi-      with other covering materials.         easier. Fastening systems are
ous problem with a small green-       Originally, glass panes for green-     available for single- or double-
                                                                             layer applications.

4 Alabama Cooperative Extension System
Fiberglass Reinforced                Step 6: Choose the                    Step 7: Choose the
Panels                               framing materials.                    foundation and side-
     Fiberglass reinforced panels         To a large extent, this choice   wall materials.
(FRPs) are rigid plastic panels      will be influenced by the type of          Greenhouses to be covered
made from acrylic or polycar-        covering material chosen, the         in PE usually do not require an
bonate that come in large corru-     desired permanence, and costs.        extensive foundation, but the
gated or flat sheets. FRPs are        Many homemade greenhouses             support posts must be set in
available in 24- to 57-inch widths   are made of wood. Be sure to          concrete footings. The problem
and up to 24-foot lengths. FRPs      purchase treated lumber for ex-       with a PE-covered Quonset
are durable, retain heat better      terior use. Metal-framed green-       house is usually not holding it
than glass does, and are light-      houses are more permanent but         up but holding it down. In
weight (less structural support      are difficult for do-it-yourself       strong wind, the shape of the
needed). Large panels are flexi-      projects from scratch. However,       house makes it function like an
ble enough to be applied to a        greenhouse manufacturers offer        airplane wing, and it may lift off
Quonset greenhouse. Light trans-     many kits that are easy to erect      the ground. Figure 4 shows a
mission may be better than glass     and are reasonably priced.            simple poured concrete footing
simply because less structural       Aluminum or galvanized framing        and a concrete block foundation.
support is needed, which creates     last a long time with little main-
less shadow. The prices of FRPs      tenance. Schedule 80 PVC pipe         Poured                 Concrete
range from $1.00 to $1.25 per        can also be used as supports for      Concrete               Block
square foot, depending on the        small Quonset-style greenhouses
guaranteed life span of the mate-    that are to be covered in PE, but                6’’ minimum
rial. Inexpensive materials may      PVC requires extra support to
be guaranteed for as little as 5     withstand strong wind.
years; more expensive types may           Whatever the choice, the                          Below
be guaranteed for as long as 20      framing must adequately support                        Frost
years.                               the covering and any equipment                         Line
                                     and/or hanging baskets suspend-
Double-Layer Structured              ed from the framing. The fram-
Panels                               ing must also be able to with-
      Acrylic or polycarbonate       stand wind, rain, snow, or ice                     51⁄ 2’’                71⁄ 2’’
double-layer structured panels       common to the geographic loca-
(DSPs) are made of two layers of     tion. The load exerted by hang-          12’’                  16’’
plastic held apart by ribs spaced    ing baskets can be greater than
1⁄ 2 to 1 inch apart. The double-    that caused by weather.               Figure 4. Simple concrete founda-
layer construction increases              Wood preservatives for green-    tions for a hobby greenhouse
structural strength and heat re-     house framing and benches
tention but decreases light trans-   should be pressure-treated, salt-         Attached greenhouses and
mission compared to single-layer     type, or copper naphthenate.          those covered with glass should
materials. Panels are 4 feet wide    Avoid creosote (no longer legal)      have a strong concrete or con-
and up to 39 feet long. DSPs         and pentachlorophenol preserva-       crete block foundation that ex-
made of polycarbonate cost           tives—these are dangerous for         tends below the frost line ac-
$1.75 to $2.50 per square foot,      people and toxic to plants.           cording to local building codes.
and those made of acrylic cost            The eave height for a gable      A 2- to 3-foot-high sidewall can
$2.00 to $3.50 per square foot.      roof greenhouse should be at          add considerably to the appear-
                                     least 51⁄ 2 feet with a minimum       ance of a greenhouse. Concrete
                                     gable height of 8 feet. However,      block, stone, or brick are the
                                     gable heights of 10 to 15 feet are    most popular materials used, but
                                     common. Roof pitch for a glass        shingle, clapboard, and asbestos
                                     greenhouse should be 6-inch           rock have also been used.
                                     rise per foot (27 degrees) to pre-    Choose the type that works best
                                     vent inside condensation from         with the overall architectural
                                     dripping on plants. Plastic-cov-      scheme.
                                     ered greenhouses require a
                                     steeper pitch of 7- to 81⁄ 2-foot
                                     rise per foot to prevent drip.

                                                                             Hobby Greenhouse Construction     5
Step 8: Choose the type                drainage and air movement.            greenhouses. Some are designed
of floor and walkways.                  Spruce or redwood lath and 14-        to hang from the structure of the
                                       gauge welded wire fabric or ex-       greenhouse; others sit on the
     In many places where              panded metal make a strong,           floor. Costs for gas unit heaters
drainage is adequate, a solid          long-lasting, open bench top.         range from $300 to $550 for
floor is not necessary. Four inch-      Benches should be 2 to 3 feet         20,000 to 60,000 BTU units, re-
es of 3⁄ 4-inch crushed stone or       wide with access from one side        spectively. Unit heaters burn gas
pea gravel will help control           or 4 to 5 feet wide with access       in a firebox. Heated air rises
weeds and provide a porous             from both sides. If using solid-      through the inside of a thin-
medium for water to evaporate          topped benches, set them back         walled heat exchanger on the
in the summer to provide some          from the sidewall of the green-       way to the exhaust chimney. A
cooling. If you want a solid con-      house by 6 inches to allow air        fan draws air in from the green-
crete floor, install one or more        movement. No setback is need-         house, across the outside of the
French drains, and slope the           ed for open-top benches.              heat exchanger, and out into the
floor toward the drains. Don’t          Benches should be 24 to 36            greenhouse. Thus, most of the
forget to plan for plumbing            inches high; for individuals in       heat is removed from the ex-
when planning the foundation.          wheelchairs, bench height             haust before it exits the struc-
     Walkways can be constructed       should be 30 to 36 inches, with       ture. The exhaust chimney must
of concrete for easy movement of       little or no surface lip. Place       be sufficiently tall to maintain an
equipment and people, especially       bench supports 6 to 12 inches         upward draft and extend above
if a family member is disabled or      back from the surface edge to         the greenhouse roof. An 8- to
in a wheelchair. Brick filled with      provide knee room.                    12-foot chimney is usually suffi-
sand, flagstone, or stepping stones                                           cient. All open-flame heaters
can be used for decorative walks.                                            must be vented to the outside
Gravel under the benches keeps         Step 10: Decide how                   and given a fresh air supply for
the walkways free of debris and        to heat your                          complete combustion. Fresh air
reduces weeds. Walkways can be         greenhouse.                           should be provided by an unob-
2 to 3 feet wide in a small green-                                           structed opening to avoid carbon
                                            Greenhouses are poorly in-
house. Adjust the width of walks                                             dioxide buildup.
                                       sulated structures, and heat loss
if a cart or wheelbarrow will be                                                 In larger greenhouses, a
                                       through the covering on cold,
used. Larger greenhouses often                                               plastic tube system may be
                                       clear, windy nights can be con-
have 2- to 3-foot secondary aisles                                           needed to distribute the heat
                                       siderable. Heat can be supplied
and a 4- to 6-foot main aisle. If a                                          evenly within the house. The
                                       using electricity (can be expen-
family member is in a wheelchair,                                            system consists of a perforated
                                       sive), natural gas or propane,
use at least 4-foot-wide aisles and                                          polyethylene tube suspended
                                       fuel oil, solar energy (unreli-
ramps for easy access.                                                       overhead in the ridge and ex-
                                       able), or kerosene (emergency
                                       use only). You can also connect       tending the length of the green-
Step 9: Choose the type                the greenhouse to your home           house. A fan connected to the
of benches to install.                 heating system. Natural gas or        tube blows warm air from the
                                       propane are probably the most         heater into the tube for green-
     Benches can be constructed                                              house distribution. This system
                                       cost-effective ways of heating a
from a variety of materials and                                              can also be used for circulating
                                       hobby greenhouse. If natural gas
arranged in many different ways.                                             internal air when heating or
                                       is available in the home, plumb-
If you plan carefully, 70 to 80 per-                                         cooling are not required.
                                       ing into the existing line saves
cent of the floor area can be de-
                                       considerable cost over a new              A variety of electric resis-
voted to growing plants. Make
                                       meter and gas line. If natural gas    tance heaters can also be used.
sure that the supports for benches
                                       is not available, check with the      Costs range from $100 to $350
are strong enough to hold the
                                       local propane gas company for         for 5,000 to 17,000 BTU units,
largest number of plants and the
                                       cost and availability. Determine      respectively. Those available in
largest container size anticipated.
                                       if the company provides a stor-       department stores and home
Wood, metal pipe, or concrete
                                       age tank free or if it can be rent-   centers are usually only ade-
block can be used as bench sup-
                                       ed or purchased. Consult the          quate in the smallest greenhouse
                                       company to determine the tank         for starting seedlings in the
     Also make sure the bench          size appropriate for the green-       spring. Larger units can be pur-
surface is strong enough to sup-       house.                                chased, but operating them can
port plants without sagging but                                              be costly.
                                            A variety of gas unit heaters
that it is open to allow water
                                       are available to heat hobby

6 Alabama Cooperative Extension System
     Heater size for a given            should equal about 20 percent         dations for warm climates call
greenhouse and geographic lo-           of the roof area. Vents can be        for a fan capacity to remove 12
cation depends on the surface           manually controlled, but this re-     to 17 cubic feet per minute per
area of the greenhouse and the          quires frequent temperature           square foot of floor area. If the
temperature difference between          checks and vent adjustment ac-        greenhouse is attached to the
the inside and outside of the           cording to outside conditions.        east, west, or south wall of an-
greenhouse. To determine the            Using an electric motor and ther-     other building, solar heat will
size heating system you need,           mostat for automatic vent control     collect inside the greenhouse
calculate the total surface area of     is much easier. Simple automatic      from this wall. If this is the case,
the greenhouse covering. Then           systems open or close the vents       add half the area of the attach-
determine the difference be-            based on a setpoint temperature.      ment wall to the floor area when
tween the minimum expected              More advanced systems open            calculating the ventilation re-
outside temperature during win-         and close vents in stages based       quirements. Work with a green-
ter in your area (see the USDA          on multiple setpoint tempera-         house supply or construction
hardiness zone map, average an-         tures. The advantage of natural       company to determine the prop-
nual minimum temperature) and           flow-through ventilation is that it    er size fan or vent for your situa-
the maximum night temperature           costs less than forced-air ventila-   tion.
you wish to maintain (generally         tion to operate; however, it may
60 to 65 degrees F). Multiply the       not be adequate to cool the
greenhouse surface area by the          greenhouse during the summer.         Step 12: Decide how to
temperature difference by the                Forced-air ventilation sys-      cool your greenhouse.
heat loss conversion factor in          tems consist of a louvered intake          One of the best ways to cool
Table 1 for the appropriate cov-        vent and electric fan(s) con-         a greenhouse in the summer is
ering. The answer will be in            trolled by a thermostat. The fans     to reduce light intensity. How
BTUs per hour. Most heaters are         pull cool air into the greenhouse     much reduction to provide de-
rated in this unit. Many green-         from outside through an intake        pends on the heat load in the
house supply or construction            vent and push warm inside air         greenhouse and the light re-
companies can help you deter-           out. Fans should be mounted in        quirements of the plants grown.
mine the proper size heater for         a waterproof housing with air-        Greenhouse whitewash, shade
your situation.                         activated louvers to protect elec-    cloth, screens of wood or alu-
                                        trical components from in-            minum, or Venetian blinds can
                                        clement weather and to keep           be used for shading. Green-
Step 11: Decide how                     cold air out during the winter.       house whitewash is a special
to ventilate your                       Be sure to install a screen over      kind of latex paint that is diluted
greenhouse.                             the inside of the fans to prevent     in water and sprayed on the
     The purposes of ventilation        injury. Be sure that the distance     covering surface. It is designed
are to exchange carbon dioxide          between the fans and adjacent         to be applied in the spring and
and oxygen, to remove hot air,          structures equals at least 11⁄ 2      gradually degrade by the action
and to lower relative humidity.         times the fan diameter. The lou-      of rain and sun so that little re-
Hobby greenhouses can be vent-          ver on the intake vent on the         mains by fall. Shade cloth is a
ed by natural flow-through ven-          wall opposite the fans can be         black, green, or white woven
tilation or by forced-air ventila-      air-activated or motorized. Costs     fabric of polypropylene that is
tion. Flow-through ventilation          of fans range from $150 to $250       laid over the outside of the cov-
relies on side and top vents that       for 1,200- to 3,000-cubic-feet-       ering. Shade cloth can be pur-
pull cool outside air into the          per-minute units, respectively.       chased with various weave den-
greenhouse through the side                  Be sure that the fan capacity    sities that result in 20 to 80
vent as warm air rises and exits        is large enough to exchange the       percent light reduction. For most
through the top vent. The com-          air in the greenhouse at least        hobby greenhouse applications,
bined side and top vent area            once per minute. Recommen-            30 to 50 percent light reduction
                                                                              should be sufficient.
                                                                                   Another method for cooling
Table 1. Heat Loss Factor for a Small Hobby Greenhouse                        a greenhouse is evaporative
Greenhouse covering        Heat loss (calm area)  Heat loss   (windy area)    cooling, which relies on air pass-
Polyethylene or fiberglass           1.2*                      1.4             ing through a porous pad satu-
Glass                               1.5                       1.8             rated with water. The evaporat-
Double-layer plastic                0.8                       1.0             ing water removes heat from the
*Heat loss in BTU/hour/square feet/°F

                                                                                Hobby Greenhouse Construction   7
greenhouse. There are two types      Step 13: Decide how to               Do’s:
of evaporative coolers: fan-and-     provide utilities to your            • Keep the greenhouse and sur-
pad systems and unit coolers                                              rounding areas clean and orga-
(swamp coolers). Fan-and-pad         greenhouse.
systems consist of a cellulose           Check all local building
pad at least 2 feet tall and ex-     codes before building a green-       • Allot enough time to the
tending the length of one wall,      house on your property. Some         greenhouse weekly to be suc-
with water supplied from the top     county or city codes prohibit        cessful.
to keep the pad wet during op-       greenhouses or place restrictions    • Learn more about greenhouses
eration. Fans are installed in the   on size, type, covering, or con-     and growing plants by reading
opposite wall to draw outside air    struction materials. Also, check     and talking to others.
through the pads.                    with the local electric company.
                                                                          • Keep the greenhouse in a
    Unit coolers consist of a        In some areas, the utility compa-
                                                                          good state of repair.
metal box mounted outside the        ny may request a utility pole and
                                     separate meter for the green-        • Discard weak, diseased, or
greenhouse and evaporative
                                     house. Be sure all electrical        badly insect-infected plants.
pads on three inside walls.
These units may be more practi-      work is performed by a licensed      • Enjoy the greenhouse; arrange
cal and attractive for small         electrician according to code.       work intelligently so it doesn’t
hobby greenhouses. A water               Water can usually be             become a chore.
connection, collection lines, and    plumbed from the home supply         • Experiment—try something
recirculating pump are built into    line as long as the volume and       new!
the unit. A fan within the unit      pressure are adequate. Install a
draws outside air through the        backflow prevention valve in the
                                     water supply line to prevent the     Don’ts:
pads and the cooled air into the
greenhouse through a duct. A         possibility of contaminating the     • Don’t take in every friend’s
vent on the opposite side of the     water in your home.                  sick plant. You’re asking for
greenhouse provides an air exit.                                          trouble if you do!
Unit coolers should provide          Step 14: Choose work                 • Don’t start with the most diffi-
about 15 cubic feet per minute                                            cult plants. Gain experience with
for each square foot of floor         and storage areas.                   plants that are easier to grow be-
area. Costs for unit coolers range       Make a work area for potting     fore trying the difficult ones.
from $125 to $600 for 500- to        and maintaining plants. This may
                                     be located inside or outside the     • Plants in a greenhouse are a
                                     greenhouse. The north wall is        responsibility. Don’t leave them
units, respectively. Determining
                                     often a good location for a work     without care.
the evaporative cooling capacity
for a given greenhouse and geo-      area inside the greenhouse. The
graphic location is difficult. Work   work area may also include a
with a greenhouse supply or          sink. Storage areas for soil and
construction company to deter-       containers can be located out-
mine the proper size for your sit-   side the greenhouse but should
uation.                              be protected from the weather
    Generally, heating, ventilat-    and not be an eyesore.
ing, and cooling are controlled
by thermostats located close to
the center of the greenhouse at
                                     Step 15: Follow the
plant level. For accurate control,   greenhouse “do’s”
be sure the thermostat is shaded     and “don’ts.”
from direct sunlight. Mounting it         Finally, consider the follow-
in a plastic or wood box venti-      ing points. They are intended to
lated by a small blower works        prevent problems and to make
well.                                life with a greenhouse more en-

8 Alabama Cooperative Extension System
16           3

     Hobby Greenhouse Construction   9
10 Alabama Cooperative Extension System
Hobby Greenhouse Construction   11
12 Alabama Cooperative Extension System
Hobby Greenhouse Construction   13
14 Alabama Cooperative Extension System
Hobby Greenhouse Construction   15
           J.R. Kessler, Jr., Extension Horticulturist, Assistant Professor, Horticulture,
           Auburn University
           For more information, call your county Extension office. Look in your tele-
           phone directory under your county’s name to find the number.
           Issued in furtherance of Cooperative Extension work in agriculture and home economics, Acts of May 8 and June
           30, 1914, and other related acts, in cooperation with the U.S. Department of Agriculture. The Alabama
           Cooperative Extension System (Alabama A&M University and Auburn University) offers educational programs,
           materials, and equal opportunity employment to all people without regard to race, color, national origin, religion,
ANR-1105   sex, age, veteran status, or disability.                               UPS, 6M30, New May 1998, ANR-1105