This cross section of a tree shows from which tree part different kinds of building lumber are produced. Elements such as annual rings, sapwood, and heartwood determine a lumber’s strength and thus use.
BEFORE YOU BEGIN building your greenhouse, it’s important to understand and consider the variety of materials that can be used in its construction. Only by learning more about these materials will you be able to make the best choice for your greenhouse style and location and for the conditions in your region.
In this chapter you’ll find information on materials for construction of the structure itself. At the same time that you plan your greenhouse, you should make decisions about building materials, the foundation (materials for the foundation are discussed in chapter 8), the glazing (addressed thoroughly in chapter 7), and the wiring and plumbing (information on wiring and plumbing can be found in chapter 10).
Included in this chapter are discussions of wood, aluminum, and steel as building materials; an assessment of sheathing and various external finishes for walls such as those on the north that may not be glazed; and a look at the kinds of insulation you might incorporate into your structure. Also considered are alternative building materials and the various paint finishes you can use.
In choosing building materials, it’s important to take into account the temperatures in your region and specific area and the effects of wind at various times throughout the year. Cooler temperatures or cold seasons (such as you’ll find in the northern United States) dictate the need for insulation, especially on the northern wall of your structure. Also consider building an unglazed wall on the north side that’s higher than the knee wall you might build on the south side. Note, however, that if the north wall is the tallest wall of the structure, the greenhouse is liable to flex in high winds, which can pound the structure from the north, even when it’s constructed according to the best building practices. To avoid this, build lower and reinforce the wall with an air lock or potting shed, which provides some structural rigidity to the wall.
Any decision to construct a greenhouse from wood should be made with some understanding of wood species and properties. It’s important to select the right wood for your construction. A nondurable wood can quickly rot, endangering you and ruining your greenhouse investment. For example, a heated greenhouse I built early on of regular kiln-dried pine lumber purchased at the local hardware store lasted seven years in total, but in the heat, some of the pine lumber rotted after only five years. Even though the lumber was painted with exterior paint, the constant humidity and heat of the greenhouse simply led to rot.
Wood can be divided into two categories: hardwood and softwood. Softwoods are usually evergreen conifers and hardwoods tend to be leaf-dropping species, but there are exceptions to this rule. For example, larches (tamaracks) and bald cypresses, which retain their leaves year-round, are hardwoods. Some hardwoods such as basswood and balsa are very soft, while some softwoods, such as long-leaf pines, are very hard, so don’t make the mistake of thinking that the classification of tree or lumber is always indicative of the toughness of the wood.
In general, hardwoods tend to be more resistant to rot and most are classified as durable — that is, they last a long time in the moist, humid conditions of the greenhouse. Durable woods such as ash, beech, elm, greenheart, hickory, locust, mahogany, maple, white oak, teak, walnut, and poplar can all be used for greenhouses, but in most areas they are likely to be expensive and difficult to obtain. You should find out which durable woods are available in your area before you start estimating and designing your project.
Though durable, hardwoods can be difficult to work. If you’ve ever tried to drive a finish nail into a maple board, you know how hard these woods can be! Some hardwoods require that you drill a hole slightly smaller than the nail before hammering the nail into the wood. Gluing these woods is also difficult. Teak and maple are notoriously hard to glue or epoxy because the wood is so dense. In addition, the oils in teak require that you wipe down the wood with a solvent such as acetone before you apply epoxy.
Rot-resistant softwoods are less expensive, more readily available (though some are scarce relative to pine and the woods more commonly used for dimensional lumber), and easier to work with than hardwoods. That said, the cost of naturally rot-resistant woods such as some cedars, cypress, and redwood has skyrocketed in recent years (due mostly to their endangered status). If you can find them, you’re unlikely to obtain enough lumber to build a greenhouse. Other woods such as tidewater cypress, western red and Alaskan cedar, and eastern larch (tamarack or hackmatack) can also be used for greenhouse structures in that they all have a moderately high resistance to decay.
You can use other, less durable woods, but you’ll need to protect them with an impermeable layer to prevent moisture from getting into the wood and causing rot. (See Painting Your Greenhouse.) You can use ordinary exterior oil-based paint or epoxy resins or marine paints designed to withstand very humid conditions, though these tend to be quite expensive. Just remember this trade-off: Using a less expensive wood means that you need to use a more expensive paint. It might be worthwhile, then, to invest in better-quality wood at the outset.
This cross section of a tree shows from which tree part different kinds of building lumber are produced. Elements such as annual rings, sapwood, and heartwood determine a lumber’s strength and thus use.
Along with the kind of wood you choose, how the wood is cut and what part of the tree it comes from are important factors. Heartwood, as may be expected, comes from the middle of the tree, while the sapwood comes from the area nearer the bark. The rings in the wood give an idea of how fast the tree has grown and offer a clue to how durable it is. For example, widely spaced sapwood rings often indicate that the tree grew quickly and that the wood is less durable. Conversely, annual rings that are very close together indicate a wood that has grown slowly and that is very dense and likely quite rot resistant. Obviously, this is the kind of wood that’s most desirable to use for your structure. Heartwood, formed when the cells in the sapwood die and the tree’s pores get plugged, is the strongest wood available. Sapwood is the living part of the tree trunk and is usually not very resistant to decay.
Another characteristic to consider when choosing wood for your greenhouse is the grain: Make sure the grain flows along the wood as opposed to across the wood. A grain across the wood may cause the lumber to snap or fracture under the weight of any load. In addition, remember to pick the pieces with the straightest grain.
If you yourself are selecting the lumber for your project, don’t be afraid to dig through the pile at the lumberyard to select the best pieces available. Reject any pieces that have gouges or are split or that come from the outer edges of the log and have rough grain along one side. Look over the entire piece of lumber to make sure that it’s straight. If the lumber is warped, you may have difficulty building with it and installing straight-edged glass or other glazing after you’ve framed the structure.
Knots are formed where branches develop from the trunk of a tree. As the tree grows, the branch remains attached to the pith in the middle of the tree and a knot forms where the sapwood grows around the branch. From a builder’s point of view, knots are undesirable because they cause irregular shrinkage and compromise structural integrity; the knot usually shrinks at a different rate from that of the surrounding wood and may drop out of the wood after it has been cut, leaving a hole and weakening the lumber. Knots are also difficult to paint over: They tend to allow resin to seep through the paint layer.
When selecting wood for your greenhouse, try to choose pieces with few or no knots. If you must accept some knots and you plan on painting the greenhouse, paint them with two coats of oil-based sealer before you start work to ensure that they will not bleed through the paint layer.
Regular building lumber, mostly pine, can be obtained at your local hardware or building supply store. It comes in standard sizes, as described.
Standard lumber widths and thickness are known as either 1× (“one-by”), as in 1×6, 1×8, 1×10, or 1×12; or 2×, as in 2×3, 2×4, 2×6, 2×8, 2×10, or 2×12. Heavier lumber may be 4 ×, as in a 4×4 post or a 6×6 post. Landscape timbers are 8×8. Typically, 1× lumber is used for trim and edging, while 2× is used for joists, rafters, framing, and other structural elements. Note that all dimensions cited are in inches. Lengths increase in 2-foot increments starting at 6 feet, although you can often get a lumberyard to cut shorter lengths. Longer lengths, up to 18 and 20 feet, usually must be specially ordered.
In considering what building materials and methods to use for your greenhouse construction, you must consider wind speeds to which the structure will be exposed. As the wind speed increases, the pressure it exerts on a tall wall increases exponentially: In a 5-mile-per-hour wind the pressure is 25 units, in a 20-mile-per-hour wind the pressure increases to 400 units, and in a 40-mile-per-hour wind the pressure reaches 1,600 units. Take wind pressure into account and adjust your structure’s design and materials accordingly.
As I mentioned, I used regular lumber for my first greenhouses, but found that spores, heat, and humidity caused the lumber to rot after four or five years, even though it was protected with an undercoat and topcoat of paint and I repainted it after several years. By contrast, the lean-to greenhouse attached to my studio was built with pressure-treated lumber and has lasted for more than 15 years without any signs of rot.
Pressure-treated lumber is created by bathing the wood in a water solution of chemicals (arsenic, chromium, or copper) and then pressurizing it so that the chemicals impregnate the wood. Generally the chemicals do not penetrate very far into the wood, but they bond with it to make it reasonably impervious to rot and insect damage. Uncoated chromated copper arsenate (CCA) pressure-treated lumber can, however, leach arsenic into the soil when the lumber gets wet. Plants grown in soil where high levels of arsenic are present can take up the element, and when these plants are eaten by humans, the arsenic can be absorbed into the body. Although arsenic is found in the body in tiny trace quantities, in larger quantities it is a known carcinogen. The Environmental Protection Agency (EPA) therefore announced that as of December 31, 2003, no wood for residential use may be treated with CCA.
Varieties of pressure-treated lumber sold today include those treated by copper azole (CA), ammoniacal copper quat (ACQ), and ammoniacal copper zinc arsenate (ACZA), although ACZA lumber is most often used in the marine environment. Use of CA-treated lumber and ACQ-treated lumber has replaced CCA lumber. Note that you cannot dispose of CCA-treated wood in landfills. Nor should you burn it. Even the sawdust from this wood can be toxic.
Because various scientific studies have shown that the elements in pressure-treated wood can leach into the soil, if you choose to use this wood, all pressure-treated lumber should be painted before installation and kept painted while the greenhouse is being used. According to test studies, no leaching occurs when pressure-treated lumber is painted regularly. If, however, the prospect of painting (and repainting) your lumber is not appealing, you might want to think of lumber treatment alternatives that produce rot and insect resistance. These include zinc and sodium borates and the organic pesticide copper 8-quinolate. These chemicals don’t remain in the wood, though, and may also leach into surrounding soil.
If you do choose pressure-treated wood for your structure, take care in the building process. According to a recent alert issued by the federal Consumer Product Safety Commission, ACQ wood may cause faster corrosion of fasteners that come in contact with it. The commission recommends using stainless-steel brackets and nails instead of galvanized fasteners.
A safer, greener alternative to pressure-treated wood is a new product called TimberSIL, which utilizes a brand-new mineralization process to both preserve wood and eliminate all the problems of CA- or ACQ-treated wood. According to the company, TimberSIL uses sodium silicate technology to produce a nontoxic, noncarcinogenic, and noncorrosive preserved wood that can safely be in contact with the ground for up to 40 years. The product earned a Building-Green Top 10 award for 2004 (see Resources for information on this product).
The actual measurements of dimensional lumber differ from those that are used to label the lumber. For example, an 8-foot 2×4, known as a stud, is actually 8 feet by 11⁄2 inches by 31⁄2 inches and a 2×10 joist actually measures 11⁄2 inches by 91⁄2 inches. The labeled dimensions reflect the size of the piece of lumber before sawing, while the actual dimensions reflect the lumber size after sawing and planing.
Any part of the outside of your greenhouse that is not glazed will be sheathed before siding is installed. The most common (and most expensive) sheathing is 1⁄2-inch plywood made for this purpose. Most greenhouses require only a few sheets of plywood, and the extra expense of this material is well worth it. The American Plywood Association (APA) uses a grading system for such wood (see Plywood Manufacturing and Grading, below) and you should familiarize yourself with it before you make any purchases.
Install plywood horizontally so that it spans the vertical structural supports and stagger each row of panels if the wall is more than 4 feet high. When nailing plywood, use 6d nails spaced at a minimum of 6 to 9 inches apart. Nailing the plywood properly to the stud walls makes the entire structure more rigid and stable.
Plywood is made by first peeling a thin veneer of wood from a log as it is rotated. Once the wood has been separated in this way, the veneer is cut to size and dried to give it a uniform 2 to 4 percent moisture content. Each dried piece is then sent to a gluing machine, where sheets are glued together in panels, and a press. When exposed to heat and pressure, the glue dries in a few minutes and the plywood panel moves to another station, where it is trimmed. At this time, the better-quality pieces are sanded.
The plywood is then graded according to the quality of the veneer. According to the American Plywood Association–Engineered Wood Association, these standards are voluntary and may follow one of two systems: The first rates panels according to the thickness or span rating, with the span rating being the measure of the plywood stiffness and strength parallel to the face grain. The number on the left side of the stamp on a sheet of plywood gives the span rating of the wood — that is, the load it will carry when placed across a given span.
The second method rates plywood according to the veneer quality. N and A are the highest grade levels. Veneers with these grades have no knots or patches. A is intended for paintable surfaces, while N is the rating for plywood intended to be left with a natural finish. Both grades are sanded very smooth, though grade B plywood has small surface knots and may be plugged where larger knots occur. All the knots in grade C plywood are plugged; it is intended for use as an underlayment or for subfloors. Grade C, with its small knots, is the lowest grade of exterior plywood. Grade D plywood often has voids or unplugged knotholes.
If you see an entire sheet of plywood graded AB, this means that the face grade veneer is A quality and the back veneer is B quality. In AC plywood, the back veneer is grade C.
Plywood that is manufactured to be used where it will be exposed to more than 18 percent moisture content (meaning it will get rained on!) comes with an X designation. This means that it was assembled with a waterproof glue or epoxy and is suitable for use in wet areas such as a greenhouse. A number of projects in chapter 13 call for using CDX plywood (grade C on one side, grade D on the other, with an X designation meaning it is suitable for moisture exposure). See Resources for more information on plywood use and grading.
Preservative-treated plywood is also available should you decide that you need to protect your structure. The APA suggests, however, that the only plywood suitable for greenhouse use is that treated with copper 8-quinolinate, an organic preservative, though plywood treated with this chemical may not be readily available in most areas.
When you install two rows of plywood sheathing on a greenhouse, be sure to stagger the pieces to avoid continuous seams, which make for a weak surface.
Also available is plywood made with fiberglass plastic as its veneers. In a situation where the inside face of the plywood will be exposed — for example, in a lean-to greenhouse with varnished rafters and a partial white ceiling or rear wall — you might investigate using this material. Its fiberglass overlay keeps the CC-grade plywood dry. Its applications include use for truck bodies.
Oriented strand board has become a replacement for plywood in many areas. It is made from smaller trees that are chopped into pieces from 2 to 6 inches long. These wood chips are dried and coated with a glue before being oriented longitudinally in a machine. As the panels come from the machine, heat and compression are applied to form them into a large “master” panel, which is then cut into the appropriate size. Like plywood, oriented strand board has a span rating to give the builder an idea of its structural applications. When fastening this material, use 6d 2- or 21⁄2-inch nails spaced 6 to 8 inches apart. See Resources for more information on oriented strand board.
Sheathing is usually covered with either Tyvek house wrap or a similar commercially available material. House wrap over sheathing helps to prevent air infiltration (and exfiltration); allows moisture to pass through the wall; and helps prevent mold, mildew, and wood rot. Other materials such as building paper, tar paper, and roofing felt can be used in its place, though these don’t have all of the properties of house wrap. Building paper and tar paper, for instance, don’t allow much moisture to pass through the wall, and if it gets wet on a regular basis, building paper can deteriorate.
For installation, Tyvek is stapled or nailed in place and the seams are sealed using a special tape that adheres to the wrap. Building paper is nailed in place with large-headed roofing nails or nails fitted with large tin washers.
Siding, installed on top of house wrap, can take many forms. So that my greenhouse would harmonize with the look of the other structures on my property, I shingled its north, west, and east sides. If your home has wooden or vinyl siding, you can use the same material on your greenhouse so that it will blend in with the house. Following is a short review of a few siding materials, though there are many others from which to choose.
Wooden side-wall shingles or shakes come in either red or white cedar. Sometimes they’ve been dipped in an oil-based preservative such as bleaching oil (and sometimes you must dip them yourself) or you can install them and then coat them with a similar preservative.
Shingles are nailed or stapled directly to the sheathing. When shingling, measure the height of windows and doors above the bottom of the area to be shingled and divide the resulting area into equal sections of 6 to 7 inches. This will give you the number of inches of each shingle that will be exposed to the air or, as a builder might say, “to the weather.” For example, if a windowsill is exactly 3 feet above the ground and the top of a door frame is 6 feet 6 inches, each course of shingles will be spaced 6 inches apart. In that way, the bottom of the window will coincide with the seventh row of shingles. If you decide to space the shingle courses 5 inches apart, you will end up with a course of shingles 1 inch wide under the windowsill. Shingles that are this small tend to split and drop off a wall quickly and easily.
When setting up the first course of shingles, nail or staple the first row directly over the Tyvek or tar paper. About every 3 feet, set one shingle 2 inches to 3 inches below the others. This hanging shingle is used temporarily to hold a piece of straight furring whose top edge is level with the bottom edge of the shingles. Next, nail a row of shingles onto the first course fastened to the sheathing. Make sure that joints between shingles on the top course do not lie directly over joints on the bottom course. Also make sure that your nails are more than 6 inches above the base of the shingles so that the nail heads will be hidden under the next course of shingles.
When the second course is complete, remove the furring strip from the temporary low shingles, reset it at a level 6 inches higher, and trim the low shingles level with the bottom edge of the course. Next, nail the furring strip so that its top edge is 6 inches above the bottom edge of the first course and begin fastening the second course of shingles in place. Repeat this process until you have shingled the entire area.
When shingling corners, on one course the end shingle from one side of the structure overlaps the end shingle from the other, and on the next course, the overlap is reversed. Of course, if you don’t want to make corner overlaps, you can install 1×4 or 1×6 wood trim on the end of the walls, overlapping the trim where the corner meets.
Wood shingles or shakes are an attractive and long-lasting siding for a greenhouse.
As its name implies, in horizontal lap siding, one board of siding overlaps the board below it. This type is probably the most common and perhaps most popular in the United States today and comes in a variety of materials: wood (usually cedar), vinyl, and fiber cement, which is resistant to rot and cracking. Like shingles, lap siding is installed by working upward from the bottom of the building.
If you plan on using cedar siding, the first step is to make sure the material has a low moisture content (by allowing it to dry or keeping it under cover) and that it’s either stained or painted before installation. If you use a clear stain or bleaching oil, the wood should be coated on all sides. If you plan on painting the boards with siding paint, they should first be covered with an oil-based primer coat (either on both sides or on the fronts alone while the backs are painted with a clear water sealer).
Before installing lap siding, nail a furring strip to the bottom of the greenhouse wall to set the first piece of siding away from the wall. Lap siding is nailed in place at 24-inch intervals using 6d ring or siding nails for siding measuring 1⁄2 inch or 5⁄8 inch, 7d nails for 3⁄4-inch siding, and 10d for M-inch siding. Siding nails have a blunt tip that tears into wood without potentially splitting it. The nails should be either stainless steel or galvanized. Note that if you must nail near the edges of siding, you should predrill nail holes to avoid splitting the board. When nailing siding, always make sure the top course overlaps the one below it by about an inch and that the nails pass through only a single piece of wood. (If you nail through two pieces, there is little room for the wood to expand and the siding will probably split.)
Lap siding and tongue-and-groove siding
Barn board is an exterior-grade, plywood-like material that comes in 4×8 sheets that are stained or painted and nailed to the sheathing. If you wish, you can bypass sheathing and nail barn board directly to studs. It also makes an easy-to-install wall covering.
With this method of siding, boards between 1 inch and 2 inches thick are nailed vertically to the sheathing, then a thin strip about 1 inch by 2 inches is nailed over the seams between the boards to cover the joints. Boards that are 16 inches wide can be nailed to the studs in lieu of plywood sheathing, but any width other than 16 inches will require the boards be fastened to sheathing or furring strips. Installing board and batten siding on furring strips may necessitate using caulking to eliminate any gaps and construction adhesive to be sure each join is firm. Construction adhesive can be used instead of or along with nails, but if your greenhouse will need to be moved or disassembled for any reason in the coming years, using nails will make this process far easier.
Board and batten siding
Other materials besides wood can be used in greenhouse construction. The most commonly employed are aluminum, plastic, and steel. Aluminum and plastic are used mostly for kit greenhouses and steel for larger commercial greenhouses. While steel is less expensive (aluminum costs about three times as much), its use in hobby-sized greenhouses has largely been eliminated as lighter aluminum and plastic extrusions have become available.
Aluminum-frame greenhouse kits may come assembled or may require assembly, which usually consists of simply bolting together the various parts. Aluminum for greenhouses is extruded, meaning the metal is heated until it is just molten and is then squeezed through a die to give it its shape. When the metal emerges from the die, it is rapidly cooled to ensure that it keeps this shape. The extruded metal is made with a channel of a specific size that is designed to accept a glazing material based on the material’s thickness, which means that if you use an aluminum frame for your structure, you may not be able to substitute glass glazing for polycarbonate. If the fixed channel size of the metal is large, it can accept double or twin wall glazing, or you may install a rubber or plastic insert if you plan on using a thinner glazing material. A rubber insert can also serve as an expansion joint and a thermal break between the glazing and the aluminum.
One of the pluses of working with aluminum is that it can be cut with the same tools that you would use for wood. Another benefit of the metal in a greenhouse application is that it has a natural oxide layer that makes it corrosion resistant. On some greenhouse extrusions, the oxide layer is enhanced by anodizing it, or treating the surface of the material to help it resist corrosion. In more upscale greenhouses, the aluminum extrusion is painted with a urethane- or epoxy-based paint that adheres strongly to the alloy. This type of paint should last the lifetime of the greenhouse, but in severe conditions it may require repainting every 10 years or so. In addition, unlike one of wood, an aluminum structure, with its consistent dimensions, rarely leaks.
Aluminum-framed greenhouses typically cost more to heat because the metal is a poor heat conductor and allows heat to pass through easily. If you plan to heat your aluminum greenhouse during a northern winter, you can decrease heat loss by suspending a layer of plastic or bubble wrap insulation inside the structure to prevent warm air from touching the cold metal framing.
As we saw in chapter 2, the earliest greenhouses were often made with cast- or wrought-iron frames and supports, but the use of cast iron has gone the way of the dinosaur. It was replaced with steel for structural supports in larger greenhouse applications.
When used in commercial structures, this metal is protected from rust and corrosion by either galvanizing or painting it, though galvanizing (coating the metal with zinc) provides a more durable finish. The zinc is electroplated to the steel after the metal is bent (bending steel after it has been plated often results in cracking the zinc layer and eventual corrosion). Nongalvanized steel will eventually rust if it is not painted regularly. In addition, steel must be protected from the humid environment of a greenhouse, which usually requires paint that must be refreshed about every 10 years. Note that if you plan to weld together your steel frame, you will have to use ungalvanized steel; welding galvanized steel emits hazardous fumes.
In home greenhouses today, steel (in the form of galvanized steel pipe) is used most frequently for framing hoop houses. The pipe frame is covered with plastic greenhouse sheeting or with thin layers of flexible polycarbonate or acrylic. (See chapter 7 for more information on these and other glazing materials.) Because steel frames, like those of aluminum, tend to lose more heat than structures framed with wood, hoop house covers made of two layers of plastic with warm air blown between them can help to reduce the heat loss of the metal structure.
Like aluminum, plastic frames can be extruded (pushed through a die) or pultruded (pulled through a die) to produce a shape suitable for holding polycarbonate or acrylic glazing. Because most plastics cannot carry heavy glazing, however, kit greenhouses with this framing are usually small (from 2 feet by 4 feet to 6 feet by 8 feet), although some of the latest styles using heavy-duty plastic for the frame and lightweight polycarbonate glazing can reach dimensions of 8 feet by 10 feet. Benefits of plastic construction are that it will never rot, it has reasonable UV resistance, it can retain heat fairly well, and frames using it tend to be inexpensive. If you can afford a wood- or metal-framed greenhouse, it will hold up longer, but if you are not sure that you want to embrace the greenhouse gardening lifestyle, a plastic-framed greenhouse is a good starting point.
As I mentioned earlier, an alternative to a plastic kit greenhouse is a homemade hoop house made from schedule 40 PVC pipe. For a cost of less than $150, I made such a house that was 40 feet long and 14 feet wide as a temporary garden structure to keep plants slightly warmer and to keep deer out of the garden. For the cover I used a 100-foot roll of construction polyethylene at a cost of less than $100. Manufacturers of higher-quality greenhouse coverings do NOT recommend using them for plastic hoops, specifying that they should be used only with metal frames. During summer, I remove the polyethylene and install deer netting on the plastic frame to protect the garden from deer and rodents. See chapter 13 (project 7) for specific information on building this type of do-it-yourself hoop house.
Properly insulating your greenhouse can cut down enormously on heating costs should you choose to operate the structure year-round. There are a number of types of insulation available to get the job done. Specific applications may depend on where and how the insulation is to be used. Following are explanations of some of the most common, along with the pluses and drawbacks of each and some tips for installing them.
Fiberglass insulation usually comes in pre-cut batts, or you can buy it in a long roll and cut it to size. Its width is designed to fit between studs that are either 16 inches on center or 24 inches on center. If you are going to lay a vapor barrier on the inside of the greenhouse, you do not need to buy paper-backed fiberglass insulation, though the backing does make it easier to staple it into place. If you do install paper-backed fiberglass between the studs, cover it with a nonflammable material such as drywall (waterproof drywall, sometimes called greenboard, is best). In addition, don’t leave the fiberglass batts or paper exposed because tiny particles of fiberglass can fall and contaminate the growing beds.
Installing fiberglass is probably one of the most unpleasant jobs you’ll ever undertake. Installation day always seems to be hot, especially in a greenhouse, when you least want to wear a Tyvek suit, respirator or dust mask, and any of the other gear that you use for this type of work. To install fiberglass insulation, I first apply a barrier cream on my arms and face, then a Tyvek suit with a hood (both the cream and Tyvek suits are available in good hardware stores). Next, I put on a pair of boots, rubber gloves, and a respirator. You can also wear goggles, but they steam up very quickly, eliminating visibility. After I’m finished with the installation, I usually toss the Tyvek suit in the trash because it is covered with so many fiberglass particles. Finally, I wash off in a cold shower (hot water opens the pores in your skin and allows the tiny needles of fiberglass to enter). I also rinse the respirator, boots, and gloves to get rid of any fiberglass shards.
Once the fiberglass is in place, you can cover it with a polyethylene film vapor barrier or install waterproof drywall directly on top of it.
Polystyrene is the pink or blue rigid foam insulation that you see at your local lumberyard or home center. It comes in panels that are 1, 2, or 4 inches thick. Extruded polystyrene has an R-value of 3 to 5 per inch, depending on how it is made, and expanded polystyrene, which is white, has an R-value of 2 to 4.
Each polystyrene panel is 2 feet wide and 8 feet long and, with the exception of white expanded polystyrene, has tongue-and-groove edges. When using this material, make sure the tongues and grooves lock properly to eliminate air gaps. On the inside of a building, the panels are generally nailed or screwed directly to the studs using large tin washers to prevent the fastener heads from breaking through the insulation. Remember that it’s easy to damage polystyrene when hammering nails into it, even if you use large washers. Furring strips can be installed horizontally along the walls at 16-inch centers so that you can screw the drywall to them. The additional air gap created by the strips between the drywall and the polystyrene ensures that the insulation is not crushed, and imparts a little more insulation value.
You can also install polystyrene on the outside of your greenhouse walls using the same method along with 1⁄4-inch or 3⁄8-inch exterior-grade plywood or oriented strand board sheathing. Lay the polystyrene on the plywood, using furring strips between each sheet for nailing, and then cover the entire furring/polystyrene layer with a second layer of 3⁄8-inch plywood. This makes for a very rigid structure, but obviously costs more than a single layer of plywood sheathing. Of course, if you plan to leave the exterior wall exposed, use barn board or plywood rated for exteriors as your outer layer.
Often known by its trade name Celotex, this material looks like rigid foam insulation but has aluminum foil on both sides, with the manufacturer’s name printed on one side. The printed face should be installed against the wall. Polyisocyanurate costs slightly more than regular polystyrene insulation, but in my opinion it does a better job. I’ve used it on the inside of my cool greenhouse, where, along with its insulating properties, the foil face reflects sunlight onto the plants. After you have installed this type of insulation, tape with ordinary duct tape all seams or, for a few dollars more, use foil tape designed to adhere to the foil surface of the insulation. The foil tape certainly looks much better than duct tape and also makes a better vapor barrier.
The drawback to polyisocyanurate is that it is highly flammable, and when it burns, it gives off toxic fumes. If you plan to use this material in a greenhouse attached to your home, most fire codes call for it to be covered with a nonflammable material such as drywall. Of course, you should also install smoke detectors in the greenhouse area.
If you decide that your greenhouse will incorporate an insulated north wall, you might want to install newer-to-the-market stress skin panels, which have an insulated core material sandwiched between two layers of plywood or dry-wall or, in some instances, higher-quality wood veneers. Such panels are made by several manufacturers (among them Winter Panel Corp. and Eagle Panel Systems, Inc.; see Resources) and have an R-value of between 4 and 5 per inch of thickness. Along with offering more insulation value than conventional stick-built construction, these panel smake for easier building. Structurewall panels, made by Winter Panel Corp., have a layer of foam insulation between two layers of oriented strand board (OSB) and are intended to carry the load of mounted shelving or cabinets. Curtainwall nonstructural panels, also made by Winter Panel Corp., have an interior surface of gypsum wallboard that can be finished to have an R-value of 25, making them ideal for the north wall of a double-glazed greenhouse. At a slightly lower cost, you can use panels with an expanded polystyrene (EPS) core, such as those made by Eagle Panel Systems. The layers of the panels are tightly bonded, so you will never have to worry about them coming apart.
If you decide to build a free-form, insulated greenhouse using either of these products, you will need to design the structure to accommodate their sizes. The panels come in lengths up to 16 feet for Curtainwall and 24 feet for Structurewall and are 4 feet wide. The framing carries the structural load and the stress skin panels provide additional rigidity. You should install the panels with 11⁄2-inch nails spaced 12 inches apart on hardwood frames or 21⁄2-inch nails on soft-wood frames, and each panel should be fully supported on all edges. Unlike regular plywood, they can be installed horizontally or vertically simply by nailing or screwing them to the structure. Panel edges are joined with splines set just inside the exterior layers to ensure good insulation and strong joints.
According to the Winter Panel Corp. Website, because ants and termites may get into the insulation between the external layers of stress skin panels, you should install a termite shield on the sill plate before installing the panels.
Note that one drawback to stress skin panels is the installation of wiring, which can’t be run through the panels themselves. If you locate outlet boxes at or near joints, however, you will be able to run the wiring through the middle of the joint and have nothing but the outlet box showing.
The anatomy of a stress skin panel
If your greenhouse is attached to your home, you should install a vapor barrier between the greenhouse wall and the house wall, which will keep moisture from entering the walls of your home. Water in the walls combined with the heat of the greenhouse will quickly cause rot.
Foil-faced insulation can serve as a vapor barrier, especially if it is taped at each seam, but most people use 4- or 6-mil polyethylene plastic sheeting. The thicker version is more durable and, because of its extra weight, is easier to install without tearing.
I have found that a vapor barrier is best installed on the side of the studs facing the interior of the greenhouse so that air outside the vapor barrier can flow away from the walls. Because the air in the greenhouse is usually more humid than the air in your home, installing the vapor barrier on the greenhouse side of the studs keeps moisture out of the walls. I do not recommend that you put a second vapor barrier on the outside of the greenhouse, because it will hold in any moisture that made it through the inside barrier and could cause the walls to rot.
The size of the greenhouse door you install depends on how you plan to use it. If you will be mainly weeding and moving trays of seedlings in and out of the greenhouse, the door can be as narrow as 30 inches, but if you want to be able to get a wheelbarrow or lawn tractor through the door, you will need to make it much wider. Some gardeners fit garage doors to their greenhouses to facilitate moving large volumes of plants and to allow easy vehicle access, though these are more commonly seen in commercial structures as opposed to hobby greenhouses.
Greenhouse doors should be as airtight as possible if you plan to use the structure year-round, to keep cold winter air and wind from blowing on your plants. Install a good-quality door with weather stripping. Wooden or foam-filled doors have the best insulation values. In addition, as we’ve learned, a screen door installed outside the main door is very useful. Not only does it provide an extra layer of insulation in winter, but in summer it can serve as a means of allowing air to circulate through the greenhouse and keeps insects off plants when the main door is open as well.
There are a number of alternative building products you can use for greenhouse construction. Many of these combine structural properties with insulation value, often using principles of thermal mass to ensure heat absorption during the day and radiation of this heat back to plants at night. In addition to possibly incorporating the more “formal” alternative methods described below, you might use the ideas of thermal mass in your more traditional greenhouse structure by painting large cans black, filling them with antifreeze, and stacking them on a north-facing wall, where they can absorb and radiate heat. Or you might make a thermal mass “wall” of old tires packed with sand or gravel to provide nighttime heat.
Note that some alternative materials do not offer the structural longevity of those materials in common use.
Straw bales are used more and more for construction, but if you use this method and decide not to wrap or plaster them in any way, you may find that critters like to nest in them and eat the products of your hard work.
If you plan to build a large structure using straw bales, you should support the roof with wood framing so that the straw walls are not bearing the entire load. A typical straw bale has an R-value of 2.5 to 3 per inch of bale; thus, a normal 18-inch bale has an R-value of about 48, which is more than adequate for most greenhouses. Straw bales that are protected by plaster or a kind of stucco applied over chicken wire can last for many decades in some parts of the country.
A temporary straw-bale greenhouse made of three walls with panes of glass or translucent glazing on the front and roof can be used to protect plants in spring. The simplest such structure has a sloping glass front. Old storm windows or old patio doors work perfectly for the front or roof of such a greenhouse.
Adobe, used primarily in the southwestern United States, is a time-tested material for home construction. Original adobe bricks are made from sand and clay with straw or grass added to bind the material. Bricks are allowed to dry in the hot sun and are then cured for several weeks before they are used. Because adobe bricks can absorb moisture and slowly deteriorate, they are often stabilized with the addition of cement or asphalt to the sand and clay. In construction, adobe bricks are joined with a mud mortar, although many builders use regular mortar mix with stabilized bricks to get the same effect. An adobe brick wall can be further stabilized with a coating of mud, lime plaster, or stucco. An adobe wall in a greenhouse provides an excellent thermal mass that may be up to 18 inches thick. The humidity of the greenhouse, however, may expose the wall to moisture, thus causing its deterioration.
Rammed earth is ordinary soil with a high clay content that is compacted in a special machine and made into bricks. Unlike adobe bricks, which are sun-cured, rammed-earth bricks may be baked in a kiln. These bricks can then be used to build a wall similar to the way concrete blocks or house bricks are used. One pronounced difference: Unlike concrete blocks, rammed-earth bricks tend to be wide and flat, producing a wall that can be 18 to 24 inches thick. An alternative to using bricks is building forms into which you pour the earth mixture, which is then compacted.
The problem with a rammed-earth wall made entirely of compacted soil is that it will be compromised by rain. Consequently, today rammed-earth walls are made with a mixture of earth, clay, sand, gravel, and up to 10 percent cement. Even with this more impervious material, however, rammed-earth greenhouses usually have large eaves. On the plus side, repair of rammed-earth construction is relatively easy and the material does not burn and is vermin-resistant.
Unfortunately, greenhouses combine the three elements that most often cause rot in wood and corrosion in metal: heat, moisture, and fungal rot spores. No matter how hard you try to eliminate these, they will almost always be present in your structure. The only option is to protect against them in some way. If your greenhouse frame is made of wood, this is most commonly accomplished by painting, oiling, or staining it. (As discussed in Pressure-Treated Wood, you must coat even pressure-treated wood or cedar to promote long life or protect chemicals from leaching into the soil.) Painting is not required if the greenhouse frame is made of aluminum, which is naturally protected through the oxidation of the metal, or of anodized aluminum, which has been treated for additional protection.
As for the paint you use, it should be durable because it will be exposed to heat, moisture, solvents you’ll use to clean glazing, and the over-spray of fertilizers, pesticides, and herbicides (if you use them). This means that you should use oil-based exterior paint both inside and outside the greenhouse. The moisture levels inside the greenhouse will cause indoor paint to peel in a short time. Don’t rely on a coat of latex paint to prevent the wood from rotting. Moisture can penetrate latex paints. Home exterior oil-based paints tend to prevent rot for a little while longer, but these do not form an impermeable layer on the wood either. Much as you need to repaint your home every few years, you’ll need to repaint your greenhouse — if the wood doesn’t begin to rot first. (Note that a heated greenhouse rots faster than an unheated one).
One way to protect wood is to encapsulate it in an epoxy resin such as that available from WEST System or Epiglass from Interlux, and then apply paint or varnish over it. The epoxy prevents moisture from entering the wood but it does not protect against UV radiation, so after applying it, you’ll have to paint the wood with a few coats of polyurethane with UV blockers. The best varnishes with the highest levels of UV blocking are Epifanes and Schooner.
If you really want to protect a less expensive wood, I recommend cutting all lumber to size and, before assembling it, undercoating it with a marine epoxy paint such as 545 base (gray) from Awlgrip or Interprime (available in white or gray) from Interlux, and then coating it with a marine topcoat such as brushed or sprayed Awlgrip, Brightside from Interlux, Toplac, or Perfection. (Make sure you use a respirator when spraying paints, and read the manufacturer’s safety data.) Brightside and Perfection are polyurethane paints that protect wood from rot; Toplac is a silicone-based paint that is easy to clean and also helps to protect wood. These marine paints are intended for use on wood that will be in a hot, humid environment and they do the job quite well, although they are expensive. Surfaces painted with them can be easily cleaned with the wipe of a sponge.
In my warm greenhouse, I used a conventional oil-based house paint undercoat and top-coated it with an outdoor latex paint. I find that this type combination lasts from 3 to 5 years. It can be cleaned, but eventually the paint gets dirty or the humidity gets to it and causes it to peel.
Note that using white paint ensures that all surfaces reflect the maximum amount of light onto your plants. White also reveals where dirt and condensation are collecting.
In a finish coat, you may prefer the warm look of natural wood as opposed to paint. My experience is that finish products of this kind made for the home stand up to the moisture and humidity of a greenhouse for only a year or two before requiring another application. For a natural look, seal the wood with epoxy and then paint it with a good polyurethane varnish with plenty of UV block for a finish that will last for a number of years without the need for recoating. In my opinion, the best high UV varnishes are Epifanes and Schooner, both boat varnishes that have the best reputation for protecting wood over the long term. The beauty of using epoxy for the first two coats is that it bonds to the wood (it can be removed only by sanding) and it dries within an hour or so.
Ideally you should paint building lumber before you assemble the greenhouse and before you install any glazing so that the wood is protected on all sides and surfaces.
If you use epoxy over teak, it will not adhere because of the oils in the wood. It’s necessary to wipe the teak with a solvent to remove oils before applying epoxy. According to one manufacturer, the makeup of epoxy is very close to that of polyurethane varnish, thus the varnish, which has UV inhibitors, can easily be applied over the epoxy. To do this, simply lightly sand the epoxy (wear a dust mask), wipe the surface with a solvent, then brush on the varnish. When it comes time to recoat, simply sand the top layer of varnish and varnish it again.
When working with solvents, epoxies, and paints, remember ALWAYS to wear rubber gloves, a dust mask when sanding, and other appropriate protective gear.