THERE ARE A VARIETY OF materials you can use to glaze your greenhouse: glass, plastic (acrylic, polycarbonate, or polyethylene sheeting), and fiberglass. The best choice for you depends on factors such as the specific material’s properties, the amount of light that must be transmitted through the glazing, cost, weight, frequency of glazing maintenance, and the location of the greenhouse. For example, you might intend to use your greenhouse to grow orchids, which favor bright light but not full sun. Consequently, clear glass may not be the best choice. A slightly more opaque material such as tinted fiberglass or a material that diffuses light such as polycarbonate would be more suitable for your plants. Of course, if you really want to use clear glass, you can use one of the shade cloths that block 10 to 75 percent of the light entering the greenhouse. These cloths are available from commercial greenhouse suppliers. (See chapter 5, Shading, for more on shade cloths.)
The frequency of necessary maintenance of glazing also factors in to your ultimate choice. If you have to clean the glazing weekly (because of smoke or other environmental pollutants, or because of falling leaves or bird droppings or other substances that might affect how much light gets to your plants), your greenhouse may become too much of a chore and you may neglect it in a short time. If your glazing requires frequent cleaning, it may acquire small scratches (as is the case with polycarbonate), which may affect the transparency of the product. Of all the glazing products available, glass is the easiest to clean. Next are polycarbonates (given the scratching caveat), but unless it is coated, fiberglass tends to collect dust and dirt, and when it is cleaned, tiny particles dislodge from the material.
How to choose between glass and plastic? Each material has unique properties that make it suitable for certain applications and not for others. For example, though glass is one of the best materials for light transmission, it is heavy and can shatter relatively easily. In an area where large hailstones are likely or in some greenhouse frames, polycarbonate or fiberglass is a better choice than glass for sloping surfaces and roofs.
While you can supplement greenhouse light with electric light, most of the light in your greenhouse will come from the sun. Solar radiation comes in many wavelengths, but plants use only the wavelengths between 400 and 700 nanometers for photosynthesis, which are approximately the same wavelengths that the human eye sees. Radiation in this wavelength is known as photosynthetically active radiation (PAR). A prism will divide light into the colors of the spectrum, ranging from violet to red. The wavelengths from the blue to the red end of the scale are the ones most used by plants. Certain wavelengths affect growth, while others promote flowering and fruiting. In general, wavelengths toward the red end of the spectrum encourage flowering, while wavelengths toward the blue end of the spectrum tend to make plants shorter and stockier. This is because the amount of energy in a wavelength is inversely proportional to the wavelength. Thus blue, a longer wavelength, doesn’t have quite as much energy as red light and doesn’t promote a plant’s robust growth. Studies have found that plants prefer light from the red end of the spectrum. If you want to grow plants under lights, then you need to look for bulbs that emit wavelengths from this end.
Studies have found that plants prefer light from the red end of the spectrum.
Light loses some of its radiation as it passes through glazing material. The plants may also experience partial shading from greenhouse benches and shelving and other structural elements. Some light is also reflected off the structure and some is absorbed by objects inside the greenhouse. A wood-framed home-built greenhouse may lose as much as 30 percent of the available light from shadows cast by the structure and the glazing. If you have any doubt about how much light is getting into your greenhouse, buy a light meter and take readings both inside the building and outside. The difference between the two figures is the loss of light as it passes to your plants in the structure. Because of the lower light levels inside the greenhouse, take care when moving plants from inside to outside. Because they become used to the lower light inside the structure, outside they cannot develop enough chlorophyll to protect themselves. If moved too abruptly or for too long a period of time, they will often drop leaves or suffer sunburned leaves when exposed to full-strength sunlight. When you bring your plants out of the greenhouse in spring, place them in a slightly shaded area for a month or so, until the leaves have had time to adapt to full sunlight.
The initial design of your greenhouse should take into account the type of glazing material that you will be using. As I have pointed out, because it is heavy, glass requires fairly strong structural members that also tend to block more light. Polycarbonate and fiberglass, however, can be installed on a lighter frame of wood, aluminum, steel, or PVC. Polycarbonate allows up to 85 percent of light through the material (the specific amount depends upon the thickness and color of the material), while fiberglass may allow through only 75 to 85 percent of available light. (For direct comparisons of light transmission of various materials, see the The Properties of Various Glazing Materials chart.)
You must also consider the expansion and contraction of glazing materials when designing your greenhouse. All glazing materials will expand somewhat as the greenhouse heats up, but the rate of expansion varies dramatically. Glass, for example, expands only a little, but polycarbonates can expand significantly.
For most people, glass is the obvious choice as a glazing material. It is plentiful, can easily be replaced, and has a long life span (some Victorian glasshouses are still in operation with most of their original glass in place). It is resistant to UV degradation and maintains excellent light transmission (as long as you keep it clean). Glass comes in different thicknesses, can be cut to suit a specific application, comes as either clear or with built-in shading, and can fit many aesthetics — you can even get rounded glass panels to make a greenhouse with curved eaves or walls.
Its weight and fragility, however, are important considerations. If you want to use glass in your greenhouse, check your building codes carefully; in many areas, walls and overhead panels are required to be of tempered or laminated glass. I find that the biggest problem with breaking a pane of glass is not so much replacing the pane as it is picking the broken glass out of the plants and growing beds. Of course, if a pane of glass breaks in the middle of winter, you will have to repair the damage as quickly as possible or you could lose plants. Until it’s replaced, cover the hole with plywood or plastic right away.
Glass comes in many types. You can choose from regular single-weight or single-thickness, double-weight or double-thickness, tempered, laminated, or double-pane or Thermopane low-emissivity (low-E) or low-resistivity (low-R) glass. Low-E glass has a thin layer on the exterior that allows short-wave solar energy through but blocks long-wave infrared and UV energy. This allows most of the sun’s energy through but restricts heat energy from escaping. A low-E coating can give a double-pane window efficiency near or equal to that of a triple-pane window. Generally, about 70 percent of solar heat and about 75 percent of solar light are allowed through low-E glazing. Low-R glazing cuts down on the amount of heat escaping from the greenhouse. (The R-value is the resistance of a material to heat flow.) A window with a high R-value has greater resistance to heat flow. Typically, a single-pane window has an R-value of 2 or 3, while a double-pane window has an R-value of 4 to 6. A triple-pane window may have an R-value as high as 8 or 9. By filling the space between the panes with a gas such as argon or krypton, the R-value can be increased to about 10. This is almost the value of a stud wall (about 11 in older homes and as high as 19 in well-insulated newer construction).
The intended use of your greenhouse should govern the type of glass you select. For example, a greenhouse used for growing understory tropicals in the winter might have triple glazing to keep down heating costs. The lower light levels of triple-pane glass will probably not affect the plants — in fact, more shading may be needed. If your plan is to grow vegetables to market, however, you might decide to use a glass that keeps in heat and allows the transmission of plenty of light.
One problem you should be aware of when you install any type of clear glass is that birds tend to fly into it, even when windows are marked with decals or ornaments to warn the creatures. Necessarily, the glass in a greenhouse must be kept clean, and the green plants inside the greenhouse naturally attract wildlife. Because vertical panes in a greenhouse can take their toll on birds, install a few pieces of reflective tape or other defensive features on many panes to discourage as many avian impacts as possible.
The most common glass is single-weight window glass that comes ‹/£™ inch thick and can be cut to almost any size. From the early days of their construction until the late 1950s, greenhouses were built with fairly small panes of glass that overlapped no more than 1 or 2 inches. Single-weight glass can now be purchased in sheets up to 48 inches by 84 inches, but such large sheets are very fragile and should not be used in a typical greenhouse. The biggest panes that can safely be used are about 24 inches by 24 inches, but again, check local building codes before using panes of this size.
Double-weight glass is 1⁄8 inch thick and can be used for slightly larger spans (it is made in sheets of 72 inches by 120 inches, but this size is generally available only to specialty glass users and may be hard to find). As with single-weight glass, you should check the local building codes to see if it is permissible to glaze a greenhouse with ordinary glass and to learn what pane size is considered too large. The best reason to use double-weight glass in a greenhouse is that it is easy to cut to size, though even if codes do allow it, I do not recommend using it in overhead greenhouse applications.
Plate glass comes in large sheets and is usually 1⁄4 inch thick, although it can be obtained in thicknesses up to 1 inch. Quite often, you can get plate glass directly from the owner of a store that is being dismantled or from a builder specializing in dismantling old stores. Plate glass doesn’t cost much more than double-weight glass and will stand up to much more impact. In addition, because it is stronger than single- or double-weight glass, the panels can span greater distances between supports, which allows more light through, even though the individual supports must be stronger to support these heavier panels. Note that plate glass is quite heavy; just one plate glass sheet may require two or more people just to lift it.
Laminated glass consists of two layers of glass bonded on either side of a plastic layer to create a single glass panel. The plastic layer is usually polyvinyl butyral (PVB), but it can also be a resin. This type of glass is also called safety or shatterproof glass because if the glass is shattered, the plastic layer between the panes holds the fragments together to prevent them from falling.
Laminated glass can be made earthquake-resistant, hurricane-resistant, and resistant to the impact of rocks and hail. It can be obtained in a variety of colors and is also fabricated with reinforcing wire running through it. Laminated glass is expensive — three to four times the price of single-pane glass — but must be used if your building codes call for it in overhead structures.
Tempered glass is created by carefully controlling the conditions under which the glass is cooled. It can resist about four times the impact of regular glass and when it fractures, it shatters into tiny fragments, known as dicing. If tempered glass falls on you, it is unlikely to cause serious injury because the glass fragments are so small; nevertheless, it can give you some nasty nicks. Because of its unique structure, you cannot cut tempered glass, and it must be ordered sized to fit. It is best used for overhead glazing and for areas that are subject to high winds.
If tempered glass shatters, it goes everywhere and will need to be carefully picked out of beds and off benches. I’ve found the best way of removing shattered tempered glass from a bed is simply to shovel it away along with a layer of the topsoil. Light transmission of tempered glass is only slightly lower than that of conventional glass of the same thickness, but installation requires great care: If it’s bent or catches in any way, it can shatter.
Often known by the trade name Thermopane, double-pane glass is composed of two layers of glass with an air space between them. The panes are held apart by a beading that contains moisture-absorbing chemicals to keep the space between the panes dry. Light transmission of double-pane glass is lower than that of single-pane glass. Note for installation that the panels are extremely heavy and must be handled very carefully to avoid breaking the seal around the edges that keeps the panes dry. Any amount of bending during installation will break this seal, resulting eventually in moisture entering between the panes and fogging them. If the seal is not replaced, the inside of the glass will become etched and gray, cutting down dramatically on the amount of light it transmits.
You can order double-pane glass in any size, but two standard sizes — 34 inches by 75 inches and 46 inches by 76 inches — are readily available because they are used in 3-foot and 4-foot patio doors, respectively. These standard panes are sometimes referred to as patio glass. The 46-inch by 76-inch panes in my heated greenhouse allow me to heat a 3,000-cubic-foot space with a small, 18,000 BTU propane heater during the worst of winters.
The biggest drawback of double-pane glass is that the seals may eventually break. In my heated greenhouse I have found that the roof panels lose their seals after about 8 to 10 years, while the vertical panels have been installed for 12 years with no failure except as a result of a hurricane. In fact, the vertical panels of double-pane glass used in my house construction have lasted for more than 20 years; only three windows have required seal replacement.
When installing double-pane glass, take care to allow the glass to expand and contract freely. Usually this means mounting vertical panels on small pieces of hard rubber and leaving an expansion gap around the edge of the mounting. Each panel is held in place with quarter round or tapered wooden molding that you can paint to keep the greenhouse looking neat and tidy. Do not allow any nails or screws on the greenhouse structure to touch the glass. This contact can form a pressure point that will eventually break the glass. This means you must build precisely so that the glass does not butt against bare wood or fasteners. I have discovered from experience that a pane of glass resting against a screw in the greenhouse will crack during the first winter, if not earlier.
Not only is triple-pane glass expensive, but it also reduces light transmission to half or a third of natural light. In addition, as a drawback to installation, triple-pane glass is extremely heavy. The seals between the panes are also broken very easily. Yet the R-value of triple-pane glass is high, often up to 8. Considering that 31⁄2 inches of fiberglass has an R-value of 11, in some climates triple-pane glass could be used on the north wall of a greenhouse instead of a wood-framed insulated knee wall without the huge heat loss of single- or double-pane glass.
Tempered low-emission (low-E) glass is often recommended for walls of solariums (which have glazed roofs) and sun spaces (which have solid roofs) because it is a safety glass that allows heat into the greenhouse but keeps out UV radiation and cuts down on heat escaping from the structure. More expensive than regular, tempered low-E glass are MC low-E glass and super MC low-E glass. Both of these are UV-resistant and tempered and are recommended for use in roof areas of solariums. Light transmission of low-E glass is 75 percent and that of MC is 65 to 70 percent. Super MC low-E glass is available from Florian. Be aware, however, that low-E glass tends to block the rays that plants need, leading to slow or stunted growth.
With the incidence of hurricane destruction on the rise in a number of areas, various coastal cities have enacted building codes that require the use of hurricane glass in new construction. Before you build, you’ll need to determine if these codes are in place in your area. Hurricane glass consists of two layers of glass sandwiching a polycarbonate laminate. The glass is capable of withstanding a blow from a 2×4 stud fired at it at 40 miles per hour. Hurricane glass is very expensive and reduces light transmission.
If you, like me, are a pack rat, you may have a collection of castoff storm windows that you’ve saved for use in a greenhouse or cold frame. You can, indeed, build a greenhouse using old storm windows, but most of these windows leak (probably why they were replaced). Rather than using the windows in their casements, it might be better to remove the glass panes from the wooden frames and use them in a frame you construct from scratch.
For many years I’ve successfully used a cold frame built of storm windows. In it I keep greens growing most of the winter. (See Hot and Cold Frames for an illustration of a cold frame built from old storm windows and project 10 in chapter 13 for building instructions for a cold frame.)
There are a number of alternatives to glass glazing. Most of these come in the form of rigid plastic panels made of polycarbonate, acrylic, PVC (polyvinyl chloride), or fiberglass, or flexible membranes of polyethylene (PE) sheeting, PVC, or ethylvinyl acetate (EVA). Flexible materials may also be reinforced with threads to reduce damage from tearing. All these materials are used in commercial greenhouses and have advantages and disadvantages for use in home greenhouses.
Fiberglass comes in two forms, flat and corrugated, which is more resistant to flexing. There are hundreds of grades of fiberglass. For a greenhouse, you need the clearest that you can buy (usually not the kind you’d find at your local lumberyard). Because of the glass fibers embedded in the material, it diffuses light well, but it can also yellow with age.
Fiberglass sheets are made either by laying chopped fiberglass pieces, known as a fiberglass mat or chopped strand mat (CSM), on a flat or corrugated surface that has been sprayed with a clear gel and then wetting the fiberglass mat by spraying it with a polyester resin, or by spraying chopped strands of fiberglass coated with resin directly onto the gel coat by using what is known as a chopper gun. The clear resin sets up hard within a few minutes. The fiberglass sheet may be coated with gel on one or both sides and may be squeezed between rollers to make it smooth on both sides. Some types of fiberglass are smooth on only one side. If you buy this type, make sure during installation that the smooth side is facing the elements. If the rough side faces out, it will trap dirt and be extremely difficult to clean. Eventually the material will hold enough dirt to affect the transmission of light. About the only way to clean old, rough fiberglass is to use a vacuum cleaner to remove the dust and loose particles of fiberglass. During this process, wear a Tyvek suit and a dust mask to keep from getting the particles on your skin and clothes and to keep from inhaling them. Fiberglass that is coated and pressed smooth on both sides is much easier to clean, but it tends to be more opaque.
To its credit, fiberglass is moderately flexible and quite strong. In general, it will last 8 to 10 years and its light transmission is 75 to 85 percent, though Charley’s Greenhouse and Garden (see Resources) offers fiberglass through which up to 93 percent of the available light can pass. Because it is flexible, fiberglass requires more support than glass, but it does have high impact resistance, is easy to handle during installation, and is relatively inexpensive. The biggest drawback of fiberglass is that unless a flame-retardant resin is used in its fabrication, the material is highly flammable. Flame-retardant resins, however, tend to make the material darker, thus reducing light transmission.
If you need an unusually shaped panel for a custom greenhouse, it can be created from fiberglass. To do this, buy chopped strand mat, make a mold from a sheet of glass or wood, spray it with mold-release wax or lay plastic sheeting over it, and lay the mat over the mold. Once the mat has taken the required shape, wet it with polyester resin or epoxy and allow it to set. You can also attach structural reinforcing such as plywood, steel, or plastic supports directly to fiberglass by bonding it to the surface with resin or epoxy. (See chapter 7 for information on installing fiberglass panels.)
Acrylic is known best by trade names such as Plexiglass and Acrylite. The material can be bent using heat (a professional installer will have specialized heating tools for this purpose), stays reasonably clear for many years, is more flexible than glass, and repels the impact of small hailstones (though large rocks or hailstones can crack it). Acrylic is also easy to cut with a slightly blunt saw and can be shaped with ordinary hand tools. Triple-wall acrylic saves up to 25 percent more energy than double-wall while allowing about 80 percent light transmission. Less expensive types of acrylics can yellow and craze, but more expensive types usually come with a 10-year warranty against discoloration and will last up to 30 years. Before buying acrylic panels, check the warranty.
One drawback to acrylic is that it can easily be scratched. When installing it, leave the covering paper on the material until all the work is done. You should also be very careful when drilling acrylic during installation. A slight twist or jamming of the drill bit can crack the material. When drilling, it is best to use a slightly blunt drill bit and to cut slowly. Additionally, acrylic is highly flammable. If you use an open-flame heater in your greenhouse, you should not choose this glazing material.
Acrylics for greenhouses come as single panels or double- or triple-wall panels in a variety of thicknesses. The layers of double- or triple-wall sheets are separated by a rib placed roughly every 3 to 6 inches. A ‹/¡§-inch sheet is often used for commercial hoop houses because it can easily be bent to suit the arch of the greenhouse frame. When purchasing acrylic panels, look at the R-value and the amount of light the material transmits. (This information can usually be found on the manufacturer’s Website or in the brochure available where you’ll be buying the product. See Resources for acrylic panel manufacturers.) In choosing the thickness of the material, take into consideration this data as well as the application. For example, on a greenhouse roof you can install 1⁄2-inch or 3⁄4-inch glazing to prevent too much heat loss but use only 1⁄4-inch or 3⁄8-inch on the walls to allow more light transmission. Acrylic is a great material to use for larger greenhouses because sheets can be obtained in 4-foot widths and up to 40-foot lengths.
Like acrylic, polycarbonate comes in either double- or triple-wall panels. There are many brand names for polycarbonate, such as Suntuf, Palgard, and Macrolux. Double-wall panels can save about 30 percent more heat than single-wall panels while allowing through about 85 percent of the available light. Also available is a white double-wall, sometimes called opal polycarbonate, that blocks about 50 percent of direct sunlight and diffuses the light that does get through. This glazing is good for greenhouses where plants need to be protected from any strong, direct sunlight. Standard sheet sizes are 48 inches long by 72 inches wide, but you can obtain lengths up to 36 feet. The R-value for double-wall polycarbonate is about 1.6 and for triple-wall it is 2.1 — close to the value for glass.
Polycarbonate is reasonably lightweight and can be installed by one person. If you install it yourself, however, you must take care because the material expands and shrinks a good deal more than glass. In addition, the ribs that separate the two or three layers of the material must run vertically to allow moisture to drain from the panels. With polycarbonate that has been coated with a UV blocker, you’ll need to make sure that the panels are installed so that the blocker faces the elements. There are also hidden costs with polycarbonate installation: You’ll need to cap the ends of the panels, install glazing tape and sealant to seal all joints, and install bar caps on the ends of the sheets.
Polycarbonate is reasonably lightweight and can be installed by one person.
Some extremely strong polycarbonate sheets such as Suntuf, Palgard, and Palsun have greater impact resistance than glass. These allow more than 90 percent light transmission, are almost unbreakable, and weigh less than half as much as glass and only 43 percent as much as aluminum. Macrolux, a polycarbonate manufactured by Co-Ex Corporation, has the highest impact resistance of any co-extruded thermoglazing. Consider these materials if you live in an area where your glazing is exposed to hailstones, golf balls, small tree limbs, small animals, or other sources of potential breakage.
Flexible sheeting such as polyethylene (PE) sheeting and polyvinyl chloride (PVC) has a relatively short life span, often as little as one season, although most sheets will last at least two seasons before UV radiation breaks down the material beyond use. If you purchase commercial-grade PVC intended for greenhouse use, it will last longer than industrial grade PVC, but as I mentioned earlier, it is for use only on metal-framed structures. The biggest drawback to flexible sheeting is that snow loads and wind can destroy the material in a few weeks. My experience with both building polyethylene and greenhouse PVC is that they tend to split along the folds, but you can use either duct tape or a commercially available repair tape to make a serviceable repair and extend the life of the covering. Another negative is that it doesn’t have a very high insulation value. In fact, it raises the internal temperature of a greenhouse by only a few degrees. Benefits of polyethylene sheeting are that it transmits plenty of light, it is inexpensive, and it is easy to install. A 100-foot roll can be purchased for less than $200 and comes in various widths up to 50 feet.
For greater longevity and heat retention of the material, you can install two layers of polyethylene sheeting on your greenhouse with an air space between them. In fact, some commercial growers who use hoop houses blow warm air between the layers. Not only does this insulate the greenhouse, but it also creates more stability for the plastic and requires just a small blower fan, although, due to air leaks, the fan will run almost continuously. To create two layers, fasten one sheet to the inside of a wooden frame with wooden strips cut to size and the other layer on the outside of the frame. Typically, two layers of plastic sheeting as compared to a single layer cut light levels by about 10 percent but increase the energy savings by up to 40 percent. One problem with a dual-sheet system is condensation between the layers, which can reduce the energy savings and cut light levels even further.
Combining glazing materials can address specific weather conditions in various areas and can add insulation value. In regions where winters are especially severe, for instance, you might install double- or triple-wall polycarbonate with an inner liner of polyethylene sheeting. You can also use an inner layer of sheeting beneath single-pane glass. The air space between the panels or panes and the sheeting helps insulate the plants. In Britain, many gardeners install a layer of bubble wrap inside their greenhouse to help reduce energy losses. Bubble wrap can readily be purchased from many packing suppliers, shipping stores, and office supply stores in widths up to 4 feet for under $20 per 50-foot roll. As I mentioned earlier, clear polyethylene or weatherproofing tape can be used to tape bubble wrap to the greenhouse structure to eliminate drafts. While bubble wrap slightly lowers light transmission levels, the additional insulation it provides protects plants from freezing and helps to promote growth in cooler conditions.
Using the right caulking and weather stripping material will prolong the life of your greenhouse and glazing, prevent water from entering the structure and heat from escaping it, make it easier to paint, and make it much easier to remove and insert a new pane of glass or sheet of plastic should one get broken. Varieties of caulking (whether from a tube or a roll) are most often applied directly onto the wood or metal frame and the glass or polycarbonate glazing is then pressed into it. It’s important to note that caulking is not glue and should not be relied upon to hold the glazing.
There are many types of caulking. Selecting the right one for the job is important to getting good adherence and a good seal. For your greenhouse, the best type of caulking is butyl or polybutyl, which allows you to install and easily remove glass for repairs. A bead of adhesive caulking will certainly do the job, but should you have to replace or repair a pane or sheet of glazing, it will be impossible to remove the piece without breaking it. If you use butyl or polybutyl caulking and mechanical fastenings to secure the glazing, making repairs will be simple.
Before you caulk, wearing rubber gloves, wipe the area with a solvent or degreaser such as acetone or 3M’s general-purpose adhesive remover to eliminate grease, dirt, and other residue. This will give the caulking better adhesion. Apply tube caulking from a caulking gun or unroll a strip before you install the glazing. (Strip caulking is most convenient to use in areas where glazing must be lowered onto caulking.) When the glass is in place, use tube caulking from a caulking gun to seal the gap between the glass and the greenhouse structure. Caulking guns can be purchased for a few dollars from any hardware store. Before fitting the tube in the gun, cut the tip of the tube nozzle at a slight angle so that the caulking will come out of the tube in a bead. You may have to use a spatula or a finger to smooth a bead once you lay it. If you decide to use your finger, wear a rubber glove for easy cleanup or, as Don Mather, a glazier and building contractor in Middleton, New Hampshire, says, wet your finger and smooth the bead. Be warned that there is a knack to this and if you don’t do it right, you can end up with sticky fingers!
After you have applied the caulking, use a rag and solvent to clean up the area. Try to clean up the caulking before it dries, or you may have to remove it with a knife or razor blade, which could scratch the paint job, the frame, or the glazing.
Silicone is the most often used adhesive caulking primarily because users are not aware of other types of caulking that could be more suitable. When choosing a type of caulking for your greenhouse, look for one that stays flexible, does not yellow, and can be painted (if you paint your greenhouse).
Silicone caulking has high UV resistance and because they are water-cured, most silicones emit no volatile organic compounds (VOCs) when they are applied. Silicone applied on very humid days will cure faster, and in most environments it will dry to a tack-free stage in 3 hours, but you should allow at least 24 hours for the caulking to cure completely. The benefits of silicone sealants are that they are clear and noncorrosive and they provide a good seal without a great deal of effort, though their adhesive strength is low compared to other types of caulking, such as polyurethanes and polysulphides. Silicone caulking cannot be painted, however; it must be applied to painted surfaces or purchased in a color similar to that of anodized aluminum. Note that the strength of the seal of silicone applied over a painted surface often relies on the strength of the paint surface itself, which may be low. When it comes time to remove a silicone sealant, often the paint surface peels away with it.
Silicone stays flexible for a long time, but brands that are less expensive can allow mold growth, which may cause rot in wooden surfaces. Some more expensive marine silicones are usually made with fungicides and UV inhibitors. While there is no real silicone remover, a lacquer remover will get it off hard surfaces or a razor blade can be used to remove it from glass after it has cured.
Polyurethane sealants have strong adhesive qualities and great strength. Typically, they are used where a permanent seal is required. Polyurethanes cure to tacky in about 24 to 48 hours and set up fully in 4 to 7 days. One drawback is that they may yellow when exposed to sunlight.
Sulfur-based with high adhesive properties, this is one of the most commonly used sealants available. Polysulphides come in a variety of colors and are priced in the middle of the spectrum of caulking. They stay flexible over their lifetime, are not affected by most liquids, and can be painted and sanded. Because they are sulfur-based, however, polysulphides can degrade some plastics and acrylics. (If you have any doubts about its effects on your greenhouse plastics, make a test in an inconspicuous place or on a spare piece of polycarbonate or acrylic glazing. Allow it cure for several days before you apply it to your structure.) Polysuphides reach the tack-free stage in times that vary from 30 minutes to 3 days, depending on the brand.
This low-adhesion, rubber-based material is used mostly as a sealant rather than as caulking. It comes in a variety of colors and prices.
Typically, compounds try to combine the best qualities of different caulking and sealants and eliminate the problematic qualities of each. One such compound is Bostik Marine 940 Fast Set, a nonyellowing polymer sealant blended from sylilated polyethylene, silicone, and urethane. Compounds made from silicone and polyurethane are designed to be low in odor and to have moderately strong adhesion.
For any greenhouse that might move with heat or wind pressure, you should use a two-part polysulphide sealant that cures to a rubbery stage. Many manufacturers make such a product. The sealant remains flexible; resists cleaners, oils, and petroleum products; and is designed to adhere well to wood surfaces and to bend or flex with the wood. These sealants were developed for marine use, where flexibility in the face of heat, wind, and water is important.
If you intend to heat the greenhouse, it pays to apply weather stripping around window and door openings. This will stop stray drafts and help cut your heating bill. The most common type is a spongy material with adhesive on one side. Pressed into place in a door or window frame, it allows the door or window to be closed tightly against it. Weather stripping costs very little in relation to the amount of energy that it saves.