Acrylic is light, easy to cut, and has more impact resistance than glass, though it scratches easily and has a high rate of expansion. Double-skin acrylic sheets are translucent, and the diffuse light is good for plants. Designer Valerie Walsh thermoforms these sheets into curved roof sections for custom sunspaces.
Commonplace now as the stuff of automobile lights, bank security window, gas-station signs, camera and contact lenses, TV screens, and even paint, blankets and carpets, acrylic plastic has been around a long time. Although development of this highly elastic substance began back in the 19th century, it was not until the 1930s that chemical firms first began producing commercial quantities of acrylic, which can be manufactured as a liquid, as fibres or in sheets. It took World War II, when the War Department started testing and using acrylic extensively in aircraft, to push the technology into the applications familiar to us today.
The larger family of plastic glazing materials has been closely scrutinized over the last decade by solar designers and builders searching for the least expensive material for collectors, greenhouses, windows, skylights and water storage. Most plastic glazings are flexible, light-weight, impact-resistant and light-diffusing. Acrylic stands out because it will not degrade or yellow in ultraviolet light. Along with high clarity and an impact resistance of 15 to 30 times greater than that of glass, acrylic offers a lifetime gauged at 20 years.
Given a burning rate of Class II in the codes, acrylic burns very rapidly, but does not smoke or produce gases more toxic than those produced by wood or paper. The ignition temperature is higher than that of most woods, but acrylic begins to soften above 160F.
Used for exterior and interior windows, doors, skylights, clerestories and greenhouse glazing, acrylic sheets can be moulded into various shapes and contours. Both single-skin and double-skin versions of the material are available. Single-skin acrylic is clear and comes in sheets or continuous rolls of various thicknesses. Extruded into a hollow-walled sheet material, double-skin acrylic has interior ribs, spaced 5/8 in. apart, running the length of the sheet. It is translucent, but not transparent.
The Debate
Builders and designers who have worked with acrylic fall into two camps: they either hate it or love it. Any type of glazing is ultimately compared with glass, and those who like acrylic, whether single or double shin, offer these reasons:
It is versatile. Single-skin acrylic can be cut into a multiplicity of shapes, either for pure design reasons or to meet the demands of an out-of-square solar retrofit. Single-skin acrylic can be cold-formed into curves; both single and double-skin sheets can be heat-formed.
Acrylics are easy to cut and can be site-fabricated. Lighter in weight and easier to carry, the double-skin sheet is more convenient to install than glass. The flexibility of single-skin sheets varies with their thickness; longer sheets of thinner acrylic require more people to handle them.
Acrylic has high transmissivity, and better impact strength than glass. Double-skin acrylic has an R-value competitive with insulated glass units. It is safe in overhead applications, because it will not shatter. Instead it breaks into large, dull-edged pieces.
On the other hand, builders who prefer glass offer these reasons: Acrylic has a high rate of expansion and contraction, requiring careful attention to keep an installation leak-proof. As it moves, the acrylic sheet makes a noise described as ticking or cracking and acrylic scratches easily. The extent, to which this is considered a problem, or even an annoyance, varies from builder to builder.
Costs
Acrylic used to be much cheaper than glass, but now single-skin acrylic is competitive only when purchased in bulk. Any cost advantage is likely to be lost if you attempt to double-glaze with single-skin acrylic. This is a labour intensive process, and it is tough to eliminate condensation between the panes. The price of double-skin acrylic is close to that of insulated glass, but the cost is higher if the price of a compression fastening system is figured in. (Some builders expect the price of double-skin acrylic to drop in the future when more companies begin to manufacture it.)
If acrylic is used for a roof in a sunspace or greenhouse, however, its availability in assorted lengths can cut down on labour costs. Designer Larry Lindsey, of the Princeton Energy Group, pints out that long pieces eliminate the need for horizontal mullion breaks, and so can be installed less expensively than several smaller ones. They are also cheaper. An uninterrupted piece of glazing can run the full length of the slope, supported by purlins underneath.
Professional use
Architect David Sellers of Sellers & Co., an architectural firm in Warren Vt., explains his extensive use of acrylic: "Our whole plastics experiment has been an aesthetic means of expanding the type of architecture we do. With acrylic we could push the house beyond what it was already, both the inside and the outside experience of it". In the process, the firm has developed a spectrum of applications for single-skin acrylic (sidebar, below right).
Designer-builder Valerie Walsh, of Solar Horizon, Santa Fe, N. Mex., uses double-skin acrylic greenhouses and sunspaces that are her firm’s specialty. She first used single-skin acrylic because it was slick, clean-looking, and did not degrade in the South-western sun. She began to explore unusual shapes, such as a wheel-spoke roof design. Then she turned to using double-skin acrylic. Walsh thermoforms acrylic in her own shop - curved pieces that are as wide as 5½ ft. and typically 6 ft. to 7 ft. long, although she has done 8-footers.
Safety and economics figured prominently in the Princeton Energy Group's decision to use acrylic glazing overhead in their greenhouses and sunspaces.
"The whole issue is a matter of expense," says Larry Lindsay. "In order to have glass products we feel comfortable installing overhead, we have to pay two penalties, one in transmittance and one in bucks. At present, there is no laminated low-iron glass available at a reasonable cost."
For those who have years of experience with acrylic, a willingness to experiment and to learn from mistakes has produced a valuable body of knowledge about working with the material, its design potential and its limits.
Movement
Leaking is a particular concern with acrylic glazing because it moves a lot, expanding and contracting in response to temperature changes. To avoid leaks, design principle number one is to try to eliminate horizontal joints, and yet glazing systems that may do a perfect job of sealing glass joints will not work at all with acrylic. Its movement will pull the caulk right out.
"Acrylic has a tremendous coefficient of expansion - you have to allow maybe an inch over 14 ft. for movement," cautions Chuck Katzenbach, construction manager at PEG where they have worked with double-skin acrylic for exterior applications and single-skin for interior ones. "No silicones or sealants we know of will stretch that potential full inch of movement." Indeed, one builder tells a story about using butyl tape for bedding: The acrylic moved so much in the heat that the tapes eventually dangled from the rafters like snakes.
Room for expansion must be left on all four sides of an acrylic sheet, because the material will expand and contract in all directions. The amount of movement depends on the length of the sheet and the temperature extremes it will be subject to.
Acrylic glazing can be installed year round, but it is vital to pay attention to the temperature when it is put in place. Katzenbach explains that if it is 30 F outside, then you have to remember to allow for expansion to whatever you figure your high temperature will be. If it could go from 30 F to a peak of 120 F in your greenhouse, you have to make provisions for a 90o change.
Cutting
To cut a sheet of acrylic, use a fine-tooth carbide-tipped blade set for a shallow cut, and move like a snail. This is important because speed will cause little pressure cracks to appear on the bottom edge. While cutting, make sure that the sheet is firmly supported on both sides of the cut. Sharpness is vital, so use that blade only for working with acrylic. When the acrylic is cut it heats up and the edges melt, but the wider kerf of a carbide blade will prevent the newly cut edges from melting back together again. After the cut, the edges can be planed, filed or sanded.
As the acrylic is cut, little fuzzy pieces will fly up. Some will reglue themselves to the edges and can be broken off when the cutting is completed. With double-skin acrylic, the fuzz tends to fill the 5/8 in. dia. columns between ribs. Use an air gun to blow it out.
If you drill acrylic, support the sheet fully, and use a very sharp spade bit, ground to a sharper angle than for drilling wood. The sharper angle helps prevent cracking. Drill very slowly, and slow down even more just before the drill breaks through the sheet. Be prepared to break some pieces, no matter how careful you are.
Acrylic sheets come protected with an adhesive masking, which exposure to rain or sunlight makes quite difficult to remove. Leave the protective masking on the acrylic as long as possible, and be prepared for a good zap from static electricity when you pull it off.
Fastening
For years it has been common practice to fasten single-skin acrylic by screwing it down. The designers at Sellers & Co. developed a pressure-plate fastening system to distribute the pressure evenly and drilled the holes for the bolts or screws an extra 1/8 in. wide to allow for movement. But after ten years or more, the hole has shifted and started pushing against the screw in some installations. Cracks developed where none had existed.
Small cracks in single-skin acrylic can be stopped if the force on them is not too great. Although his firm now uses installation details that do not involve drilling the sheets, Jim Sanford at Sellers & Co., recommends stopping cracks by drilling a 1/4 in. dia. hole at the end of the crack and filling it with silicone. Designer-builder Alex Wade, of Mt. Marion, N.Y., who still uses screws, suggests drilling a tiny hole at the end of the crack, too. He then widens the crack slightly with a knife and fills it with silicone. Finally, he removes the offending screw. Wade suspects that many builders do not take into account the season of the year in which they are working when they drill the holes for screws. When installing acrylic in the extremes of summer or winter, Wade drives the screw either to the inside or the outside of an over-sized hole in the acrylic, to allow for subsequent contraction or expansion when the temperatures change. It is important to space the holes evenly (about 2 ft. apart) and to tighten the screws uniformly to distribute the pressure equally. The sheet must be held down firmly, but still be able to move.
To avoid taking a chance with cracking, however, most builders have abandoned screw. Instead they use a compression system of battens that hold the plastic sheet down on a smooth bed of ethylene propylene diene monomer (EPDM). It comes in strips and is supplied by several manufacturers. The acrylic can easily slide across the EPDM as it moves.
Manufacturers recommend a 3x rafter to support the bedding in the compression glazing system. PEG installs an interior condensation gutter on greenhouse rafters that doubles as a smooth, uniform bed for the EPDM gasket in the glazing system. The 20-gauge sheet metal straddles the rafter and is bent into a 5/8 in. lip for the gutter on each side.
PEG has also developed a system for a standard 2x. A clear all-heart redwood 1 x 4 trim piece is screwed on top of the 2x, widening the bed and providing a smooth surface (inset drawing, facing page). Concentrated stress on the acrylic sheets is as important to avoid with a compression system as it is with screws. If one point is fastened tighter than the others, the acrylic will bow in and leak or crack.
Larry Lindsey recommends aluminium battens on south-facing roofs, because wooden ones will eventually cup upward, creating a leak. Aluminium battens can be purchased with various finishes, or they can be capped with a strip of red wood.
Double-skin acrylic needs to be supported at its base or it will bow instead of moving within the compression glazing system.
PEG lets the sheets hang over the roof's edges as a shingle would sealing it underneath. For the bottom edge on installations with curved roofs, Valerie Walsh has developed a system with no damming problems. She slides on aluminium terminal section (ATS) from CYRO on the bottom of the double-skin acrylic, then snugs the acrylic into a larger aluminium U-channel that is in turn screwed into the wood beam. Walsh then caulks the inside and rills weep holes through the outside of the U-channel.
In one of his designs, David Sellers decided to glaze the south-facing roof area with long strips of single-skin acrylic. To avoid leaking, he encouraged the tendency of the 1/4 in. sheets to sag slightly. Small blocks under the edges of the sheets accentuate the dip. Melting snow or rain flows to the center of each panel and then drains off the roof. On the bottom edge, an angle keeps the acrylic from slipping.
Glazing materials
Acrylic is fussy stuff. Chuck Katzenbach reels off a list of materials to avoid using with this plastic. Vinyl leaches into acrylic and weakens its edges. Some butyls have plasticizers that may also leach into acrylic. In these cases, either the acrylic will eventually fail or the butyl will become very hard. The plasticizer in most neoprenes is not compatible, so check with the manufacturer.
Compatible glazing materials are few: EPDM heads the list. Silicone caulk is okay, but sooner or later the acrylic's movement will pull it loose. If it is installed on a cold day, the silicone may pull out on the first really warm one. There are many urethane foams you can use, but you would be well advised to consult the manufacturer directly about compatibility.
Support
The double-skin acrylic can bow in over the length of the roof, and the sheet could conceivably pull out of the glazing system under a very heavy snow load, according to Larry Lindsey. So as a cautionary measure, PEG figures on a 30-lb. snow load and installs purlins 4 ft. o.c., about 5/8 in. below the sheet.
Condensation
Acrylic transpires water vapour, so a double-skin unit typically will have some cloudy vapour inside. Double-skin acrylic should be installed with its ribs running down the slope so that any condensation inside the channels will collect at the bottom edge of the sheet. This edge needs to be vented. Double-skin sheets arrive with rubber packing material in both ends of the channels, to keep them free of debris. PEG's construction crew just leaves it there and perforates it with a scratch awl to allow air movement.
Cleaning
Never use abrasives, ammonia-base glass cleaners or paint thinner on acrylic glazing. Mild detergent, rubbing alcohol, turpentine and wax-base cleaner-polishers designed especially for plastic are safe when applied with a soft cloth.
Cleaning brings up the controversial issue of scratching. "The scratching drives me crazy, though other builders do not seem to care as much," says Valerie Walsh. Whether she is storing the sheets she has heat-formed into curves or transporting them to the building site, she keeps thin sheets of foam padding wrapped around every piece.
"With double-skin acrylic, people's tendency is not to expect to be able to see through it. They are not looking through it as they would through a window, and so they are not seeing the small imperfections in the surface itself," argues Chuck Katzenbach.
Even though single-skin acrylic is transparent, many builders who work with it say that scratching just is not a significant problem, particularly if the glazing is kept clean. The only serious scratching problem is likely to be the work of a dog. Most scratches can be easily removed with a Simonize paste-wax buffing. And now some single-skin acrylics are available with a polysilicate coating that makes them abrasion-resistant and also improves their chemical resistance.
