The following is a contribution from Northern Greenhouse Sales, a supplier of many types of coverings since 1980. Here they explain the characteristics of super woven poly.
Successfully tested in the cold Canadian Yukon and Northwest Territories, in Alaska, and in all areas of the United States and of Canada, from the Arctic Circle to South America, from the Atlantic to the Pacific and from one end to the other of the great central regions of our continent. Used by hobbyists, backyard gardeners, farmers, and commercial growers.
Expect at least 3-4 years from quality UV-treated polys; some get 7 years or more in shaded or cloudy areas.
Poly life depends on quality of installation, operation and weather factors. Specifically: placing the rafters’ close enough to minimize flapping in wind, crossbracing the structure and/or rafters to prevent vibration in the wind (mentioned elsewhere), sanding the rafters smooth, wrapping the rafters with plastic, installing on a warm day to get plastic tight, keeping kids and shovel blades off the poly, protection from severe thunderstorm winds, freedom from large, jagged, gale-driven hailstones, and amount of sunshine.
Resistance to the elements
Triple layered woven poly is water-proof, rot and mildew resistant, and resistant to most chemicals (inorganic acids, alkalis, aqueous salt solutions) (particularly important in coastal areas like B.C., the Maritimes, Washington, Oregon, Atlantic states, and Gulf Coast).
Resistance to punctures, stress and… animals!
Punctures are the bane of unwoven plastics. Pinholes or punctures can easily expand like a tear in your pants. Thick, dense & cross-weaving is very resistant to punctures. I sometimes strike my greenhouse covering with a rake handle and so far, no holes. It has a higher impact resistance than glass!
Also, for some reason, my cats love me. When I’m in the greenhouse they try to break through. They covered my regular poly with pinholes from their claws as they climbed it. However, our woven poly resists them due to its toughness. An added bonus is that due to its translucence, they can’t see through it.
Superstrong Woven Poly is stronger than the most non-woven plastics, even the thick ones. Thus its thickness, weave, high density, and lamination all add up to extra toughness.
Superstrong woven poly is highly resistant to stretch due to the dense tight weave. Try stretching it & stretching regular polys. Once regular (non-woven) poly is stretched it can sag, inviting destructive flapping in gales. Woven poly will resist stretching & keep its original tautness well under stress.
Resistance to Mother Nature
In cold weather: most materials, including plastics, contract in cold weather like mercury in a thermometer. At low temperatures regular 6 mil poly turns very brittle and shatter-prone. On a frigid day winter gales lash at ordinary plastic, shattering it into thousands of shreds blowing them all over the countryside. Woven poly has less contraction and less brittleness than non-woven polys. The dense cross-weave fibers resist contraction and give it added strength to resist cold brittleness.
Thickness and lamination also add strength. Most greenhouse polys in use are 2, 4, or 6 mils. Put some 10.5 mil woven poly and regular poly in your freezer. While both are frozen solid, try bending and flexing them. It’s an even more effective test outdoors when temperatures dip to 30 or 40 below zero. A quality woven poly has resistance to cold down to minus 100 degrees F.
In hot weather: plastic expands in the heat. One year before starting to sell woven poly, I covered my greenhouse with a less expensive 6 mil plastic. That plastic expanded and sagged in the heat. Along came a prairie thunderstorm and the winds started flapping that loose plastic like a rag doll. It was over in minutes. The 6 mils covering hung in shreds.
The dense poly weave of woven plastic stands up well in summer storms. If you want to test it, mount it and brand X on two identical frames corresponding to the space between 2 of your greenhouse rafters. Turn your house temperature up to 95 degrees. Turn your teen’s mini jet aircraft engine up to a wind speed of 80mph (just kidding) and watch the results.
Wind resistance: woven poly is highly resistant to wind stress at any temperature due to its strength. This is important on hot days (Hot Weather Resistant) when the greenhouse is even hotter inside than out. It’s even more important on cold days (“Cold Weather Resistant”) when the winds of winter howl mercilessly.
Hail resistance: many greenhouse owners fear hail more than heat, cold, or gales. Large jagged wind-blown hailstones can destroy non-woven coverings & the crops in seconds. Small hailstones have taken paint off houses. Woven poly is resistant to hailstones due to its thickness, lamination, high density & the tight weave. Small hail bounces off it like a kid on a trampoline. If you live in a hail-prone area, a steeper angle is better ideally. It is cheaper to cover a greenhouse with plastic than with glass if you get repeated devastating hailstorms. It’s also easier and faster to install or replace in an emergency.
Woven poly and light
Most plants require 65% to 85% of available light for optimum growth. Woven poly’s light transmission is very favorable compared to greenhouse fiberglass. Commercial greenhouses are growing high yielding tomato and cucumber crops under it, crops needing maximum light. Rafters can be farther apart than for glass permitting more light as well. Optimum light transmission remains high throughout our poly life due to protection from the yellowing or discoloring common to many other coverings.
Woven poly is not clear like glass. It is translucent, diffusing light in all directions. This characteristic is beneficial to a growing environment.
Glare-free light is produced by woven poly. Plants thrive, the carpet doesn’t fade, and strangers can’t see inside. Some even have photographic studios and woven poly creates the ideal conditions for color work and editing because of the bright clear well-diffused light.
The weave reflects and “traps” light, bouncing it around like a frosted mirror. Many comment how bright it is in a woven poly greenhouse on a cloudy day.
Ultraviolet protection: quality polys contain ultraviolet stabilizers and here’s why: ultraviolet solar radiation quickly breaks down untreated plastic, sometimes in 5 months in high radiation areas such as the prairies and plains where there is less cloud and water vapor in the air. A quality woven poly is treated on both sides and in the middle to resist UV breakdown. Some are UV stabilized on only one side, or only in the outside coatings. Some brands are not UV stabilized at all.
Translucence: leaf burn is a common problem under clear glass or plastic, especially on hot, clear, windless days. The solution used to be to whitewash and/or add light-shading materials such as shade cloth or lath. An undesirable side effect of whitewash is that the light reduction is permanent, even on cloudy days when you don’t want it.
In many cases shadecloth or whitewash are not even needed with woven poly’s nice even light distribution. Woven poly provides an excellent diffused light, which is free of glare, hot spots, and sun shafts, thus reducing or eliminating damaging leaf burn. The extra labor cost & extra shading-product cost are eliminated, all with no reduction in light.
Hot spots and varying temperatures can lead to flower malformation and decreased growth. Diffused (scattered) light reduces such fluctuations and the moisture content of the buds and fruit becomes more uniform.
Additional benefits of woven poly
Less leakage: the large width and length of poly panels mean less leaky joints and less edge/perimeter length. This cuts down on leakage of water and air. Little sealing is needed, compared to glass. It eliminates most of the potentially expensive and drafty leaky joints typical of many glass greenhouses.
Minimized heat loss: the flat surface has 20% less surface than corrugated types thus reducing heat loss and requiring less energy to maintain the desired temperature.
Speed of installation: it’s far quicker to install a wide sheet of woven poly than the smaller widths and lengths of glass or solid plastics. Extra sealing time is eliminated (just nail edges down instead of using a caulking gun). Also eliminated is the continual re-puttying of edges that glass can require.
Lighter supporting structures: glass requires a stronger, heavier more rigid structure, to carry the dead weight. Poly is lighter and can thus be attached to a lighter frame.
Less expensive foundations: woven poly weighs less than glass and is not breakable. This eliminates the need for heavy foundations which frequently have to be extended beneath the frost line (6 feet deep in this area) to prevent the heaving in spring which could break glass. Such a foundation may cost more than the greenhouse itself. With woven poly I can use free/cheap railway ties for a very strong but cheap foundation, which “floats” on a few inches of gravel.