Greenhouse Glazing
Tuesday, January 13, 2015 at 2:51PM 
Like choosing the color you paint your house, or the new countertops in the kitchen, greenhouse glazing is a decision you’re may have to live with for a while. Once the greenhouse location has been selected, the clear material used to cover the greenhouse, or glazing, should be the next selection in the design process. The type and weight of glazing material will then dictate the strength requirements and design of the greenhouse’s frame.
Building a high tunnel style greenhouse with a PVC frame to get through one winter? A roll of lightweight polyethelene sheeting will do the trick.
Creating an elegant lean-to structure built onto the south facing side of a luxury home? A high performance, multi-pane glass would be more appropriate.
But whatever the greenhouse glazing material, ultimately the goal is the same: To create an indoor grow environment that allows for control of temperature and humidity while allowing natural sunlight to reach the plants. While it is certainly feasible to use supplemental lighting inside a greenhouse, most growers will rely on sunlight, thus the greenhouse should be directionally located to receive the maximum about of available sunlight.
Though solar radiation has a broad spectrum of wavelengths, studies have shown that only wavelengths between 400-700 nanometers are useful for photosynthesis in plants. Known scientifically as photosynthetically active radiation (PAR), radiation in this range is about the same as what is visible to the human eye, and inevitably, sun light will lose some of its radiation as it passes through glazing into a greenhouse. Shading from the actual greenhouse frame, benches, and shelving can also reduce the amount of light that reaches plants by as much as 30 percent.
In addition to light transmission, several other variables should be carefully considered when selecting greenhouse glazing:
Maintenance : how easy is it to clean dirt, leaves, bird droppings, etc. from the glazing material. How difficult is it to repair damaged glazing?
Life Span: How many years will the glazing material perform at an optimum level before it must be replaced?
Structural Value: How strong is the glazing material, and how substantial must the greenhouse frame be in order to support it?
Insulation(R – Value): How effective is the glazing material at retaining heat? R value is a measure of a material’s thermal resistance, and in theory, the higher the R-Value number, the better the material’s insulation value.
Degradation: How quickly will the glazing material break down and lose performance from UV exposure?
GLASS
The first and only greenhouse glazing option for many people, glass has many advantages. It’s plentiful, strong, long lasting, and has high R-value and as much as 90% light transmission. It comes in different thicknesses, and is made with high performance coatings that allow even more control of the conditions inside the greenhouse. It’s the ideal glazing in so many ways, but it does have a couple of drawbacks that should be considered. Glass is fragile, and sometimes a broken pane is unavoidable. At the very least this can make for a tedious morning spent picking pieces of glass out of garden bed or gravel floor, and worst this can post a significant risk to personal safety. Another consideration for glass glazing is weight. Single pane glass isn’t especially heavy, but it adds up quickly when building with double or triple pan high performance glass sheets, requiring careful consideration of the strength of the frame design.
Single Pane Glass (R-Value 2-8)
This plentiful and inexpensive glazing material offers about 90% light transmission, but should be handled with care and considered carefully along with other, less fragile(dangerous) glazing options. While 3/32” and 1/8” panes are commonly used, investing in thicker, ¼” single pane glass will greatly increase impact resistance. Remember that with thicker glass, of course, comes more weight, requiring a more substantial greenhouse frame capable of supporting the heavier load.
Laminated Glass
Made by sandwiching a layer of plastic or resin between two layers of glass. It’s frequently called safety glass or shatterproof glass because the plastic layer holds fragments together if the pane is shattered.
Tempered Glass
Created by controlling the cooling phase of production, tempered glass can resist about four times the impact of regular glass. In the event that it does break, tempered glass shatters into tiny pieces, called dicing. While it’s a tedious process to clean up a myriad of tiny bits of glass, the danger of being seriously hurt by a large shard falling from overhead is dramatically reduced.
Double and Triple Pane Glass
Two or three layers of glass with an air space in between them are held apart by a special, moisture absorbing beading. While R-value is improved, light transmission is substantially reduced with double or triple paned glass glazing. Eventually the seals between panes will break down allowing moisture between the panes, producing a clouded effect and further reducing light transmission.
Low E and MC Low E Glass
Frequently used for solariums (glazed roofs) and sun rooms ( solid roofs), tempered low emission, or low e glass is available as an upgrade by some high end greenhouse manufacturers. It allows heat to enter the greenhouse, helps retain heat inside, and also blocks UV radiation from the sun. Tempered Low e and MC low e glass can reduce light transmission to as low as 65%, also blocking much of the light spectrum that plants need for vigorous growth, making it a better choice for a solarium application than a greenhouse.
FIBERGLASS (R-Value 1-4)
Available in flat or corrugated sheets, fiberglass is flexible and strong and delivers about 90% light transmission. It’s inexpensive, lightweight, easy to install, and impact resistant. Besides a more homely appearance than other glazing material, however, fiberglass tends to yellow with age, and is highly flammable. Expect about 8-12 years of useful life before it will have to be replaced.
ACRYLIC (R-Value 2-6)
Often called by the trade name Plexiglass, Acrylic has some key advantages in greenhouse glazing applications. Double walled acrylic sheets have excellent insulation value, and yield about 85% light transmission. Acrylic sheets are structurally stronger than fiberglass, and are easier to clean and maintain. The useful lifespan is similar to fiberglass, though : about 8-12 years. Keep in mind that cheaper acrylic material will quickly yellow and cloud from UV exposure. Premium acrylic panels designed for use in greenhouses are often treated with a UV resistant coating and come with a 10 year warranty against discoloration. The nature of acrylic material makes it easy to cut, and bend into custom shapes with special heating tools, but these same properties also mean it is also easily scratched and cracked. Use care when drilling, and leave protective films in place until installation is complete.
POLYCARBONATE (R-Value 3-6)
Like acrylic, polycarbonate is sold in twin wall sheets specially made for greenhouse applications. An exterior coating that blocks UV light helps prevent discoloration and the sheets typically come with a 10 year warranty against fading. Often the material will perform for as long as 15 years however, and polycarbonate is very shatter resistant, making it a safer choice where kids or seniors may be present. It has excellent impact resistant to damage from objects like hail stones, tree branches, acorns, etc. Twin wall polycarbonate Sheets are sold in 48” and 72” widths, in thicknesses from 6mil to 12mil or higher. Although polycarbonate is a lightweight material easily handled by one person, it has significant structural value allowing for greater spans between rafters and reducing sun light exposure lost to shadows.
POLYETHELENE & PVC SHEETING (R-Value 1-3)
Both inexpensive and easy to install, PE and PVC sheeting also offer excellent light transmission, and are still the greenhouse glazing of choice for professional nursery growers. With a useful lifespan of just a few years, and low insulation value, plastic sheeting is not a long term glazing solution for the typical home grower, and despite even the most meticulous installation few would consider it an attractive enhancement to a residential backyard landscape.
Reader Comments (2)
on your ref to polycarbonate panels..it should be mm not mil (mil is .001)
There are plenty of various Greenhouse glazing Components that are available.Most budget mini greenhouses are clad in twin wall polycarbonate or very thin acrylic or polythene sheets.Twin wall polycarbonate does provide some rigidity and insulation, but prevents you easily viewing your plants.