46
Table 11: Comparison of greenhouse glazing materials.
| Material | Life | R value | U value | Advantages | Disadvantages | Cost |
|---|
| Glass Single layer, tempered | (Until it breaks) | 0.95 | 1.13 | Strong, attractive, good seal, 90% light penetration | Breaks, difficult to install, heating | $ $\n- |
|---|
| Rigid plastics Fiberglass | 6-15 years | 0.83 | 1.20 | Lightweight, strong, light penetration | Opaque, degrade/yellow over time-6yrs, needs resin recoat | - - |
|---|
| Rigid single-wall | | | | | | - - |
|---|
| Rigid double-wall acrylic | 20 yr | 1.4-1.9 | 0.75-1.0 | Transparent, 30% energy savings with double layer, Bends | Extra layer reduces light penetration to about 80% | - |
|---|
| Rigid double or triple wall polycarbonate with UV coating | 10-15 yr | 1.4-1.9 Triple 2.5 | 0.53-0.70 | Same as acrylic, may bend more easily than acrylic | Same light as acrylic, but will yellow without UV coating, about 80% | - |
|---|
| Plastic films (rolls) Single layer, 6 ml | 1-4 yr | .85 | 1.20 | Good light penetration (90% 1-layer) inexpensive | Needs to be replaced frequently, condensation | - |
|---|
| Double layer, 6 ml (polyethylene) | | 1.4 | 0.5-1.0 | Reduces condensation, increases warmth | Reduces light 10& with each layer (80% total) | - |
|---|
Heating: For small or backyard-size producers, implementing a passive heating system can help reduce heating costs during cold months. In this type of system, sunlight enters the south wall. The north wall has reflective material to trap and store heat. Black barrels filled with water absorb heat from sunlight during the day and slowly release the heat during the night. Thermal curtains can be hung on the south wall to trap heat during the night (Figure 26). While helpful to reduce heating costs, this practice would not be practical for large producers as it takes up valuable production space in the facility
and is not able to maintain a consistent and reliable temperature.
Larger producers that have year-round, consistent production will need to maintain a temperature independent of what can be gained from the sun. Forced air heaters powered by natural gas, propane, or electricity are most commonly used in the U.S. These heaters control the air temperature by a thermostat. Radiant heaters such as wood or natural gas broilers control the temperature by pumping hot water through pipes located throughout the structure. Broilers are popular, as wood is a cheap source of fuel compared to oil or natural gas.