1. Greenhouse Management
Greenhouse Glazings
Greenhouse Management

2. Greenhouse Glazings
The greenhouse glazing is the “skin” or covering of the greenhouse.
There are 4 basic types of glazings:
Glass
Rigid Plastics
Semi-rigid Plastics
Plastic Films

3. Light Transmittance
Light transmittance is the most important property of a glazing.
No glazing material transmits 100% of the light that strikes it.

4. An Even-span Greenhouse
Light Transmittance
Light striking a greenhouse glazing has 3 fates:
Some of it will be reflected by the glazing.
Some of it will be absorbed by the glazing
The rest of it will be transmitted through the glazing.
An Even-span Greenhouse

5. Glass Prior to the 1950’s, all greenhouses were glazed with glass.
Longevity: 25+ years.
Advantages:
Best light transmittance; approximately 90%.
Not affected by UV radiation from the sun. Because of this glass is long-lived.
Not subject to temperature related expansion and contraction.

6. Glass Disadvantages: Moderately expensive. Breaks easily.
Because of it’s weight it requires more support. The additional framing materials add to the cost of the greenhouse and create shade.
Low heat retention.

7. Glass Additional comments:
Available as a thermopane, although most new construction is single pane.

8. Cross-sectional view of a rigid plastic
Rigid Plastics
Most of the rigid plastics used on greenhouses today are multi-walled.
The 2 most common rigid plastics are acrylic and polycarbonate.
2 layers
ribs
Cross-sectional view of a rigid plastic

9. Acrylic

10. Polycarbonate

11. Rigid Plastics Longevity: 20 – 30 years. Advantages:
Good light transmittance; approximately 80-83%.
Do not break easily.
Lightweight compared to glass.
High heat retention.
Uses 50-60% less fuel for heating than a glass greenhouse.

12. Rigid Plastics Disadvantages: Most expensive.
Subject to temperature related expansion and contraction.
While polycarbonate glazings are flame resistant, acrylic glazings are flammable.

13. Semi-rigid Plastics
FRP, fiberglass reinforced plastic, consists of glass fibers embedded in acrylics.
It is available in flat sheets or corrugated sheets. Corrugated sheets are stronger.
Corrugated Sheet

14. Corrugated FRP

15. Semi-rigid Plastics Longevity: 10 – 15 years. Advantages:
Suitable for even-span and quonset greenhouses.
Good light transmittance in the first 3-5 years; approximately 88%.
Does not break easily.
Moderately expensive.
Lightweight compared to glass.

16. Semi-rigid Plastics Disadvantages:
Requires a surface treatment/cleaning every 5 years to increase longevity.
Medium heat retention.

17. Quonset style house glazed with plastic film
Plastic Films
Most of the plastic films used today are made from polyethylene.
Two sheets are used. An inflation motor blows air between the sheets and “inflates” them.
Quonset style house glazed with plastic film

18. Plastic (polyethylene) Film

19. Inflated Double-Polyethylene Film

20. Plastic Films
Plastic films are slowly permeable to oxygen and carbon dioxide.
Not permeable to water.
Thickness of the sheets is measured in “mils”; one mil is one thousandth of an inch.
Standard sizes are readily available, or custom sizes can be ordered.

21. Plastic Films Longevity: 3 – 4 years. Advantages:
Very lightweight compared to glass.
Conforms to trusses and bows of all sizes.
Inexpensive and easy to install.
Double-poly has high heat retention.

22. Plastic Films Disadvantages:
Damaged by UV radiation and consequently needs to be replaced on a regular basis.
Reduced light transmittance, especially through double-poly; approximately 84%.

23. The End