Information on greenhouse glazing
(from http://attra.ncat.org/attra-pub/solar-gh.html)
You need to understand four numbers when selecting glazing for solar greenhouses. Two numbers describe the heat efficiency of the glazing, and the other two numbers are important for productive plant growth. Many glazing materials include a National Fenestration Rating Council sticker that lists the following factors:
• The SHGC or solar heat gain coefficient is a measure of the amount of sunlight that passes through a glazing material. A number of 0.60 or higher is desired.
• The R Value is the measure of heat loss through a wall or glazing. The higher the R value the lower the rate of heat loss. the U Factor is the inverse of the R Value.
• VT or visible transmittance refers to the amount of visible light that enters through a glazing material. A number of 0.70 or greater is desired.
• PAR or photosynthetically active radiation is the amount of sunlight in the wavelengths critical for photosynthesis and healthy plant growth. PAR wavelength range is 400-700 nanometers (a measure of wavelength). Note : When choosing glazing, look at the total visual transmittance, not PAR transmittance, to see whether the material allows the spectrum of light necessary for healthy plant growth.
In addition to energy efficiency and light transmission, you should consider the following when choosing glazing materials for your greenhouse:
• Lifespan
• Resistance to damage from hail and rocks
• Ability to support snowload
• Resistance to condensation
• Sheet size and distance required between supports
• Fire-resistance
• Ease of installation
Glazing Characteristics
Glass—single layer
Light transmission*: 85-90%
R-value**: 0.9
Advantages:
• Lifespan indefinite if not broken
• Tempered glass is stronger and requires fewer support bars
Disadvantages:
• Fragile, easily broken
• May not withstand weight of snow
• Requires numerous supports
• Clear glass does not diffuse light
Factory sealed double glass
Light transmission*: 70-75%
R-value**: double layer 1.5–2.0, low-e 2.5
Advantages: • Lifespan indefinite if not broken
• Can be used in areas with freezing temperatures
Disadvantages :
• Heavy
• Clear glass does not diffuse light
• Difficult to install, requires precise framing
Polyethylene—single layer
Light transmission*: 80-90% - new material
R-value**: single film 0.87
Advantages:
• IR films have treatment to reduce heat loss
• No-drop films are treated to resist condensation
• Treatment with ethyl vinyl acetate results in resistance to cracking in the cold and tearing
• Easy to install, precise framing not required
• Lowest cost glazing material
Disadvantages:
• Easily torn
• Cannot see through
• UV-resistant polyethylene lasts only 1–2 years
• Light transmission decreases over time
• Expand and sag in warm weather, then shrink in cold weather
Polyethylene—double layer
Light transmission*: 60-80%
R-value** double films: 5ml film 1.5, 6ml film 1.7
Advantages: • Heat loss significantly reduced when a blower is used to provide an air space between the two layers
• IR films have treatment to reduce heat loss
• No-drop films are treated to resist condensation
• Treatment with ethyl vinyl acetate results in resistance to cracking in the cold to tearing
• Easy to install, precise framing not required
• Lowest-cost glazing material
Disadvantages:
• Easily torn
• Cannot see through
• UV-resistant polyethylene lasts only 1–2 years
• Light transmission decreases over time
• Expand and sag in warm weather, then shrink in cold weather
Polyethylene—corrugated high density
Light transmission*: 70-75%
R-value**: 2.5-3.0
Advantages:
• Mildew, chemical, and water resistant
• Does not yellow
Disadvantages:
n/a
Laminated Acrylic/Polyester film—double layer
Light transmission*: 87%
R-value**: 180%
Advantages:
• Combines weatherability of acrylic with high service temperature of polyester
• Can last 10 years or more
Disadvantages:
• Arcrylic glazings expand and contract considerably; framing needs to allow for this change in size
• Not fire-resistant
Impact modified acrylic—double layer
Light transmission*: 85%
Advantages:
• Not degraded or discolored by UV light
• High impact strength, good for locations with hail
Disadvantages:
• Arcrylic glazings expand and contract considerably; framing needs to allow for this change in size
• Not fire resistant
Fiber reinforced plastic (FRP)
Light transmission*: 85-90% - new material
R-value**: single layer 0.83
Advantages:
• The translucent nature of this material diffuses and distributes light evenly
• Tedlar-treated panels are resistant to weather, sunlight, and acids
• Can last 5 to 20 years
Disadvantages:
• Light transmission decreases over time
• Poor weather-resistance
• Most flammable of the rigid glazing materials
• Insulation ability does not cause snow to melt
Polycarbonate—double wall rigid plastic
Light transmission*: 83%
R-value**: 6mm 1.6, 8mm 1.7
Advantages: • Most fire-resistant of plastic glazing materials
• UV-resistant
• Very strong
• Lightweight
• Easy to cut and install
• Provides good performance for 7-10 years
Disadvantages:
• Can be expensive
• Not clear, translucent
Polycarbonate film—triple and quad wall rigid plastic
Light transmission*: 75%
R-value** triple walls: 8mm 2.0–2.1, 16mm 2.5
R-value** quad wall: 6mm 1.8, 8 mm 2.1
Advantages: • Most fire-resistant of plastic glazing materials
• UV-resistant
• Very strong
• Lightweight
• Easy to cut and install
• Provides good performance for 7-10 years
Disadvantages:
• Can be expensive
• Not clear, translucent
Sources: (2, 6, 7, 13, 14)
* note that framing decreases the amount of light that can pass through and be available as solar energy
** R-Value is a common measure of insulation (hr°Fsq.ft/BTU)
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You need to understand four numbers when selecting glazing for solar greenhouses. Two numbers describe the heat efficiency of the glazing, and the other two numbers are important for productive plant growth. Many glazing materials include a National Fenestration Rating Council sticker that lists the following factors:
• The SHGC or solar heat gain coefficient is a measure of the amount of sunlight that passes through a glazing material. A number of 0.60 or higher is desired.
• The R Value is the measure of heat loss through a wall or glazing. The higher the R value the lower the rate of heat loss. the U Factor is the inverse of the R Value.
• VT or visible transmittance refers to the amount of visible light that enters through a glazing material. A number of 0.70 or greater is desired.
• PAR or photosynthetically active radiation is the amount of sunlight in the wavelengths critical for photosynthesis and healthy plant growth. PAR wavelength range is 400-700 nanometers (a measure of wavelength). Note : When choosing glazing, look at the total visual transmittance, not PAR transmittance, to see whether the material allows the spectrum of light necessary for healthy plant growth.
In addition to energy efficiency and light transmission, you should consider the following when choosing glazing materials for your greenhouse:
• Lifespan
• Resistance to damage from hail and rocks
• Ability to support snowload
• Resistance to condensation
• Sheet size and distance required between supports
• Fire-resistance
• Ease of installation
Glazing Characteristics
Glass—single layer
Light transmission*: 85-90%
R-value**: 0.9
Advantages:
• Lifespan indefinite if not broken
• Tempered glass is stronger and requires fewer support bars
Disadvantages:
• Fragile, easily broken
• May not withstand weight of snow
• Requires numerous supports
• Clear glass does not diffuse light
Factory sealed double glass
Light transmission*: 70-75%
R-value**: double layer 1.5–2.0, low-e 2.5
Advantages: • Lifespan indefinite if not broken
• Can be used in areas with freezing temperatures
Disadvantages :
• Heavy
• Clear glass does not diffuse light
• Difficult to install, requires precise framing
Polyethylene—single layer
Light transmission*: 80-90% - new material
R-value**: single film 0.87
Advantages:
• IR films have treatment to reduce heat loss
• No-drop films are treated to resist condensation
• Treatment with ethyl vinyl acetate results in resistance to cracking in the cold and tearing
• Easy to install, precise framing not required
• Lowest cost glazing material
Disadvantages:
• Easily torn
• Cannot see through
• UV-resistant polyethylene lasts only 1–2 years
• Light transmission decreases over time
• Expand and sag in warm weather, then shrink in cold weather
Polyethylene—double layer
Light transmission*: 60-80%
R-value** double films: 5ml film 1.5, 6ml film 1.7
Advantages: • Heat loss significantly reduced when a blower is used to provide an air space between the two layers
• IR films have treatment to reduce heat loss
• No-drop films are treated to resist condensation
• Treatment with ethyl vinyl acetate results in resistance to cracking in the cold to tearing
• Easy to install, precise framing not required
• Lowest-cost glazing material
Disadvantages:
• Easily torn
• Cannot see through
• UV-resistant polyethylene lasts only 1–2 years
• Light transmission decreases over time
• Expand and sag in warm weather, then shrink in cold weather
Polyethylene—corrugated high density
Light transmission*: 70-75%
R-value**: 2.5-3.0
Advantages:
• Mildew, chemical, and water resistant
• Does not yellow
Disadvantages:
n/a
Laminated Acrylic/Polyester film—double layer
Light transmission*: 87%
R-value**: 180%
Advantages:
• Combines weatherability of acrylic with high service temperature of polyester
• Can last 10 years or more
Disadvantages:
• Arcrylic glazings expand and contract considerably; framing needs to allow for this change in size
• Not fire-resistant
Impact modified acrylic—double layer
Light transmission*: 85%
Advantages:
• Not degraded or discolored by UV light
• High impact strength, good for locations with hail
Disadvantages:
• Arcrylic glazings expand and contract considerably; framing needs to allow for this change in size
• Not fire resistant
Fiber reinforced plastic (FRP)
Light transmission*: 85-90% - new material
R-value**: single layer 0.83
Advantages:
• The translucent nature of this material diffuses and distributes light evenly
• Tedlar-treated panels are resistant to weather, sunlight, and acids
• Can last 5 to 20 years
Disadvantages:
• Light transmission decreases over time
• Poor weather-resistance
• Most flammable of the rigid glazing materials
• Insulation ability does not cause snow to melt
Polycarbonate—double wall rigid plastic
Light transmission*: 83%
R-value**: 6mm 1.6, 8mm 1.7
Advantages: • Most fire-resistant of plastic glazing materials
• UV-resistant
• Very strong
• Lightweight
• Easy to cut and install
• Provides good performance for 7-10 years
Disadvantages:
• Can be expensive
• Not clear, translucent
Polycarbonate film—triple and quad wall rigid plastic
Light transmission*: 75%
R-value** triple walls: 8mm 2.0–2.1, 16mm 2.5
R-value** quad wall: 6mm 1.8, 8 mm 2.1
Advantages: • Most fire-resistant of plastic glazing materials
• UV-resistant
• Very strong
• Lightweight
• Easy to cut and install
• Provides good performance for 7-10 years
Disadvantages:
• Can be expensive
• Not clear, translucent
Sources: (2, 6, 7, 13, 14)
* note that framing decreases the amount of light that can pass through and be available as solar energy
** R-Value is a common measure of insulation (hr°Fsq.ft/BTU)
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