Greenhouse Covers

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Greenhouse Covers

• By
• Jay Holcomb

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Glass-Float Glass

• Greenhouse use double strength glass
• Trend toward larger panes
• Light transmission 88-90
• Bar capped for low maintenance

• Initial expense-1.50-2.50 per sq ft
• Easy to break
• Not very energy efficient
• No bar caps mean frequent painting and puttying

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Glass--Tempered Glass

• 5 Times stronger than float glass
• Can use larger pieces
• Heavy so must be supported on 4 sides
• More expensive than float glass
• Flat glass can be bent to increase strength

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Glass--Low-iron glass

• Iron is removed from glass so you get 5-6 more
  light transmission
• Can be double strength glass
• Can cost 4.00 or more per sq ft.

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Plastic--Fiberglass

• Made from glass fibers imbedded in acrylic resin
• Light transmission 78-88 when new
• Resin degraded by UV light
• Estimated life is 10-15 years

• Cost similar to glass but comes in larger
  pieces-installation less
• Light through FRP is diffuse
• FRP does not shatter with hail or stones, but
  will burn rapidly under some conditions

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Plastic--Fiberglass-2

• Longer life if Tedlar coated. Tedlar can
  delaminate
• Install so condensate can flow to gutter
• Grades of FRP from 4 to 8 oz (8 oz is best)
• Cost 1.00-2.00 / sq ft

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Fiberglass-Weatherability

• UV breaks down polyester resin
• Oxidation of components
• Surface erosion
• Thermodegradation of FRP

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Plastic--Acrylic

• Usually purchased as structured sheets
• Light transmission 83-87
• Acrylics do not discolor with age but do get more
  brittle
• Life expectancy can be 20 years or more

• Maximum width is 4 feet as it is for FRP
• Acrylic has a high expansion coefficient so must
  use special hardware
• Cost is about 3.00-4.00 /sqft
• Fairly flammable

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Plastic--Acrylic-2

• One commercial product is Exolite
• 16 mm for straight and 8 mm for curved
• Condensation can reduce light and drip on
  plants--use Exolite No Drip

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Plastic--Polycarbonate

• Structured sheets with widths of 2-6 and 1-3
  layers
• Light transmission of 2 layers 75-85
• Polycarbonate is not very flammable
• Flexible for Quonset shaped greenhouse

• Can yellow over time
• Life of uncoated polycarbonate is 8 years
• Acrylic layer can be bonded to polycarboante so
  it will not yellow as fast
• Cost about 2.00/sqft

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Plastic-Polyethylene

• Film fits over most greenhouse shapes
• Light Transmission 88-89 for single and 81 for
  double poly
• Width of poly to 40
• Transparent to longwave radiation

• Generally use double layer for heat retention
• Poly broken down by UV light
• Can have UV stabilized or UV inhibited poly
• Life is 1 to 3 years occasionally 4 years

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Plastic-Polyethylene-2

• The cost for a 3 year 6 mil poly woud be about
  0.07 /sqft
• Replace cover every 1 to 3 years
• Installation may cost 0.25/sqft each time the
  greenhouse is covered

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Plastic-Polyethylene-3

• IR Reflective film
• Reflects longwave radiation back toward plant
  saves heat
• Can get anti-condensate film and some colored
  films