07 cover

VI. GREENHOUSE COVERINGS: 

VI. GREENHOUSE COVERINGS A. Selection - factors to consider 1. Photosynthesis Transmission vs plant reception 2. light quality 400-800 nanometers

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3. durability Initial vs long term 4. Initial & maintenance cost 5. energy savings 1 layer vs 2 layers

B. Covering types: 

B. Covering types 1. Glass 40 years; high cost transmission 90%-97% Size 18" x 18", 24-39" wide x up to 65" long Frame: usually aluminum; galvanized iron, wood low maintenance Energy air leaks 2 layers ? Reglazing every 15-20 yrs

2. Plastic film: 

2. Plastic film a. Polyethylene CH2 = CH2 short life 2-4 years deterioration- UV light,O2 and heat Prevention: UV inhibitors anti-oxidants eliminate black surfaces Transmission 1 layer 90% 2 layers 80-83%

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structure light weight aluminum or steel Loss of heat: I.R. radiation loss high Condensation Tight house, little air exchange a. Polyethylene, contu.

b. Vinyl: 

b. Vinyl 1) Polyvinyl chloride CH2 = CH – Cl 2) Polyvinyl acetate CH2 = CH - OCCH3 - 0 4-5 yrs non UV resistant attracts dirt

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Polyvinyl fluoride (tedlar) CH2 = CH - F 10-15 yrs stretched over frame

3. Rigid plastics: 

3. Rigid plastics Polyvinyl chloride – CH2 = CH - Cl corrugated 4-5 yrs with UV inhibitors more expensive than polyethylene

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b. Fiberglass reinforced plastic (FRP) -C-O-C-O-CH2 CH2-O l l l l 0 0 corrugated panel transmission 90-92% surface may degrade treated with tedlar 5-6 yrs; 15 with tedlar light transmission scattered

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c. Acrylic profiled sheet transmission 80% Energy savings: 40% over 1 layer glass Strong structure Expensive

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d. Polycarbonate profiled sheet transmission 80% UV inhibitors increases life e. Polycarbonate corrugated panel transmission 90-92%

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4. New developments inert gas between layers of glass Chemical solutions in rigid plastic channels

C. Comparison - coverings: 

C. Comparison - coverings

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Plant Growth 1. Light transmission a. quality All allow 400-800 nanometers

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b. Transmission 1 layer 90%; 2 layers 80% direct vs diffused

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obstructions

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Heating Tight vs loose Polyethylene, fiberglass, acrylics and polycarbonates .5-1 air exchange per hour Glass .5-2 air exchange per hour 2 air exchanges/ hour 10-15% of energy infiltration through cracks, vents, doors etc. Greater heat loss

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Greenhouse Construction Factors, C, for the Common Types of Greenhouses in Use Today All metal (good tight glass house -20 or 24 in. glass spacing) 1.08 Wood & steel (good tight glass house -16 or 20 in. glass spacing) (Metal gutters, vents, headers. etc.) 1.05 Wood houses (glass houses with wood bars, gutters, vents, etc.- up to and including 20 in. glass spacing) Good tight 1.00 Fairly tight 1.13 Loose 1.25 FRP covered wood houses .95 FRP covered metal houses 1.00 Double glazing with 1. air space .70 Plastic covered metal houses (single thickness) 1.00 Plastic covered metal houses (double thickness) .70 -------------------------------------------------------------------------------------------------------- Standard heat loss values for transparent components of greenhouses such as gables and roofs transparent side walls and ends as well as covering are multiplied by a factor (C) to correct them for the type of construction.

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Polyethylene double layer Glass CO2 CO2 CO2 CO2 CO2 CO2 CO2 Air with. > CO2 Air depleted CO2 Air with. > CO2 Air with. < CO2 Air with. < CO2 Air depleted CO2

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b. Conduction & Radiation Heat transfer coefficient BTU / hr / ft2 / 10 F temp. differential 1 layer same for all materials 2 layers 40% energy savings polyethylene polycarbonate Acrylite

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Heat transfer through Transparent coverings * BTU/ hr / ft2 / 10 F

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c. Thermal radiation (radiant energy) loss low Glass, fiberglass, acrylic, polycarbonate high polyethylene condensation reduces losses

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New film blocks thermal radiation loss New films also reduce dripping

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D. Air Inflated Double Layer Plastic 1. Attachment 2 layers of polyethylene Air inflated with small 1/10 hp fan Air tight Ideally like a balloon 2. Purpose 40% less energy cost 3. Principles Air tight Ideally like a balloon Create dead air space Static air Reduce heat transfer

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4. Installation a. calm, cool day b. tightness expansion and contraction warm day - too loose cold day - too tight c. Inflate with outside air Principles % Relative humidity Dew point

Attachment of polyethylene to frame: 

Attachment of polyethylene to frame Older method New systems Polylock

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d. Space between layers .75 - 4" ideal; least convection 4 - 18" greater convection e. Inflation pressure .2 - .5 water column greater in high wind deflate in snow storm Reduce pressure by closing vent

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D. Air inflation management Replace leaked air Source of air leak Gaps in locking system Puncture Nails, Splinter, Metal frame