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

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

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 –Energyair leaks –2 layers ? –Reglazing every yrs

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

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 1) Polyvinyl chloride CH 2 = CH – Cl 2) Polyvinyl acetate CH 2 = CH - OCCH yrs non UV resistant attracts dirt

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

3. Rigid plastics a. Polyvinyl chloride – CH 2 = CH - Cl corrugated 4-5 yrs with UV inhibitors more expensive than polyethylene

b.Fiberglass reinforced plastic (FRP) -C-O-C-O-CH 2 CH 2 -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

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

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

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

C. Comparison - coverings

Plant Growth 1. Light transmission a.quality All allow nanometers

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

obstructions

2.Heating a.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

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 tight1.00 Fairly tight1.13 Loose1.25 FRP covered wood houses.95 FRP covered metal houses1.00 Double glazing with 1. air space.70 Plastic covered metal houses (single thickness) 1.00 Plastic covered metal houses (double thickness) 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.

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

b. Conduction & Radiation Heat transfer coefficient –BTU / hr / ft 2 / 1 0 F temp. differential 1 layer same for all materials 2 layers 40% energy savings –polyethylene –polycarbonate –Acrylite

Heat transfer through Transparent coverings * BTU/ hr / ft 2 / 1 0 F

c. Thermal radiation (radiant energy) loss low –Glass, fiberglass, acrylic, polycarbonate high –polyethylene –condensation reduces losses Film typeThermal transmittance % Single glazing Glass4.4 Polyethylene70.8 Polyvinyl Chloride12 Fiberglass1 Acrylic Polycarbonate

New film blocks thermal radiation loss New films also reduce dripping

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

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 Older method New systems Polylock

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

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