Greenhouse shapes and structures (continued)
Reflective wall greenhouse
· Lower 48 and N temperate regions
· Insulated/reflective N wall
· Wall angles - maximum winter light
o No shading in the summer
· Winter light max can exceed outdoors
· Energy savings up to 25%
· Best in sunny regions
· Higher cost
· Structure not as strong
· Won't work well in Alaska--sun angles are too low
Alaskan favorites
· Detached, freestanding
o Quonset
o Even span gable end
o Gothic arch -- all year houses
· Ridge and furrow
o Mostly in south central Alaska
o Less snow fall
o Natural gas available for heating
Greenhouse
frameworks
· Characteristic of good framing material
o Light weight
o Strong
o Durable
o Casts little shadow
o Cheap
· Most common frame materials today
o Wood
o Wood & iron (semi iron
o Iron (steel) alone
o Steel and aluminum
o Aluminum alone
Wood frames
o Easily fabricated
o Whatever is available locally
o Considerable blocking of sunlight (shading)
o Plans commonly available
o Must be able to prevent rot--2 common methods:
o Use decay resistant wood such as (unfortunately all these are expensive!):
§ Black locust
§ Osage orange
§ Red-cedar
§ Redwood
§ White oad
§ Cypress
o Chemically treat the wood
§ Creosote
· May cause toxicity for more than 2 years
§ Pentacholorophenol ('penta')
· May cause toxicity for more than 2 years
§ Best wood preservatives for greenhouse work (these double the life of the wood they are put on):
· Copper napthanate in 10% fuel oil (which evaporates)
· Zinc napthanate ('Cuprinol')
§ Avoid oil-based preservatives
· the fumes may decay plastics commonly used in the greenhouse & greenhouse coverings
o Pressure treated wood
§ Process called Womanizing
§ Chromated copper arsenate
· (Osmose K-33, Green salts)
§ ammonical copper arsenite (chemonite)
§ fluorchrome arsenate phenol (Tanalith, Wolman, Osmosalts)
§ Will last 20-30 years
§ May kill plant tissue immediately after treatment
§ Slow leaching of salts into ground water
§ Concern with ground beds due to poisons leaching into them
o Wood contributes to a uniform greenhouse temperature by absorbing heat (giving off cold) when it is hot, and radiating heat when it is cold.
o Must be painted
§ Use water based paints--white preferably
§ Add fungicide
Semi-iron pipe framing
o Iron or steel used for support structure
o Wood used to hold glazing (glass)
o Has characteristics of both wood and iron
o Common in hobby greenhouses
o Must be painted or galvanized
o Acids used to clean glass may be corrosive to steel
Aluminum and its alloys
o Cheap
o Lightweight
o Durable
o Can be formed into any shape or size
o Common in prefabricated units
o Minimum shading
o Greater heat loss than wood or semi-iron
Greenhouse
coverings
Glass
o Most transparent of all coverings
o Longest life
o Weighs a lot
o One of the higher priced coverings
o Easily broken
o High replacement costs
o Common source in Alaska: old window sashes
Plastic
A. Flexible = non-rigid = sheets of film
o Polyethylene film
o First type used
o Cheap, good for seasonal greenhouses
o Most common covering in US
o Best thickness is 6 mil
§ If you have a double layer, use 4 mil inside and 4-6 mil on the outside
o Get rolls with the fewest folds; it usually fails along crease marks
o Sensitive to UV and ultraviolet
§ Yellow, brittle in one season
§ Must be treated with UV stabilizer
o 1.5 mil -- very thin, extra clear, hard to find
o Double layers:
§ Energy efficiency
§ Squirrel cage fan blows air between 2 sealed sheets
o Lower NRG costs
o Higher relative humidity
o Lower light intensity
o Can have mixed layers
§ Rigid plastic on inside
§ Sheet of poly on outside
o Can get condensation between layers
§ Blocks out light
§ Algae growth, which blocks more light and provides 'spores' for infecting your soils
B. Rigid Plastic
o FRP: fiberglass reinforced plastic
o Glass threads embedded into plastic resin
o Cheap
o Available locally
o Light weight
o Cheap stuff must be replaced every 10 years
o Problems
§ Threads wear through (resin wears off)
§ Surface becomes pitted
o Collects dirt & blocks sun
o Very difficult to clean
o May be re-coated with resin
§ Turns yellower and yellower if not UV stabilized
§ Shorter lifespan in Alaska due to long sunlight hours in the summer
o Polycarbonate plastic
o 2 layers, 1/4 inch apart
o Ribs spaced about 1/4 inch apart
o Energy efficient
o Good light transmissivity
o Types available (samples handed around in class):
§ Polygal
§ Acrylite by Cyro
§ Exolite by Cyro
§ Cryoflex by Cyro
§ Lexan
§ IBG Acrycel
o Very expensive
o Lifespan 10+/- years
o May crack if not installed properly
§ Expands and contracts with temperature changes
o Condensation may occur between sheets…and algae growth
o Acrylic plastics
o Similar to polycargonates, and somewhat more expensive than they are
o Very expensive
o Much longer lived
o Excellent light transmissivity
o Lighter weight than glass
o Can't see through it
o Highly flamible
National Greenhouse Manufacturer's Association
NGMA standards:
o NGMA
o P.O. Box 567
o Pana IL 62557
Outstanding info from:
Aldrich Ra and J.W. Bartok. Greenhouse engineering Department of agriculture
o Engineering U of Conn
o Storrs, CT 06268
Also see:
o Connecticut Greenhouse Newsletter
o Department of Horticulture at the University of Connecticut
Handouts
o Comparison of different greenhouse glazing materials
o Details of an air inflated polyethylene covered greenhouse
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