VI. GREENHOUSE COVERINGS

A. Selection - factors to consider

1. Photosynthesis–Transmission vs plant reception

2. light quality

–400-800 nanometers

3. durability–Initial vs long term

4. Initial & maintenance cost

5. energy savings

–1 layer vs 2 layers

B. Covering types1. 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

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%

• 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

CH2 = CH – Cl

2) Polyvinyl acetate

CH2 = CH - OCCH3 - 0

• 4-5 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 –CH2 = CH - Cl

• corrugated

• 4-5 yrs with UV inhibitors

• more expensive than polyethylene

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

c. Acrylic profiled sheet

• transmission 80%

• Energy savings: 40% over 1 layer glass

• Strong structure

• Expensive

d. Polycarbonate profiled sheettransmission 80%UV inhibitors increases life

e. Polycarbonate corrugated paneltransmission 90-92%

4. New developments

–inert gas between layers of glass

–Chemical solutions in rigid plastic channels

C. Comparison - coverings

Plant Growth

1. Light transmissiona. quality

All allow 400-800 nanometers

b. Transmission

1 layer 90%;

2 layers 80%

direct vs diffused

obstructions

2. Heatinga. 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) 1.08Wood & 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.

Polyethylenedouble layer

Glass

CO2

CO2

CO2 CO2CO2 CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

Air with. > CO2

Air depletedCO2

Air with. > CO2Air with. < CO2

Air with. < CO2

Air depletedCO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2 CO2

CO2CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2

CO2CO2

CO2

CO2

CO2

CO2

CO2

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

Heat transfer through Transparent coverings

* BTU/ hr / ft2 / 10 F

c. Thermal radiation (radiant energy) loss• low

– Glass, fiberglass, acrylic, polycarbonate• high

– polyethylene– condensation reduces losses

Film type Thermal transmittance % Single glazing

Glass 4.4

Polyethylene 70.8

Polyvinyl Chloride 12

Fiberglass 1

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. Installationa. 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 systemsPolylock

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

D. Air inflation management– Replace leaked air

– Source of air leak

• Gaps in locking system

• Puncture • Nails, Splinter, Metal frame