HRT 221
Greenhouse Structures
and Management
John Biernbaum
Department of Horticulture
Topics for Week 4
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Review types of Structures
– Alternative or less common types
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Functional Features & Design
Criteria
Structural Materials
Greenhouse Coverings
– Selection criteria
– Materials
Alternative Greenhouses
Sawtooth
Only found in warm
climates without snow
like southern Florida.
Designed to capture
wind for cooling, to
reduce light and limit
rain/water on foliage
plants for indoor use.
Old greenhouse for botanical collection.
Near the San Francisco Bay Bridge.
Geodesic dome for botanical collection.
Other Types of
Greenhouses
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Gothic
Pit or in ground
Air inflated
Standardization
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National Greenhouse
Manufactures Association
http:/www.ngma.com
Published Guidelines
Not all Companies are members
Function: Strength
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Dead Load
– Structure and long term weight
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Live Load
– Short term weight (15 lbs/sq ft floor)
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Snow load
– 15 lbs/sq ft of floor minimum
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Wind load
– Lateral and lifting forces to resist 80 mph wind
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Foundation
– Vertical, horizontal and lifting forces.
Structure and Light
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Orientation covered previously
Roof slope
– 35 degrees preferred
– 26 to 28 (6:12 slope) degrees
common
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Minimize structural members to
minimize shading
– Also minimize equipment
Structure and Climate
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Condensation and snow removal
Pad/vent to fan distance (100200’)
Passive ventilation
– Vents, end walls, side walls
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Heating efficiency
– Surface area to floor area
– Infiltration or air leakage rate
Working Height
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Gutter height has increased from as
low as 8’ to 10 to 12’ as common and
14’ for some structures.
Impact of higher gutter:
– Improved working environment
– Greater air mass reduces rapid
temperature fluctuations
– Equipment movement in greenhouse
Floors
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Humidity Management
– Dry floors reduce humidity
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Crop movement by carts, etc
Prevention of runoff to groundwater
– Minimize application is best prevention
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Cost of Cement is an issue
Structural Materials
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Strength
Weight
Durability
Maintenance
Cost
– Initial
– Long term
Structural Materials
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Wood
– Availability, treatment, maintenance
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Steel – Galvanized
– Most common material (Gatorshield)
– One steel company supplies most of the
greenhouse manufacturing companies
– Avoid reactive chemicals – fertilizer
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Aluminum
– Light weight, strong, low maintenance
– High cost; used for glass.
Coverings:
Selection Criteria
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Cost
– Initial and annual
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Glass - $1.00 to $2.00/sq ft (surface) –
lasts 20+ years
Plastic – at $0.10/sq ft x 2 layers x 5
replacements in 20 years = $1.00/sq ft
+labor
Structure, heating and light
differences must also be considered.
Coverings:
Selection Criteria
Life Expectancy
– Photodegradation is the primary
breakdown of plastics
– Greenhouse plastics have UV or
ultraviolet stabilizers needed for
multiyear use
– Oxidation
– Surface erosion
Coverings:
Selection Criteria
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Strength and Weight
– Structural support weight varies
„ Dead
load
– Shattering – resistance to breakage
by rock, hail, etc
„ Crazing
– star like cracks at point of
impact reduces light diffusion
– Tensile strength – resistance to
tearing
Light Transmission:
Light
Reflected
Transmitted
Absorbed
heat energy
Indirect
or
Diffused
Direct
Definitions
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Transparent
Translucent
Opaque
Direct light
Diffuse light
Light Transmission
Amount passing through (inside)
X 100 = Percent Light Transmission
Amount of light at Surface (outside)
Thermal Qualities
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Expansion and contraction
– Plastic films from winter to summer
– Rigid plastic sheets
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Heat retention – conductance
Thermal radiation
– Infrared or long wave radiation
„ Glass
opaque, plastic file transparent
– Plastic films can have IR additives
Other Factors
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Flammability
Insurance
Shading – How is light reduced?
Taxes
Heating and Fuel Costs
Labor to change plastic film
Coverings
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Glass
Rigid Structured Sheets
– Fiberglass
– Polycarbonate
– Acrylic
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Films
– polyethylene
Types of Glass
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Float Glass
($0.60/sqft) (Nelson Prices)
– Single strength (homes) double for greenhouses
– Shatters into sharp shards
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Tempered Glass (($0.75/sqft)
– Stronger
– Shatters to small pieces
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Laminated tempered Glass ($4.00 sq ft)
Low iron tempered Glass ($0.85/sqft)
Hammered Glass – diffuses light
Older structures had many small panes or lites of glass
That required more sashbars and sealing.
New structures have larger pieces of glass.
One piece of glass
from gutter to
ridge.
Glass Characteristics
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Light Transmission
– One of the highest – 88% to 92%
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Strength
Cost
– Initial high, justified for high light crops
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Thermal Properties
– Single layer, high conductance
– Needs a thermal blanket
Rigid Structured Sheets
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Polyvinylchloride (PVC)
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Fiberglass
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at Home Depot
Not recommended – short life
Being replaced with polycarbonate or acrylic
Polycarbonate
– Less rigid, able to curve more
– Lower price, flame resistant, less hail
damage
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Acrylic
– Higher price, flammable, hail damage
Rigid Structured Sheets
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Forms
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Single layer flat
Single layer corrugated
Double layer ribbed
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8 mm - $1.90
16 mm - $2.60
Aluminum extrusion to hold costs an
additional $1.50 per sq ft of panel
Corrugated Fiberglass Panels
Diffuses light
Fiberglass Bloom: resin
wears away exposing
the fibers. Collects
dirt and reduces light.
Not available in double
layers so low insulation
and heat retention
value.
Double Layer
Polycarbonate or Acrylic
Attaching Panels
Thermal expansion and contraction requires special mounting.
Large Roof Panels
Sheet Characteristics
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Light Transmission
– Depends on thickness and type
– high – 88% to 92%
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Strength – good except crazing
Cost
– Initial high, justified for high light crops
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Thermal Properties
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Single layer, high conductance
Double layer provides good insulation
Polyethylene Films
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Many sizes (widths) available
– Length typically 100’ or 150’
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3 to 4 years longevity
– Longer with lower light
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6 mil (0.006 inch) thick
Typically used as an air inflated
double layer for the insulation
and structure
Inflation Fans
One Large Fan Distributed
Moisture Between Layers
Pressure Between Layers
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Manometer to measure pressure
¼ inch difference in loop
Effects resistance to wind
Too much pressure stresses film
Polyfilm over glass was investigated at Penn State around 1980.
Light reduction reduced yield of roses more than money saved.
Triple layer in spring for
condensation drip
reduction. Not really used.
Reduce energy loss
of end walls.
Film Options
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Polyvinyl chlorides, polyvinyl fluorides,
polyesters
– Only narrow widths available
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Some longer life films being tested.
Infrared additives
Anticondensate additives
Light diffusing additives
Greenhouses do get
damaged and destroyed
Wind damage
Snow Damage
Next On Schedule
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Light and Lighting Systems
Benches (Wednesday, 9/29)
First Exam (Friday, 10/1)