Insulation set into the ground around the perimeter of the greenhouse will increase thermal resistance and decrease heat loss. The percentage of heat loss through the perimeter will be greater for relatively
small individual houses than for large gutter- connected houses. Figure 2–3 (page 26) shows details for perimeter rigid foam insulation. Rigid foam insulation can also be used on sidewalls below bench height and, in many greenhouses, on the entire north wall. Two inches of foam polyestyrene or 1.5 inches of foamed polyurethane or isocyanurate will have a thermal resistance of about 10 ft.2-hr-°F/Btu.
Steel and aluminum have very low thermal resistance and are difficult to insulate. Gutters and other metal members can be insulated with materials that foam on mixing and adhere to the member.
Blankets
Movable blankets add thermal resistance during the night and are stored during the day. Most are made of thin materials that will pleat and store in a relatively small space. Several commercial systems are on the market, and plans are available for grower-installed units. Two systems are shown in Figures 8–3 and 8–4 on the following pages. Thermal blankets can be installed in any greenhouse, but most easily in clear span units. There must be a good seal around the edges to prevent warm air from moving into the attic space created above blankets. Heating pipes may have to be moved, and there is generally some growing space lost to blanket storage. A detailed analysis should be made for each
Figure 8–2. One method for covering a glass glazed greenhouse with a single layer of polyethylene film.
COVERING SYSTEM RESISTANCETHERMAL
(ft.2-hr-°F/Btu)
DAILY HEAT LOSS REDUCTION
(%)
Single layer lapped glass 0.91 0
Double layer glass 1.43 36
Single layer polyethylene 0.83 (Heat loss increased 10%)
Double layer polyethylene 1.25 27
Double polyethylene over glass 2.00 54
Double skin structural plastic panel (b) 1.70 46
Single layer lapped glass plus thermal blanket 1.78 36 Double layer polyethylene plus thermal blanket (c) 2.50 57
(a) Compared to single layer lapped glass.
(b) Thermal resistance is an average value for commercial panels.
(c) Thermal blanket in place during 15-hour night; thermal
resistance is an average from tests of several blanket materials.
Cutaway View
Hold tube in place with cord or plastic band every 20' to 30'
Air Inflated
Polyethylene Tubing to create air space Single layer of
Polyethylene, fastened at all edges
Figure 8–3. A track-supported thermal blanket stores against one endwall and works well for a free-standing greenhouse. Blanket clip Blanket Trolly Track Support wire End support
Support wire or strapping
Thermal blanket Track
Heat seal
Heat pipes
Figure 8–4. Curtain system for gutter-connected greenhouse.
Support Truss for plants and equipment
Drive Cable
Curtain Drive System Intermediate Support Cables
INTERMEDIATE SUPPORT CABLES
Polypropylene Monofilament
Line Cable Tightener Bracket
DETAILS Double Pulley Bracket Continuous Rectangular Tubing U-bolted to each post Pipe Curtain Drive Polyethylene Pipe slit to fit over steel pipe
Cable Tightener Curtain Pipe Drive Holder Drive Shaft Bracket Bracket CURTAIN DRIVE SYSTEM
Curtain 3/8" Pipe Nylon Pipe Holder Curtain Anchor Guide Cable Guide 2" Drive Shaft Pipe Drive Shaft Mounting Bracket bolted to posts Drive Cable Double Pulley Bracket 12'–20' Bays typical
installation to determine the cost/benefit potential. Table 8–3 illustrates potential savings for a typical installation in the Northeast. Most studies have shown the internal thermal blanket to be a good investment.
It is not possible to predict the thermal resistance of a material. Each must be tested and the resistance calculated. A standard testing method used for all samples allows comparison. Full-scale testing in a typical greenhouse operation provides additional performance data.
Table 8–3. Effect of energy conservation practices on fuel use and economics for a 40,000 ft.2 greenhouse in the Northeast. (a)
Table 8–4. Overall heat transmission values for thin thermal blanket installed in three glass greenhouses.
Radiative properties are transmission, reflection, absorption, and emission. A good reflector is a poor absorber and poor emitter; a good absorber is a poor reflector and a good emitter. All properties are functions of the temperature of the radiating source. An ideal material would have a highly reflective surface facing the plants and a surface with low emissivity (high reflectivity) facing the outer greenhouse cover. An aluminized surface with no protective coating has a high reflectivity and low emissivity. A black surface has low reflectivity and high emissivity. Table 8–4 gives overall heat transmission values for several thin materials installed in a single glazed glass house.
CONSTRUCTION SYSTEM REDUCTIONFUEL USE
(%)
COST REDUCTION
(%)
Single layer lapped glass 0 0
Single layer lapped glass plus thermal blanket 41 29
Double wall plastic structural panel 38 27
Double wall plastic structural panel plus thermal blanket 59 39
(a) Adapted from Rotz, C.A. and R.A. Aldrich. Computer predicted energy savings through fuel conservation systems in
greenhouses. Trans. of ASAE.
BLANKET MATERIAL HEAT TRANSMISSION VALUE
(Btu/hr.-°F-ft.2)
HEAT LOSS REDUCTION (a)
(%) Mobile air curtain (double layer polyethylene film) 0.68 20 Stationary air curtain (aluminized polyethylene tubes) 0.54 36
White-White spun bonded polyolefin film 0.51 40
Grey-White spun bonded polyolefin film (lightweight) 0.56 34
Clear polyethylene film 0.45 47
Black polyethylene film 0.48 44
Grey-White spun bonded polyolefin film (heavyweight) 0.43 49
Aluminum foil-clear vinyl film laminate 0.40 53
Aluminized fabric 0.39 54 (b)
Aluminum foil-black vinyl film laminate 0.63 26
Double layer spun bonded polyester (tobacco shade cloth) 0.53 38
(a) Compared to single glass for the same greenhouses for night time loss.
double surface extruded plastic structural sheets to produce a wall or roof with high thermal resistance. The beads are pulled out at daylight and stored until pumped back at dusk. The system has been used in Japan and in Ohio to a limited extent. By controlling which cavities are filled, the system can be effective for summer shading.
Controls
Blankets may be opened and closed manually or set automatically with a photocell or time clock. Light activated operation is most desirable for crop production, and requires no change in settings with the seasons. A light level of 50 fc is a good threshold point. At this level, blankets will remain open even on dark, cloudy days. Time clocks are the most common method of control, even though growers must change the clock settings as seasons change. If a blanket is opened rapidly, cold air from the attic area will drain rapidly onto plants, many of which are susceptible to damage from the cold. A percentage timer opens blankets slowly over half an hour to eliminate this problem. Another solution is to wait for the sun to warm the attic before opening the blankets, or partially open the blanket 6" for an hour before opening the blanket completely. In most snowy areas, blankets are left open during snow storms to reduce potential structural damage from accumulated snow. When the integrity of the greenhouse structure depends upon melting the snow with the heating system, consider installing a snow alarm which will automatically open the blanket. Provide a manual control option for all systems in the event of a power or motor failure. Blanket systems are more cost effective if additional uses can be found, such as summer shading. Use a white porous material, providing about 50% shade, rather than a material providing the maximum energy saving potential. Summer shading reduces the radiant energy load on the crop, reduces leaf temperatures, reduces the time fans must operate, and provides a more comfortable temperature for people working in the greenhouse. However, a blanket may inhibit adequate summer ventilation in naturally vented houses with ridge vents.
some crops. The blanket can also be used for energy conservation if condensation can be drained away. The drive and support systems for photoperiod control curtains are identical to those for thermal blankets and are normally controlled by time clocks.
Cost
The most expensive parts of a thermal blanket system are the mechanical system, blanket sewing and grommet insertion, and labor costs for installation. The blanket material is probably the least expensive part, so single- and multi-layer blankets have similar costs.
Installation
Retrofitting older greenhouses can be difficult. Heat pipes, irrigation lines, plant light, roof support columns, and other obstructions may have to be moved before a system can be installed. Blanket ends and edges must be sealed tightly to reduce air exchange between the growing area and the attic. Figure 8–5 shows typical edge sealing methods. A retracted blanket will partially shade the greenhouse by as much as 10%. Storing it along the north wall or under the gutter will minimize shading.
Figure 8–5. Blanket edge sealing methods. Track is supported by
wires attached to the ceiling
Insulating blanket Hinged board Insulated sidewall Insulating blanket Insulation board Insulated sidewall Insulating blanket Insulated bench Uninsulated sidewall