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Is your greenhouse drafty, guzzling energy?

Wear and tear eventually take a toll on a greenhouse, and rising fuel costs take a toll on the purse. To overhaul a drafty, inefficient greenhouse and lower fuel costs at the same time, consider a midwinter tune-up like the one shown here. Thanks to new insulation materials and techniques, this 30-year-old greenhouse is now energy-efficient.

Before, many panes of glass were missing, so the owner decided to replace the roof with a transparent material of high insulating value, and to insulate the north wall with polystyrene board.

For the roof, the owner used double-walled sheets of transparent plastic. Three basic types of insulating plastic are sold--acrylic, polycarbonate, and polypropylene. All have essentially the same structure: thin transparent sheets are separated by thin, closely spaced ribs of the same material (see the lower left photograph on the opposite page). The ribs strengthen and stiffen the sheets without noticeably reducing the passage of light. The channels formed by the ribs trap dead air--an excellent insulator.

Light transmission is about 80 to 85 percent that of glass--but the insulation value is twice that of glass. The sheets are light, rigid enough to span several feet without sagging, and nearly unbreakable. The material shown here is .236-inch-thick (roughly 1/4-in.) polycarbonate; at 65 cents a square foot, it's less expensive than glass, and much easier to use. It can be cut with a hacksaw or a circular saw with a carbide-toothed blade. Sheets come in 4-foot widths; because the greenhouse panels were only 16 inches wide, the plastics firm sliced each of four sheets into three widths; this cost $10 extra.

Double-walled plastic sheets expand with heat, so allow 1/16 inch per foot of width for expansion. Failure to do so may cause plastic to crack. A plastics dealer can specify appropriate adhesives, sealants, or caulking materials. This installation used silicone caulking.

After removing remaining panes of glass, the owner proceeded as shown. Removing old putty was the most time-consuming part of the job; thoroughness is important here, or panels will not lie flat.

To find insulating plastic, consult the yellow pages under Plastics--Sheets, Rods, Tubes. You may have to make several calls; not all suppliers keep these sheets in stock. Ask about available materials, thicknesses, sheet sizes, and especially cost; thicker materials are quite expensive, and usually unnecessary in a small greenhouse.

For the north wall, the owner decided transparency was not as important as insulation. The north wall of a greenhouse gets little light, especially in winter, and loses much heat through radiation. Insulating this wall cuts heat loss, and, if the inner surface of the insulation is reflective (white-colored or foil-faced), light to plants can actually be increased.

He chose 1-inch thick polystyrene board. Lightweight and inexpensive (about 20 cents per square foot), it is easy to work with: just cut pieces to fit snugly between studs, then push into place.

Insulating the side walls below bench level can save additional heat.
COPYRIGHT 1985 Sunset Publishing Corp.
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Copyright 1985 Gale, Cengage Learning. All rights reserved.

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Date:Dec 1, 1985
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