Building the Sun Cottage

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There are two kinds of insulation that are readily available for application to the outside of the Sun Cottage: urethane foam and expanded polystyrene. The former material is sprayed on by roofers and insulating companies and normally has an R-factor of about 7 per inch of thickness. Expanded polystyrene, often called EPS or beadboard, comes in rigid panels of various sizes, and offers an R-value of about 4.5 per inch. Waterproof polystyrene (extruded rather than expanded) has a higher R-factor but is also more expensive per R than is beadboard. Because the Sun Cottage's insulation is protected by a waterproof membrane, the pricier material is unnecessary. You might also keep in mind that, while sprayed-on foam provides a seamless surface, EPS can be applied by an amateur using a strong, flexible adhesive called contractor's glue that's made for the purpose. Another potential advantage to polystyrene is that the boards can be custom-cut to form slopes in flat roofs or to conform to other construction parameters.

WATERPROOFING

Then again, earth-sheltered surfaces must be protected from moisture by a carefully applied waterproof membrane that extends from the eaves right down to the footing bottom. I recommend that you use a material that remains slightly elastic . . . so that it can be applied over insulation laminated to the walls without cracking or spalling off under pressure from the backfill. In any event, don't use cement-based products to coat EPS board or urethane foam: Such materials are so brittle that they'll crack when the foam underneath gives.

I've gotten good results using PennKote Two-Part Membrane—usually called TPM — manufactured by PennKote Inc., Dept. TMEN, 819 West 22nd Street, Tempe, Arizona 85282. The compound is mixed on site to form a permanently elastic roof and subgrade wall coating that can be applied by spray gun, brush, or roofer's broom. (Insulation and waterproofing detail is shown in Fig. 2.)

Before backfilling, the foundation drain — a 4" corrugated and perforated plastic pipe—should be placed in a bed of gravel located around the footing so the pipe will slope gently to screened ends at a lower grade. This drain must be low enough to relieve any water pressure against subgrade walls, so its high point will usually be at floor level. Also, spread at least one foot of additional gravel over the drain to keep the area from becoming plugged with silt. (Some contractors go so far as to place building paper over the structure to keep backfill from entering the gravel bed.)

Both the waterproofing and the foundation drain should be installed with great care, taking special precautions to cover all joints and angles of the structure. Be sure, too, to follow the manufacturers' directions very carefully when applying insulation and waterproofing: There are few experiences more heartbreaking than moving into a beautifully finished earth-sheltered home only to discover leaks during the first rain.

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