Foam Sheathing Will Help Keep Your Home Warm And Dry

HOME CLINIC: I'm building my retirement home and plan to use foil-faced foam sheathing on the outside of the walls underneath the siding. I've heard this can lead to moisture problems and structural damage within the wall. Is this true? - G.M., Tenino

DEAR G.M.: Foam sheathing is a good way to increase the energy efficiency of your walls. Recent research by Dr. George Tsongas of Portland State University showed foam board-covered exterior walls actually decreased moisture problems within the wall. The foam helps in two ways. First, it acts as a weather barrier to help keep rain water away from wall framing. Second, it helps keep the wall cavity warmer, which reduces moisture condensation within it.

More builders are using foam board insulation because it provides high insulating value in thin materials.

-- Types of foam board insulation: Rigid foam sheathings are usually made of polystyrene. They are generally applied as 2-by-8, 4-by-8 or 4-by-9-foot panels. They have thicknesses ranging from 1/2 inch to more than 2 inches. These boards are usually blue, gray, pink, or green and offer an insulation value of R-5 per inch.

Another type is almond colored and is covered with foil facing. The material is called polyisocyanurate and has an R-value of about 7.2 per inch. A new polyisocyanurate board made by Celotex Corporation is called Blackore. As its name implies, the carbon black in the product gives it a black color and increases its R-value to 8.7 per inch.

-- Using foam with wood siding: Builders generally use foam sheathing in one of three ways with wood siding. They can place it directly over the structural sheathing. Wood strip siding, such as bevel or channel siding, can then be nailed over the foam sheathing.

The foam also can be placed directly over the framing studs, then covered with textured plywood siding. Siding nails must be long enough to penetrate the wood framing 1 1/2 inches.

Another, less costly method replaces the structural sheathing with foam insulation. Shear strength must then be provided by some type of diagonal let-in bracing at corners and on long walls. Metal braces are about $5 each. A 1/2-inch by 4-by-8-foot sheet of foil faced insulation board is about $8. Compared to 1/2-inch-thick plywood at $10 a sheet, a system using foam board and metal bracing is about the same cost, but provides better insulation and moisture protection.

-- Using foam on interior wall: Foam sheathings also can be installed on the interior of the wall. Since they have a perm rating less than 1, they also make an excellent vapor barrier. By taping the joints and caulking around penetrations such as electrical boxes, interior foam sheathing provides protection against air leakage.

-- Controlling air leakage: Researchers have found that air leakage from inside the home is one of the greatest causes of structural damage. In winter, warm, moist indoor air moves into wall and ceiling cavities through electrical, plumbing and other openings. It condenses on cold surfaces and causes moisture problems. Whether or not you use foam board insulation, it is extremely important to seal all air leakage sites, then provide a mechanical ventilation system to provide fresh air.

-- Advantages of foam board insulation: Foam board insulation provides some excellent advantages in reducing energy bills and moisture damage as well as increasing homeowner comfort. But, before you choose any of the installation options, be sure to check out complete instructions with the manufacturer and your local building jurisdiction. The Western Red Cedar Lumber Association and the Western Wood Products Association publish a flier, "Guidelines for Installing Wood Siding Over Rigid Foam Sheathing." You can get a copy by writing to 522 S.W. Fifth Ave., Portland, OR 97204-2122 or by calling (503) 224-3930.

Home Clinic answers questions about home maintenance, repair and energy conservation. It is prepared by the Energy Extension Service, a division of the Washington State Energy Office. It appears Sundays in the Home/Real Estate section of The Times.