Q&A: Wood Siding Over Rigid Foam

A.David Utterback responds: When installing wood siding over rigid foam sheathing, you must protect the siding from moisture and heat buildup, and use proper nailing.

Moisture protection and heat buildup. Since foil-faced foam is a moisture barrier, it should be used with a continuous-film vapor retarder on the inside wall under the interior finish to keep condensation out of the wall cavity.

However, you may get some condensation between the sheathing and the siding, and some heat buildup beneath the siding. These conditions can cause cupping, splitting and peeling if the siding is not installed and finished properly.

Siding should be backprimed to prevent moisture that accumulates between the foam sheathing and wood siding from being absorbed into the back of the siding. If the back of the siding is left unprimed, it will absorb moisture at a faster rate than the finished front side, causing cupping. And without backpriming, that same moisture can be drawn through the siding when the sun comes out, causing the finish to blister. Building paper, applied between the foam and the siding, will absorb some moisture, which can help minimize problems. It can also absorb some of the heat that builds up there, helping to prevent excessive drying and splitting. The splitting problem is usually compounded by improper nailing practices.

Another way to prevent both the moisture accumulation and the heat buildup is to fur the exterior walls with 1x2s over the rigid foam sheathing before applying the finish siding. This creates a

3/ 4-inch air space between the sheathing and the siding, providing a natural way to ventilate the excess heat and moisture from this space. Some type of vent strip or screen should be used at the top and bottom of the wall to keep out insects. Door and window jambs must also be extended to allow for the extra thickness of the exterior walls.

Nailing. Proper nailing will ensure quality performance of the wood siding. Always use non-corrosive fasteners and never doublenail solid wood siding materials. Nails should penetrate at least 1 to 1 1/2 inches into studs or blocking under the foam sheathing. Ring-shank or spiral-shank siding nails are recommended for increased holding power.

Nails should also be set flush with the surface of the siding but not overdriven. If the nails are set too hard, the foam sheathing will compress, causing waviness in the siding.

— David Utterback is a field representaive for the Western Wood Products Association.