Foam sheathing panels vary in moisture permeability but are generally considered as good moisture barriers. They should be used with a continuous film vapour retarder on the inside wall under the interior finish or with foil-backed gypsum board. Rigid foam sheathing can cause moisture to accumulate on the back of siding and cause staining, buckling and damage to finish coats. The following recommendations are meant to minimize potential problems when wood sidings are applied over rigid foam sheathing:

• Use thicker siding patterns in widths of 8" or less. Thick, narrow siding is more stable than thinner, wider patterns and better able to resist dimensional changes.
• Use kiln-dried siding over rigid foam sheathing. This is the most stable cedar siding.
• Proper prefinish is essential. Follow the priming recommendations given in this publication before installing siding.
• Use light color finish coats to maximize heat reflection and reduce dimensional movement.
• In severe climates, an air space between siding and rigid foam sheathing can be created by fastening furring strips to the sheathing before installing siding (see Wall Construction). Air spaces allow for the venting of accumulated moisture.
• In all cases it is recommended that building paper or an equivalent building wrap be applied over the foam sheathing.