Last Updated: , Created: Thursday, October 14th, 1999

DOES STYROFOAM ON THE OUTSIDE OF THE HOUSE FORM A VAPOUR BARRIER?

Styrofoam (which is actually a Dow Chemical brand name of extruded polystyrene) is a fairly good vapour barrier. It will not cause problems in the Canadian climate if you put on at least 1-1/2 inch thick and preferably in 2 ft. x 8 ft sheets applied horizontally. This actually permits some moisture escape through the joints (which you do not want to caulk shut), but most importantly the thickness of the foam will provide significant insulation to keep the inner face of the foam fairly warm. This reduces if not eliminates any condensation. Thin sheets of insulation that are not permeable can cause problems by trapping the moisture in the cold wall.

Dow specifically dealt with this problem in Canada by making a special foam board called CladMate, which is more permeable than most extruded polystyrene products, and is not classified as a vapour barrier. In fact when the cut joints are sealed and the half lap joints are properly nailed, this product is an approved Air Barrier, eliminating the need for the house wrap.

(Extruded Polystyrene that IS a vapour barrier can be useful on the inside of the basement wall because here when you do caulk the joints, you will have created a useful vapour barrier on the warm in winter side of the insulation.)

 


Keywords: Damage, Vapour Barrier, Insulation

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