Expert: Daniel Humphrey - 12/31/2008

Question
is there going to be a moisture problem inside my walls if i apply a 1.5" Styrofoam to the outside of my house and a ploy vapor barrier to the inside? we get very cold temps. and i have heard that Styrofoam on the outside might cause condensation on the inside of the wall.

Answer
Ab,

Yes, you are right that styrofoam added to the outside can cause moisture to condense inside the wall, and the best way to prevent this is to create a perfect air and moisture barrier on the inside.

The moisture that would condense inside the wall is of course coming primarily from cooking and bathing inside the house. If you can prevent that moisture from entering the wall system, you can avoid the condensation problem. One way to help this is to use kitchen and bath exhaust fans, and have an air exchange system that creates an equal pressure inside the house at the ceiling level. Obviously, if you can put up a perfect poly moisture barrier, that would be enough. But it is very difficult to get a perfect seal around all edges and openings.

You probably realize that moisture is probably condensing in there anyway, without the stryofoam, but the frost evaporates from the slight air movement and seldom appears as dripping water. With the styrofoam, the moisture condenses and perhaps freezes against the styrofoam surface and then melts all at once when there is a warming spell. This is what people notice. If you provide a small air gap between the styrofoam and the tarpaper covering the rest of the wall, this moisture will drain out or evaporate without causing damage. You lose a little R-value with that air gap, but some consider it necessary.

--Daniel