Vapor barrier over rigid insulation in bath of unheated cottage?
We have a cottage in Northern Ontario that is mostly uninsulated, and also unheated except for about a month in May and also in September/October. The cottage is wood frame on piers. Recently I’ve begun to upgrade the bathroom by installing 3.5″ thick styrofoam between the studs, allowing about 1/2″ gaps around the edges that were then filled with one part foam.to provide a good seal to the studs and top/bottom plates. Outside I have 1/2″ plywood wrapped in Typar. Since it is a bathroom, I’m particularly concerned about moisture. My questions is whether I should also install a vapor barrier on the inside or is the styrofoam enough of a barrier?
I didn’t think I needed to do this, but some advice I read last fall suggested that the styrofoam may not be sufficient.
Thanks much for any advice that can be provided.
Replies
1) The foam is probably sufficient, though placing a well-sealed plastic vapor barrier directly over it (on the inside) wouldn't hurt.
2) Given how little you are heating the place, you probably don't need to worry about moisture (except, of course, for leaks/splashes in the bathroom). That simply not a long enough period of time (in relatively mild weather) for "moisture drive" problems to develop. You do want to do a good job of air-sealing the inside walls, however, to prevent the migration of shower/bath moisture (and also, of course, to improve comfort).
I wouldn't bother w/ another VB as the styrofoam is itself a VB w/ a low perm rating (as is any plastic foam insulation).
Thanks for your response!
Addtional VB is probably not needed, as other have advised, but if you want a little more insurance with no extra labor and almost no extra cost, use vapor-barrier paint, some of which can qualify for a Class I VB (0.1 perm or less.) Ordinary latex paint is rated as a Class III VB (1.0 perm--10 perm.)
Thanks for the additional information.