Condensation on Polyiso sheet foam
Houghton123
| Posted in Energy, Heating & Insulation on
We’re preparing to insulate the cathedral ceiling of our shed conversion with 3″ of polyiso sheet foam. Each rafter bay will be ventilated with a large soffit vent and continuous ridge vent.
Polyiso foam comes standard with aluminum foil on each face. Our son has heard from some roofers he’s talked with that this construction detail creates problems with condensation on the upper face of the foam, running down to the lower edge where it drips off to start rotting the wall.
Is this in fact an issue? A local supply house can get me tarpaper-faced polyiso, but this doesn’t serve as a vapor barrier (may not be an issue anyway; I will probably install a vapor barrier or use vapor barrier primer).
Bill Houghton, weekend worrier
Replies
The top surface
of the foam insulation would be cold, of course, but where would the moisture come from to cause condensation? Inside the house?
I can see this being a problem if the foam is simply placed into the rafter bays and somehow held in place until the drywall is installed, without foaming or otherwise sealing the edges. A quality installation calls for cutting the foam with consistent, small gaps around the edges, and completely foaming all of them with canned foam. If you leave air gaps you are leaving paths for moisture-laden air to move up thru from the interior. If you cut holes for can lights, ductwork, exhaust fans, or anything else... same thing.
3' of foam isn't much R value. Why not more?
As David said ... there should be no moisture condensation w/out a source of moisture ... which generally has to be from the inside of the house and assuming you install and seal it properly, there shouldn't be issues (i.e. condensation). I also have to ask ... why only 3"?