Poly over faced rafter insulation?
I have a Northern NJ 1930s cape with a single dormer. The rafters and gable walls have foil faced insulation. The unfinished areas of the attic have wood plank floors and are used for storage.
To tighten up the house I was considering stapling poly sheeting directly over the insulation, from where the rafters meet the attic floor, up to the top plates of the knee walls. In other words, the entire accessible rafter area. I would tighten the inaccessible areas by sealing holes in the finished ceiling & walls.
Some of what I’ve read has me concerned about adding the plastic, but also confused. It’s hard to find a clear answer.
An alternative approach I considered would be to seal only within the finished space and try to weatherstrip the access doors to the unfinished spaces (there are three).
What would you do?
Replies
If you cover the faced batts with poly sheeting it is best to put slices in the foil facing, so you dont create a double vapor barrier, trapping moisture between the two ( facing and poly).
You say its a storage area, but you may want to check on the need for using flame retardent poly if it is used for anything more.
If the poly film goes directly over the foil, with nothing between them, then the two function as a single vapor barrier, with nothing between them to be a place where water is trapped.Slicing the foil won't really affect the effectiveness as a vapor barrier. If 99% of the foam underneath still is covered with foil, then the surface overall is still 99% as impermeable as before.
The OP stated it was foil faced insulation. If it is foam or batts, he really doesnt say. If its foil faced foam, it is a vapor barrier in itself. If its faced batts, I would still go with slicing the facing, because of the two vapor barriers, or no poly at all.
Thanks for your responses. It's batts. I understand the foam slicing but I think it's true that slices won't be enough stop mosture from accumulating between foil and poly, if that's what's going to happen.
I've slept on it and now I'm thinking it's best to just weatherstrip the access doors this year. If I want to add more insulation i'd have to rip down all that poly.