Insulating my attic cathedral ceiling, but I'm trying to be (a little) cheap about it.
I'm planning to insulate the roof of my 2.5 storey Victorian house. There's currently no insulation whatsoever in the cathedral ceiling! Unfortunately the rafters are ridiculously small (real 2x4s, 16" O.C.) so there's not much room for insulation (and I don't want to lose what little headroom I've got up there). I'm removing the plaster Saturday, so the clock is ticking.
I have already had the spray foam quoted. 800 square feet of 3" thick polyurethane (about R18) would cost me almost $5000! That quote seemed high to me, but it was the cheapest of three quotes. I checked out the DIY urethane foam kits too. 800*4 board feet worth of the home kit polyurethane works out to $3500 (http://www.tigerfoam.ca/
), which is a little cheaper but doesn't include labour.
I figure I can get R20 using 4" of rigid polystyrene foam (xps) for no more than $2500, without losing too much ceiling height. (The last inch would be across the bottom of the rafters.) My only question is how to ensure a good seal between the foam sheets and the rafters on the side. Would Great Stuff or PL300 work well? If so, how many inches of foam "bead" does a typical can cover so I can add it to my cost estimate?
For added detail, I was planning, if I used spray foam, not to leave any gap for ventilation (i.e. a "hot roof"), but to leave a 1" space for ventilation if I use XPS board.
Anyone have any experience with this?
I've reposted the question from a DIY forum which was getting little response. My apologies if that's not kosher