Posted By greenseattle on 10 Dec 2011 11:36 PM
I have cathedral ceilings in my house in Seattle, built in 1971, 2x4 walls, 2x8 ceiling joists, foil-faced R14 fiberglass insulation.
I am thinking of adding 6" or 8" of rigid foam insulation to the roof deck when reroofing. Heating costs are about ~$200/mth during the heating season (heat pump), so this might take a while to pay off, but at least it will provide a uniform layer of insulation over lighting and junction boxes.
1. EPS, XPS, or polyiso, any difference in durability or moisture resistance?
2. Should I remove the foil faced fiberglass so the roof deck can dry to the interior? I don't mind some drywall repairs but I'd like to avoid removing the interior ceiling.
3. Should I use 2x furring strips to ventilate the nailer deck?
4. How best to attach this to the existing roof dock if I use 6" or 8" of foam?
5. Any alternatives I should be thinking of?
You have foil-faced f.g. in the rafters, with a vent gap between the f.g. and roof deck, you need to either rip 'em out and start over, or use only semi-permeable solutions on the exterior, with a vented nailer deck. This means not even 30# felt, and EPS only. XPS is too vapor retardent at 2"+ and even fiber-faced iso might be too vapor tight (foil/poly/vinyl faced rigid board is DEFINITLELY out of the question.) You'd ideally want to keep the foam over 1-perm to allow outward drying year round, since it can't dry toward the interior through the foil facer on the batts at ALL (even if they're ripped & torn, but not removed.)
If that's the way you're going to go, you have to fill the cavity full-up and block any soffit-ridge venting to prevent thermal by passing. It's pretty easy to blow in cellulose (borate fire retardents only) from the exterior before laying down the foam, compressing the batts in place. If the rafters are 2x6 that would give you center-cavity of ~ R18-20. (If 2x8s it would be R25+ ) Then two layers of 3" EPS with seams-staggered (and taped with housewrap tape) would add another R24 bringing it up to R44 (R50+, if 2x8 rafters) , putting an R24 thermal break over the rafters.
It might be tough to make an economic rationale for an R50 mostly-foam solution. A more modest 2 layers of 2" EPS (R16) would put you at about code-min center cavity, but would outperform a code-min by quite a bit due to the thermal break over the rafters.
If it's a plywood roof deck in good condition you can long-screw the 2x furring for the nailer deck through to the plywood at 24" o.c. and you're good. (The Hunter panels fastener guides are a good reference too.) The furring need not be continuous- there's a good argument for creating gaps for cross-venting it, particularly if there are hips & valleys, or if the pitch is below 3:12.