Posted By FredWalter on 17 Mar 2010 06:31 PM
I plan on having a non-vented attic on the 26'x30' one-story addition on my house, with the attic space being heated/conditioned. We won't be using the attic space as living space (the roof as a 4:12 pitch so it's only around 4'-5' high in the center), but we will have the heat recovery ventilator/etc up there.
I've read:
http://www.buildingscience.com/documents/digests/bsd-102-understanding-attic-ventilation
I'm currently looking at one of two options:
1) put ice+water shield on top of my plywood sheathing, then (R36) 6" of Johns Manville AP Foil-Faced foamboard on top of that, with a plywood nailer on top of that, then roof paper (probably more ice+water shield), then metal roofing
OR
2) put ice+water shield on top of my plywood sheathing, then metal roofing, and spray (>R36) ~6" of 1.5 lb foam underneath my roof plywood sheathing
My roofer guy says that with ice+water + metal roofing, that I won't have a leak, so spray foam underneath the plywood would make more sense (especially since I've already got the metal roofing).
Anyone have experience doing either of these, that can comment?
You'll get better insulation performance out of it with the insulation above the roof deck, but don't use foil-facers on your iso or EPS or you run the risk wicking snowmelt/ice water into the insulation at the nail & screw points with no means of letting it dry. Foil facers are about as strong a vapor barrier as you can find, and you need to have something that can shed the moisture that it surely WILL take on in various amounts. While both iso & EPS are hydrophobic, the will absorb considerable moisture in a saturated environment, with negative impact on R-value. Between snow loads or sustained wind-driven rain, moisture has a way...
Atlas,
Hunter,
Cornell and several others make ventilated nailer-deck iso panels to apply above structural roofs, and the performance is good. With the ventilation gap under the nailer high vapor drives from sun on a hot-wet roof and/or snow-saturated seepage has a space to dry toward, and the foam never takes on moisture. Most have specs for what is/isn't allowed under the shingles or over/under the panels (download their installation manuals), but IIRC ice & snow shield isn't recommended except as flashing in valleys, etc.
If you prefer to do your own vent-gap with purlins under a metal roof they all have half-sip type 4x8 panels with pre-applied that you can mount the purlins for metal roofs onto, for similar performance, but some have special version of vented-nailer decks designed specifically for metal roofs. (I'd have to read their literature to see what's metal-roof specific about it.)
With these panel systems even leakwater has a chance to escape without damaging the structural roof.
If the permeance of 1.5lb foam at 6" is less than 1 perm (and I suspect it is), a layer of ice & snow shield above the roof deck with the spray foam below risks creating a moisture trap at the roof deck. Any moisture that finds it's way in, be it permeated from below during the winter, wicking/dripping through imperfections in the ice & snow layer, etc, will have a hard time EVER getting back out, and in 10 years you'll have punky saturated wood in spots. If you went with half-pound foam (which takes 10" to get to R36-ish levels) it would have a chance of drying. We all HOPE that a roof will never leak, but NEVER say "never"- the all eventually do, in the most unexpected of ways.