Gluing rigid insulation to interior of roof sheathing
My home is in Northern Michigan and I wish to insulate an attic that has storage trusses and considerable open spaces behind the perpendicuilar truss members. There is soffit venting around the entire attic perimeter. The attic floor is insulated to R 30. (Peaked roof built over original flat roof)
My first choice was to spray polyurethane to the cathedral underside of the roof sheathing, however I am having difficulty scheduling a contractor, most of whom have bigger jobs they are chasing, so I am considering other options.
Hybrid insulation is attractive for its lower cost but still requires a contractor to do the spray in. I am wondering if it would be feasible to glue rigid insulation possibly 1-2 inches thick directly to the roof sheathing and then install fiberglass batts directly over? Ovbiously it would require a perfect bond between the rigid insulation and the sheating with no gaps or air pockets, in order to lock out all moisture. I imagine covering a piece of rigid insulation with a foamy type glue, and pressing it in position, and seeing the glue material seeping around the edges to seal the insulation board perimeter.
I do not know if such a gluing material exists? I am aware of spray foam products for the DIYer, but those I have reviewed are expensive and deliver inconsistent results, judging from the user blogs.
I am reaching out to readers of the Building Advisor to advise me on the feasibility of this approach and suggestions.
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Replies
The long term prospects for cut'n'cobbled rigid foam in this application is a bit uncertain, but it's do-able. Cutting the foam 1/2" narrower than the rafter bay width allows you enough gun-tip room to seal it in place with can-foam.
You still need to have sufficient foam-R to comply with IRC chapter 8:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_8_sec006.htm
In the MI- U.P. you're in US climate zone 7, which implies you would need R30 of foam for it to work. But that value is based on a total-R of R49. It's the ratio, not the absolute R that is important here. R30/R49 is about 60%- as long as 60% of the total R is the foam you install under the roof deck, you're in pretty good shape. If you're only installing R30-ish you can go with R20 foam/R13 fiberglass.
That's doable with 5" of EPS under the roof deck, or 3" of closed cell polyurethane. A mere 4" of XPS would work too, but it does more environmental damage and loses R-value over time. Don't use foil-faced polyiso in this application- it underperforms at the temperature extremes, and creates a moisture trap due to the extremely low vapor permeance of the facers.
Leaving the roof deck vented and building up rigid foam underneath the rafter edges eliminates the thermal bridging of the rafters and there is much less foam-cutting & air-sealing labor involved.
Wes,
It sounds like you are considering the cut-and-cobble approach. You should know that there have been reports of failures when the cut-and-cobble approach is used for an unvented roof assembly.
For more information, see Cut-and-Cobble Insulation.