Unvented roof assembly remodel -- technical question
I'm in building zone 5, in northern IL about an hour south of WI. We are restoring a usonian style house from the 1960's which due to a raccoon infestation (courtesy of the previous owners) has to have the roof removed, the attic cleaned out and new insulation/new roofing installed. The roofline is fairly complex so a vented attic is not possible. I am not comfortable with using site-applied sprayfoam in the house for various reasons. I've been planning the remodel based on the unvented roof assembly recommended by professionals on GBA and the Joseph Lstiburek article on the subject. The plan for the roof assembly is:
Roofing material, self-adhering membrane,1/2" plywood, 3 layers of 1" dow polyiso 6.5R foam (R19.5), 3/4" CDX, R-30 denim cavity insulation, drywall/plaster and or cedar tongue and groove ceiling (original design).
However we've encountered a few technical problems:
The cavity is 12" (2x12 beams), and the denim insulation is only 8" - which leaves a 4" gap above the cavity insulation if it is installed with the roof removed. Adding additional cavity insulation will cause an increase in condensation, but the 4" gap is going to cause convection and condensation as well. The denim is heavy and attempts to use insulation supports to keep the denim insulation up against the underside of the sheathing haven't been very promising. What are our options here?
Our other problem is that on one section of the roof there is not enough physical space for 3" of foam plus all the other layers (plywood/roofing material/etc) and 2" of foam would be the maximum amount that could fit and still match the existing architectural details. With the high 6.5 number used for the polyiso (I've read about polyiso drift in cold climates) that puts us at just R12/R-13 for the foam above the deck and R30 for the cavity insulation which from what I've calculated is awfully close to creating problems with condensation. What are the options here? Control indoor humidity levels in the winter?
Attached is a rough drawing of the roof if it helps. Thank you,
Posted Aug 2, 2014 8:57 AM ET
Edited Aug 3, 2014 7:30 AM ET
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