Interior continous insulation
buildsbydavid
| Posted in Green Building Techniques on
Rebuilding a 1995 bungalow north of Toronto in Canada. Roof structure is a parallel chord cathedral truss. Will be R40 batts between the trusses a 2 foot centre.
Roof is vented
Roof assembly will built prior to any interior walls. Current plan is insulate and smart vapour barrier and the strap for service cavity and the drywall.
Has anyone added a layer of continuous foam insulation. Foam could be vapour barrier and eliminate the thermal bridge at each truss.
Thus assembly would be truss, 2” of foam taped at joints, R8-10 depending on brand, 2x strapping and drywall.
Thanks
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Yes, commercial project. Zone 7A , so colder. 2” foam, reflective foil vapour, then 2x3 fastened horizontally for a service cavity. Drywall over this. Receptacles mounted to 2x3. Near zero envelope penetrations. Existing wall was 2x6 with fiberglass.
9000 square foot project which cost less to heat then our 1800 sq/ft home.
I have used polyiso under the rafters before in vented assemblies. I usually only use 1/2" or 1", and the primary purpose is usually to be an air barrier behind something like a T and G ceiling. The polyiso does help to cut down on thermal bridging though, and it's a vapor barrier too. Since you have a vented roof, you don't have as much to worry about and I see no problem putting polyiso under the roof assembly as you describe.
I would skip the smart vapor retarder here and just use foil faced polyiso with the seams taped. The foil facer is a vapor barrier, so you don't need anything else. I'd put the strapping over the polyiso so that the drywall can be fastened directly to the strapping, avoiding the need to run long screws through the drywall and foam to get to the structural supports.
Bill