Roof insulation and air sealing; advice and input appreciated…
I’m thinking ahead about a room addition to my house. This would be a 16′ x 15′ room with a gable roof of at least 6/12 pitch with no dormers, chimneys or other obstructions. I’d like to do a cathedral ceiling for the aesthetics, and since I can plan to insulate well, I’m okay with the small energy inefficiency of the higher ceiling (not really that high).
I’m thinking unvented with rigid foam above the exterior decking. I live in zone 6, so would need R-25 there. Type 2 EPS has about an R 4.2 value, so I would need 6″ of thickness, a bit daunting but I think unavoidable with this design. I don’t think using XPS to save 2″ of thickness is warranted, but I could do 3″ of polyiso with 2″ of EPS above to keep polyiso warmer.
I know I would need another layer of sheathing above the foam, or could use horizontal purlins since I will be using screw down metal roofing. Between the rafters I’m thinking of 12″ fiberglass batts (6″ batts are short on R-value). I can use 2″ x 10″ or 2″x 12″ rafters that would compress the batts a bit but not enough to threaten my getting more than an overall R-49.
What about below the batts (underside of rafters)? I don’t think I can use taped rigid foam as an air seal here as that would be an inappropriate moisture barrier and limit drying to the interior, correct? (would even 1″ of taped EPS be risky here)? Should I use a smart membrane here for air seal, and what would be good options? The ceiling will be sheetrock. so maybe no need for the membrane, but I’m okay with that insurance unless a complete waste of money.
If I went with all interior insulation (avoiding exterior rigid foam), could it work to have 12″ fiberglass batts (R-38) between the rafters, and then perhaps 2″ of polyiso on underside of rafters, well taped? The felt faced roofing stuff is said to be R-13, giving me R-51. Must this design become a vented one?
Thanks for listening!
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