Air permeable insulation in unvented attic
I am designing my new home. I am building in zone 2 (Orlando) and plan to build an unvented attic.
I plan to use rockwool or fiberglass batt against bottom side of roof deck (both insulations are air permeable). IRC R806.5 5.2 allows this. I will comply with all requirements including vapor diffuser port and conditioned attic with 50cfm/1000 sqft.
Quesiton: How do I install the 9” of Rock wool or the 10.4” of fiberglass to get the required r-38 value. I don’t have a roof truss design yet. It roof truss is deeper than the insulation (ie 9”Rockwool and 10” rafter) or if roof truss is not as deep as insulation (ie 10.4” fiberglass batt and 8” rafter). Also – regardless of rafter depth, is it important that insulation contact underside of roof deck or can there be an air gap (such as could be the case with 9” of rockwool and 10” rafter)?
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