Foil-faced polyiso-fiberglass roof sandwich
Happy 4th Everyone!
I slightly altered my roof insulation plan. Originally I planned two 1” layers of polyiso against the rafters with fiberglass batts on top.
I am now considering one continuous layer against the rafters, foil taped for air sealing, then R-38 batts, then the second layer of polyiso between rafters glued to the underside of the I-joist flange to also act as a vent baffle. The advantages are that this acts as a “free” vent baffle, allows use of 3” drywall screws for the ceiling, and gives a bit more headroom in the loft.
– Is it a bad idea to reduce the vent channel to 1-1/8” (the width of the flange)?
– Are there moisture concerns with trapping fiberglass between two layers of foil faced polyiso? This is reclaimed, so the foil is not immaculate (lots of little tears and holes, etc.) but is on both sides of each sheet.
– Is a foil faced vent battle worth the extra work? I know you get about R-2 from foil facing an airspace, in this case the vent channel. The roof is blue propanel and I was hoping to use the foil to reflect some of the summer heat out of the roof assembly.
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