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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Replies
>"– Is it a bad idea to reduce the vent channel to 1-1/8” (the width of the flange)?"
Code minimum is 1", which should be enough, assuming soffit to ridge venting and a roof pitch steeper than 3:12.
>"– Are there moisture concerns with trapping fiberglass between two layers of foil faced polyiso?"
As long as the continuous layer at the ceiling is made air tight and the channel under the roof deck is vented at both ends it should be fine. Dings tears & holes in the facers don't affect the vapor permeance by very much- the enemy is (as always) air leakage. Tape the seams with the appropriate tapes, as well as any dings that are full punctures.
>"– 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"
It isn't going to make a significant difference on the cooling load or peak ceiling temperatures. The real benefit of a foil facing a 1" air gap is less than R2, closer to R1. The 1" air gap is worth R1 with or without the foil. While the combination adds up to R2, only half of that would be attributable to the foil.
Thanks so much Dana for your thorough and comprehensive response!
The plan was to seal the foil faced layer under the rafters with foil tape and all the way around the perimeter to the top plates with 3M tape. Acoustical sealant between the top plates. That way I was hoping to connect the taped OSB to the polyiso in a continuous air sealing envelope.
The roof is 4/12 single pith, vented continuous from the soffit to ridge. Along each rafter bay.
Is the baffle a performance boost? It seems air not sweeping over the unfaced fiberglass would improve the assembly. It is often very windy here.