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Double Foil-Faced Polyiso Roof Insulation

davekentucky | Posted in Energy Efficiency and Durability on

Hello I am building my first house by myself and with building materials costs sky rocketing it has forced me to make some changes in my plans and where I source my materials. The main question I have is about the efficiency of my roof and durability. I am building an exposed rafter/ceiling like a cathedral just not super steep. It will be a 5/12 pitch, and I am using metal roofing. My layers are now planned as such:
from inside house looking up
1. Rafters (4×10’s spaced 4 ft OC)
2. True rough sawn 1x boards not tongue and grooved so there will be gaps
3. 15 or 30# felt
4. perimeter crib to hold layers of polyiso foam panels
5. foam polyiso staggered and seams taped
6.vertical 1x’s screwed down through polyiso into corresponding rafters
7. horizontal 1x’s strappings for metal
8. metal roofing
there will be a ridge vent and soffit vents to vent decking
I am in Kentucky so zone 4 mixed climate gets humid and was planning on having roughly close to r40 in the roof which would be about 6 inches of polyiso foam I think. I am currently trying to source materials on a tight budget and have found a couple of companies that offer factory seconds at a large discount…the only issue is that they either have felt backed or double foil faced. I have only seen people use single foil faced in roofing with the foil facing the metal roof.  Is there an issue with using double foil faced in my application? Should I use felt backed instead? The felt backed is also explained as black fiberglass mat in a few of their item description. We are going to try to do this all off grid without air conditioning but just in case we decide to condition the space later with a potable ac it would be nice for it not to cause issue in the ceiling with moisture. In the winter we will heat with wood stove. The living space will be 35×35 and mostly open. The roof with overhangs will be 48×50. Thank you for all the information and help, this site is a huge wealth of knowledge!

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  1. Expert Member
    PETER Engle | | #1

    With your description, all of the insulation is foam, so it really doesn't matter if you have several layers of foil in your stack, especially if you tape seams. I find that foil tape sticks very well to the foil facings on polyiso. It is best to wipe the face first with alcohol or acetone to get it really clean. It sounds like you are out in the sticks, so code enforcement might not be an issue. But FWIW, IECCode for zone 4 is R49, going to R60 soon. You might also want to consider a synthetic underlayment rather than felt. More airtight and durable.

    1. davekentucky | | #2

      Thank you Peter! yes it will all be foam in the roof. Great tip about wiping down the face first, thank you for that. I am in the sticks so code is not an issue but I was not aware of the r49, wow! Thats a lot of insulation! Is that overkill for this zone? Although if I dont want to use an air conditioner r49 might be needed? If I put the synthetic underlayment over my 1x roof decking and there is a leak that gets under that and on top of my 1x roof decking boards will it cause a mold issue? I was under the impression that felt would allow boards to dry out if they did get wet. I'm new to this so I totally appreciate your expert advice and explanation. Im sure I am just confused on how it works. If synthetic underlayment is better, which one would you suggest? And are the felt backed polyiso panels an option? Only reason I ask is because they are significantly less expensive.
      Thank you again!

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