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Community and Q&A

Future Proofing a Roof

stolzberg | Posted in General Questions on

I’m planning to insulate my attic ceiling with 5″ of rigid foam in 6″ rafter bays plus 3″ continuous foil faced insulation across the rafters (taped as air barrier).  I’m leaving about a 1 inch gap behind the board sheathing, which is covered in slate (my understanding is that the gap does not need to be vented, just enough space for drying behind the slate).  My question is what happens in the future if I want to replace the slate and there’s that unvented gap between the interior insulation and the sheathing.  Would it be a problem if some day I wanted to put more insulation on top of the sheathing and/or put down plywood sheathing for metal roofing, asphalt shingles or synthetic slate tiles?

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  1. Expert Member
    Michael Maines | | #1

    Insulating on two sides of a gap like that isn't ideal because they aren't really inside or outside and it's hard to predict what will happen with moisture. It may be fine or it may condense. If the gap is open to bare sheathing or the outdoors it will also be a thermal bypass, rendering most of the value of the exterior insulation moot. Could you leave a 2" gap instead? That would be easy to fill with blown cellulose or fiberglass at a future date.

  2. user-6184358 | | #2

    Future proofing is hard sometimes. Why the concern about redoing a slate roof? Slate is a 50 to 100 year material if done correct.

    1. stolzberg | | #3

      It's a 1900 farmhouse with what I assume is the original slate roof. We just took out the chimneys and the roofer thought the existing slate was in rough shape. I've been trying to decide whether to spend about $6000 on slate repair and assume regular maintenance costs cost going forward versus biting the bullet and putting on new roof now.

      Although I can see daylight in some places through the slate, the attic is currently dry as a bone with little evidence of water intrusion. But I'm bringing the attic into the conditioned space, which is why I'm concerned about the insulation and moisture.

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