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Interior roof/ceiling rigid insulation

user-993901 | Posted in Energy Efficiency and Durability on

I am considering installing 1″ xps and new drywall on the interior of the second story roof/ceiling slope, over the rafters. Our climate requires a .5 perm vapor retarder so I have a couple of concerns. 1.) The existing condition has 2×4 rafters with R-13ish faced insulation with no circulating air channel. It has seemingly done fine over its 85 years moisture wise (although it gets stifling hot up there in mid summer) so by adding the rigid and retarder am I changing the assemblies nature enough to cause problems (moisture) that dont presently exist? 2.) have you had any problems with the rigid insulation compressing and causing nails to pop or joints to crack and would 1×3 strapping help this in any way?


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  1. jklingel | | #1

    I'm not a pro, but I'd not seal that foil up this way. I suspect you've had no problems because of air leaks, and if you close those off, you'll have trouble. Do a search here and on Spray foam may be what you are seeking. It sounds like this is not an attic??? Where are you, too?

  2. GBA Editor
    Martin Holladay | | #2

    I disagree with John Klingel. I have no idea what kind of trouble he is anticipating -- perhaps inward solar vapor drive during the months when air conditioning is turned on? That's not going to happen -- so what type of trouble are you thinking about, John?

    Anyway, Brian: it would be helpful to know your climate. But your plan is fine. You can install gypsum drywall after you put up the new foam; it's possible to screw right through the foam into the rafters. I've done it. Some drywallers don't like screwing through foam, however, and prefer strapping over the foam. But if you are worried about reducing the ceiling height, you can skip the strapping.

    The basic problem with your roof assembly, as you correctly note, is that there is no ventilation channel between the insulation and the roof sheathing. I'm taking your word for it that there is no existing moisture problem. Adding the foam as you propose will lower the risk of a future problem rather than increase the risk.

  3. user-993901 | | #3

    thank you for the response. I'm in Portland, Oregon where I'll need a 1 perm vapor retarder not the old .5 I scribed earlier. I think I'll give it a go and see if I can upgrade from a stifling upstairs to just a plain uncomfortable one.
    thanks again

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