Insulated roof question
I have a cape and have torn off the roof, added eaves and overhangs, moved the thermal boundary from the knee-walls to the gable walls and now am tweaking my approach on the roof/rafter bays.
We will be adding 3″ rigid foam to roof deck before installing aluminum shingles. To achieve something close to r-50 and a good air-seal, I could clad the underside of the rafters (in the small attics of which there are 9)with sheetrock or rigid foam and densely pack cellulose into the cavities. Having spent many days in these attics moving the thermal boundary, I am not interested in this approach because it is SO DIFFICULT to work in there and thus my question:
I want this roof assembly to be able to dry to the interior. Can I install open cell foam alone or in combination with un-faced batts, aka flash and batt, into the rafter bays in the small attics and still have the perm I need? If so, what thicknesses would achieve this?
Many Thanks!
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Replies
Robert,
Either open-cell spray foam or fiberglass batts are permeable enough to use in this application without any worries, even if you achieve a total of R-50.
谢谢马丁 -
Then I could install 6" open cell in bays and in combination with 3" rigid foam on deck achieve about R-50 and still dry to interior, right?
Robert,
I disagree with your math.
6 inches of open-cell foam = 6 x 3.7 = R-22.2
3 inches of XPS = 3 x 5 = R-15
Total R-value of the assembly = 22.2 + 15 = R-37
I mistakenly thought open cell had a higher R value. I have a 2x8 bay that if full of OC would yield R-25. At that depth, do i compromise perm to the point of concern?
As an alternate, 3 inches of OC plus an un-faced R-19 batt would yield R-30. Would this approach perform as well? I imaging it would cost a lot less. I'm not a fan of fiberglass, but would it perform better in combination with the OC?
Robert,
As I said in my first answer, any of those choices is fine. Don't worry.
If you are putting the 3" of rigid foam on top of the roof deck, does it even make sense to use foam under the roof deck. Open cell foam's R/inch isn't much different than fiberglass. Is the rigid foam on the roof not enough to seal the assembly?
Fiberglass in an 2x8 rafter should get about R-25 +/-, plus the R15 from the rigid foam would be in the R40 range and I would think would be cheaper than spraying the foam.
当我在下春天做一个屋顶更换时，我认真考虑在家里做这件事，如果我没有足够的想法从内部封入密封（或其他原因让我从下侧喷洒泡沫）。就此而言，底面上的开放式电池泡沫厚度会降低厚度的诀窍（我认为我可以利用DIY型坦克的足够小的音量，您可以购买大约600bdft并避免必须动员另一个承包商现场）。
谢谢，
Ben