I am planning on spraying polyurethane foam in my attic ceiling which will be covering up any roof venting. I will spray 1″ of foam then add fiberglass batts. I am making the attic into a bedroom. I don’t want to add roof ventilation. Do I even need it after sealing the underside of the roof with spray on foam?
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Have you investigated what will happen to your roof (shingles, decking, etc.) once you seal it with spray-in/on foam? I only ask since this is a modification to an existing roof structure.
Situation
The roofing needs to be replaced in the next 2-3 years. The sheathing is tounge & grove over 2x8 joists. The roofing is ashpalt shingles.
What could happen to the existing roof?
It seems to me that with the foam insulation all of the air infiltration problems will go away, so i won't have to worry about condensation. And the batts will keep the costs down for a well insulated attic.
Am I right in my thinking or is my thinking flawed?
You may need thicker foam
The foam has to be thick enough so that it does not reach the dewpoint on the interior side.
What climate zone are you in?
climate zone
I live near lansing, mi which I think is zone 6 or 5
Would 1in of poly and 5 1/2 in batts work?
Thanks
We did this....
Corbond below between the rafters - IIRC about 6-8 inches of the stuff. Grace Ice & water shield on the outside on top of the original sheathing (which was built for slate but the slate had been replaced by asphalt roofing at some point). We put Duraslate (i.e. plastic slate) over all that. No soffit, attic, ridge, etc. vents - this is a sealed roof with no issues.
Despite keeping the majority of old windows this 138 year-old girl is only doing about 0.20 ACH. I believe the roof sealing (i.e. making the attic a part of the conditioned space) as well as the spray foam in all walls made that possible (because I know that the windows leak a lot).
attic venting
Scrap the batts. The batts are a very bad idea. Cheaper, BUT yu get less, and enter the "risky" catagory. Don't go there!! The so-called hot roof problem is not a real issue, and many manufacturers no longer have a problem with it. Further, the foam installer is already there and set up. Dilute your set up costs by letting him do his job. You get high R value with no risk (if your installer is good). I would suggest a minimum of 4 inches of application in two sprayed lifts. The whole roof should be spayed with 2 inches, and then another 2 inches. This will allow the foam to disipate heat between coats. Another consideration of batts is that the R value is only correct if it is encapsulated, and should also be derated. It only has 50% of its R value at 0 degrees F. If you are looking forward to the longer term viewpoint (5 to 25 years of a mortgage), I would suggest 5 inches or perhaps 6 inches. Beyond that though, the return on investment will become somewhat extended. You won't regret this choice when BTU's become expensive. Pay for insulation now, or fuel forever.
open- or closed-cell foam? remember, with closed-cell foam, any water that makes it through the roofing system will collect on the cold side of the foam. this moisture could, in turn, rot out the structure. this is why some municipalities only allow open-cell, e.g., .5lbs foam, for roofing applications. i wonder if cross-strapping and ventilating the cold-side of the foam would help alleviate any moisture build-up that does occur? not an option for some retro-fits, i guess...
good luck
tirfond