Air-Sealing Nail Holes in Existing Roof
I am building an unvented roof in CZ5 on a retrofit/existing roof. The shingles were stripped off – which left holes in the sheathing (rookie mistake). I didn’t have anything to seal the holes and I went ahead and had the roofer tape the sheathing seams and install a mento 3000 membrane.
Question: Is it worth it to air seal the holes from inside ? The scissor truss ceiling/attic is currently all open, so relatively easy to access. All of the fluffy insulation will be in the rafter bays. Since the old roof is 1 1/2″ lower than my new roof addition, on the old roof I will have 5 1/2″ of polyiso, which should keep the sheathing warm.
2nd question. With the attic being conditioned, how important is it to air seal at the vaulted ceiling? . I plan to reinstall a drywall ceiling (vaulted) and it will have some recessed lights in the kitchen. I will have an air handler / duct work up there too. Maybe it would be best to go with open cell in the rafter bays – I believe open cell acts as an air barrier
Thanks, Mike
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
I’m giving your question a bump but I suspect Martin answers your question with “I would say you can ignore the fastener penetrations, since the amount of air leakage associated with these penetrations is minor.” You can read the rest of his comments on a similar question here.
Kiley,
Thank you for the bump and referenced link. However, I think the linked question/article is referring to fasteners that are present, whereas I am referring to nails that have been removed and therefore have a hole (with no fastener). I'll edit my post so that it's more clear.
Mike
Hi Mike,
Quick FYI: I plan to share your situation and questions on the FHB podcast, should you want to hear what folks on that team have to say--it's likely listeners, many of whom are pros, will write in with suggestions too. (Episode 497, and possible feedback on 498 and/or 499)
Hi Kiley - that's awesome. Thank you for letting me know. I'll be sure to tune in.
I don't have any numbers to back this up, so this is mostly hand wavy.
You'll be installing the rigid plus a new deck over the existing roof and fastening it down with a bunch of big screws. This should do a pretty good job of clamping the mento down against the roof deck, I can't see those holes leaking much air. As an extra precaution you can also tape the seams of the mento. If you have some extra time, you can also pick up a pale of duct seal mastic and brush the holes on the inside as a final piece of mind.
A hybrid insulated roof assembly still needs a reasonable warm side air barrier plus a minimum of class III vapor retarder (ie painted drywall). A couple of lights here and there won't matter much. If you want to put ducting above the ceiling but under the fluffy, I would go with spray foam in those rafter bays.
You can skip any air barrier and vapor barrier but the fluffy/exterior rigid ratio changes, you'll need much more rigid for condensation control.
Open cell is about 30-50 perm/inch, so you need 5"plus of it to be a class III vapor retarder if you want to go with open cell only and no drywall.
AKos - Thanks again for your feedback. I'll have 5 1/2" of polyiso on the existing roof since it's 1 1/2" lower than the new roof/ addition (which will get 4"). It looks like I can entirely fill the 5 1/2" rafter bays with open cell. This would put the total assembly at approx R53 with the exterior accounting for approx 60%.
If I'm understanding you correctly, you are saying is that if I fill the rafter bays with open cell, this will act as my interior air & vapor barrier, correct ? And I'm guessing the open cell would further solve my nail holes concern
Last question : with this method, I don't think I have to be concerned about air sealing the vaulted ceiling below ?
thanks
Mike