How to properly air seal and insulate a cathedral ceiling composed of structural T&G cedar
This is an existing roof structure currently experiencing moisture problems most likely due to air leakage. This roof is in Winnipeg, Manitoba Canada, a very cold climate. Right now it has approximately 4″ of EPS foam and tarpaper. One of the primary reason to address this roof is that there has been significant carpenter ant activity over the last few years. Would air sealing with caulking on the inside be enough of an air seal? (the cedar is the interior finished ceiling as well). A much more expensive approach that has been suggested is to strip the entire roof – apply a peel and stick membrane to the entire roof deck for a continuous air seal, then 10″ of polyiso insulation in 2″ layers for an R-65, then plywood sheathing and conventional shingles after that. Any thoughts or suggestions?
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Jon,
Clearly, the expensive option is the best approach.
I agree that it isn't cheap. But if you have carpenter ants, you clearly have a moisture problem -- one that will only get worse with halfway measures.
We've been asked several times to build homes in B.C. with structural T&G exposed room-side. In every case the roofs were detailed as your *expensive* option suggests: moisture barrier on top of the T&G.
A few thoughts:
• Air barrier continuity from roof to eves and along top of rake walls may be difficult to execute in a retrofit situation, but if it's not paid attention to, you'll just move the problem.
• Ants may be in your foam because they find it to be a comfy place to set up shop. May not have anything to do with moisture. Depending on how big the colony is and how much tunnelling they've done in the foam, you may find it preferable to rid yourself of ants from the room-side by drilling holes and injecting a foam-friendly ant killer. We've encountered ants in EPS roofs a few times where there was no water infiltration and this technique surely worked. Use a stethoscope to find the buggers. If you can get your hands on a thermal imager, so much the better. One client chose to inject expanding foam into the ant gallery after they'd left.
• If you do strip the roof down to the T&G, consider nail-base insulation as a replacement: typically EPS laminated to OSB. What's called a 12" nail base panel has 11 13/8" of EPS core, which gets you ~ R 43, and screws to the decking with 13" long SIP screws & washers. 11 3/8" core allows you to trim rakes and eves with a 2X12. By using peel-n-stick on top of the T&G, screw penetrations aren't an issue for the moisture barrier. Layering up thinner sheets of insulation is difficult to detail: most times I see this the designer calls for a grid of framing on top of the T&G decking, which provides fastening for the roof sheathing, but also interrupts insulation.