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Insulating a vented cathedral ceiling

Trying2doitright | Posted in General Questions on

Hello, my name is Adam and I live in Southern Maine. I built a Cape style second floor over a heated space below and I am planning to leave the walls above the kneewall cathedral style instead of making it flat. 

I vented the soffit and ridge. 2×10 rafters. I have read just about every article I could about insulation and venting a cathedral ceiling. I am hoping that my plan is solid. Here it is:

i intend to site build my vent chutes using 1” rigid Polyiso and then R-30 Roxul. That will take up all available space in the stud bay. I then plan to put 1” polyiso on the underside of the rafters to prevent some thermal bridging. That should give me an overall of R43 correct?

sheetrock and paint. No vapor barrier. Does this all sound ok?

final question: do I seal the vent chutes or leave them “leaky” so any vapor that does make it through will have a means of escape? if I am to seal it, can u explain what happens to vapor that manages to find its way through the ceiling?

thanks so much,

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  1. Jon_R | | #1

    Polyiso can be your Class I or II vapor barrier/retarder on the interior side. Maintain much higher perms (ie, not foil faced polyiso) on the exterior/vent side . See here for some of the recommendations:

    In terms of air sealing, best if the interior side is better sealed than the exterior side. Do this and the more air tight the better.

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