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Dense-Pack Cellulose Cathedral Ceiling

My roof (zone 3B) needs insulation, but between Martin's concerns and the inexperience of the contractors in this market, I am conceding that 5" of polyiso foam isn't going to happen. I think that 3.0" nail base (R15) can though (with taped seams). I'd like to get closer to the code minimum of R30 for the assembly. I would rather not tear out the finished interior vaulted ceiling (cedar shingles on battens on felt paper, I know, strange), but the lousy interior air barrier (i.e. the felt paper) concerns me. Given that 4" of dense-pack cellulose is air permeable, how big of moisture risk would I be taking in this climate with this assembly?

Asked by Keith Richtman
Posted Thu, 08/14/2014 - 14:44


3 Answers

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You're talking about putting R15 continuous over the roof deck, and then re-roofing over that? If so, you should be fine--do the exterior work first, including the roof cover, and then have an insulator strategically remove shingles on the interior side and dense-pack the rafters. If there is currently venting at the eaves you might need to close it off.

Answered by David Meiland
Posted Thu, 08/14/2014 - 16:04

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In a zone 3 climate you need to have about 13% of the total center-cavity R to be exterior to the roof deck for dew point control if you had an air-tight 5-perm interior. Doubling or more that would prudent if the interior side can't be made air-tight. If the interior tar-paper is in pretty good shape and only torn or leaking in a few spots (rather than falling apart, crumbling into flakes everywhere) it will be a sufficient vapor retarder once you dense pack it, so you may not even need to double it for dew point control. (#15 felt runs under 5 perms when dry.)

At 4" true depth(as opposed to a milled 2x4, which is 3.5") the most you'll get out of dense-packed cellulose is about R15. A 3" nailbase will probably be OK, but know that you're going to be relying a bit on the buffering capacity of the cellulose (which is fine- it works.) It's lower risk in zone 3B than it would be in 3A, but it would probably still work in 3A.

IRC 2012 code minimum for zone 3 is R38, but that is for insulation thermally bridged by joists/rafters. The performance of your continuous nailbase at R30 total center-cavity R will be comparable or better performance than a well-implemented R38 joist/rafter batt-job.

Answered by Dana Dorsett
Posted Thu, 08/14/2014 - 17:13

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Thanks guys!

Answered by Keith Richtman
Posted Thu, 08/14/2014 - 18:03

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