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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 Aug 14, 2014 3:44 PM ET


6 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 Aug 14, 2014 5:04 PM ET


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 Aug 14, 2014 6:13 PM ET


Thanks guys!

Answered by Keith Richtman
Posted Aug 14, 2014 7:03 PM ET


Continuing the discussion from last year about the assembly described above ... The roofer I've selected would default to a 0.06 perm synthetic underlayment. That would leave no way for the OSB to dry to the exterior, and the foam limits the ability to dry to the interior. Felt paper would be a more conservative approach, right?

Answered by Keith Richtman
Posted May 22, 2015 5:18 PM ET


You didn't tell us what type of roofing you plan to install. Most types of roofing (including asphalt shingles and standing-seam metal roofing) don't allow roof sheathing to dry to the exterior. However, some types of roofing -- including cedar shingles, slate, concrete tiles, and corrugated metal roofing -- do allow outward drying.

If your chosen roofing doesn't allow any outward drying, your concerns are baseless, because you won't get any outward drying anyway.

That said, manufacturers of low-perm synthetic roofing underlayment forbid the use of their products over unvented roof assemblies. That reason alone is enough for you to tell your roofer that the wrong product has been selected. Most building codes require that all building products be installed according to manufacturers' instructions.

Asphalt felt should work fine.

Answered by Martin Holladay
Posted May 23, 2015 7:36 AM ET
Edited May 23, 2015 7:37 AM ET.


Thanks Martin. Roofing will be steel shingles, which will allow outward drying.

Answered by Keith Richtman
Posted May 23, 2015 9:58 AM ET

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