0 Helpful?

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


3 Answers

Select your preferred way to display the comments and click "Save settings" to activate your changes.

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

Other Questions in Green building techniques

Non-typical insulation of barn / garage

In Energy efficiency and durability | Asked by James Williamson | Mar 22, 15

Fresh air distribution for hydronically heated house?

In Mechanicals | Asked by John Charlesworth | Mar 26, 15

Insulating scissor truss

In Energy efficiency and durability | Asked by Tom Smith | Jul 19, 14

Attic insulation reality check

In Energy efficiency and durability | Asked by Nathaniel G | Mar 26, 15

Old house, wet brick

In Energy efficiency and durability | Asked by Chris Ermides | Mar 26, 15
Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!