unvented cathedral ceiling and Air Krete
I live in North Carolina (zone 6) and am about to put a roof on a newly constructed small house. I propose to have a steep cathedral ceiling, unvented, consisting of (from the inside out) wood decking for ceiling, Air Krete between 10″ rafters, ply sheathing, underlayment, standing seam metal. Questions: 1) since the Air Krete is itself impermeable to vapor, is separate vapor barrier over the decking superfluous?, 2) would a “breathable” roof underlayment work better than impermeable so as not to trap the sheathing between 2 impermeable surfaces? and 3) since walls are clay/straw which are supposedly vapor permeable (rel. humidity equilibrates inside and out) will I have condensation problems above the decking?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Anton,
What makes you think that AirKrete is impermeable to vapor? The last time I called R. Keene Christopher at AirKrete, he said that the product had never been tested for permeance, and no perm ratings are available. Since the cured product is crumbly and cementitious, I imagine it is highly permeable.
Anton,
AirKrete's own AIA specification guide specifically states that a minimum 6 mil vapor barrier is to be used with their product. It is in paragraph 2.2, sub-section C found at this link: http://www.airkrete.com/pdf/072101specification.pdf
So you might be able to count on it as an AIR barrier, but certainly not as a VAPOR barrier. And based on concerns in numerous reviews about post-cure shrinkage combined with the fact that it doesn't actually bond to the cavity surfaces, I'm not sure how much I would depend on it to be the primary air barrier either.
Given the cost per board-foot installed, you may be able to get a better application with the same R-values by foaming polyiso board in place instead. This would act as a vapor barrier and won't be subject to shrinkage concerns. You can also achieve the same (approx) value in ~5" of space vs. the 9.5" for AirKrete which would leave you about 4" of space behind your wood ceiling in the event that you needed to fish wires (or anything else) at a later date.
Granted, AirKrete has some nice features re: fire resistance, non-toxic ingredients, chemical sensitivity, etc. so that may play into your decision as well. But if that's the case, then you'll need to rethink your envelope performance profile regarding vapor movement.
Anton,
As others have indicated, AirKrete is not vapor impermeable and is likely highly permeable to water vapor. For that reason, and particularly because you're choosing a wood ceiling and an unvented roof (a triply bad combination, made even worse by an impermeable roofing membrane), you absolutely need a perfectly-sealed interior air/vapor barrier with no penetrations.
Given the difficulty in achieving that, and given that your walls are clay/staw (is that clay bales or light clay slip straw infill?), I would suggest a fully vented roof with dense-pack cellulose installed behind a taped-drywall air barrier over which you can install wooden ceiling boards.
With such a roof/ceiling assembly, your roof will perform similarly to your walls: hygroscopic and breatheable and able to dry in both directions. Most straw-bale builders here in New England complement their bale walls with cellulose ceilings - it's the perfect match, since they are both natural cellulosic materials with similar performance and similar low ecological footprint.