Vapor retarders for double stud cellulose-filled wall assemblies
miroland
| Posted in Building Code Questions on
Following up on last month’s blog about building codes, am I correct that the code allows interior vapor retarding with latex paint on sheet rock? Also, does the vapor permeability of ZIP sheathing prevent inner wall condensation without the use of exterior foam?
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Replies
What works and what is allowed by code is country/climate-zone dependent. Where are you?
Michael,
For information on vapor retarder requirements in the IRC and IECC, see this article: Vapor Retarders and Vapor Barriers.
Since 2007, the IECC has required (in section 402.5) that walls [with some exceptions] in climate zones 5 (e.g., Nevada, Ohio, Massachusetts), 6 (e.g., Vermont, Montana), 7 (e.g., northern Minnesota), 8 (e.g., northern Alaska), and marine zone 4 (Western Washington and Oregon) have a Class I or Class II vapor retarder — in other words, kraft facing or polyethylene.
The most important exception is for walls with an adequate thickness of continuous insulation on the exterior side of the wall sheathing.
Dana and Martin,
Thanks for your responses. I believe I understand vapor diffusion through walls, but since some of the GBA details for cellulose-filled wall cavities don't show an interior vapor retarder (and some do), I was hoping to avoid that step. Seems like it's up to the local adopted code and inspector and/or my ability to persuade.
Dana, I'm in upstate NY (Albany area), climate zone 5.
In climate zone 5 you can use standard latex paint as the interior side vapor retarder on a cellulose-filled wall if you have a vented "rainscreen" gap between the plywood or OSB or fiberboard sheathing, per IRC 2012 chapter 7:
http://publicecodes.cyberregs.com/icod/irc/2012/icod_irc_2012_7_sec002_par025.htm
This works with standard vinyl siding too (since it's inherently back ventilated) but not the foam-filled vinyl siding (unless you build in a rainscreen.)
Cellulose is fairly protective of plank sheathing even when the siding is tight to the housewrap due it's ability to take on substantial moisture without damage or loss of function, but if the sheathing is OSB it's not worth the risk to skip it. The risk to plywood is somewhat in-between.