[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: GBlist: vapor permeability



Hi, Frazer--

When the insulators are hanging poly as a "vapour retarder" they just
staple it up with some overlapping, but no taping or sealing, since it
doesn't have to be an air barrier to meet code. It's much quicker than
stapling all those paper flanges, and there's no chance of the paper
bunching up and distressing the drywall hangers.

The code does not require a vapor retarder in an attic ceiling because of
former misapprehensions about moisture, and because the code committee
believed that the attic ventilation would be sufficient to remove diffused
moisture (since it apparently removes moisture transported by air leaks).
Heck, our code still permits bath fan ducts to be hung near a roof jack
instead of positively vented out. Why? This is Oregon; mold is our friend
and companion!

Mike

>Mike see my correction by Marc Rosenbaum -
>
>I too was under the assumption that icynene was a VR, since I was told that
>once it is scrubbed, it looses its perm sealing abilities. I have also been
>told that if a cavity is not filled completely on the first go around that
>one cannot go back & fill in - it won't stick to itself.
>
>We can debate the various merits of the material, be it green, cheap, or
>recycled - but like so much of the insulation business, the thermal quality
>and effectiveness is in the hands of the installer.
>
>Anyway - the perm issue:
>
>The 1 perm or pelow seems to be standard - I believe FHA requires a max of 1
>perm on the warm side of both wall (except unfurred masonry) and ceilings
>and that is the rating for kraft faced f/g batts. If you remember we used to
>call it a 'vapor barrier' and it was some Canadians that taught us it was
>only a 'vapour retarder' - we may all have to pop for Lstiburek's book or
>get him to put it up on the web.
>
>I'm surprised that you all don't have a ceiling VR requirement, one might
>think that might be where a high amount of the mositure might get to. Maybe
>out there you don't have a stack effect ... And are your f/g guys taping w/
>expensive tape or caulking with messy caulk or lapping & by how much the
>poly ?
>
>best,
>--------------------------------------------------
>Frazer Dougherty                doughf@mindspring.com
>North Fork Retrofit             doughf@aol.com (fallback)
>website: http://www.home-performance.org
>
>
>
>__________________________________________________________________
>This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
>and Environmental Building News (www.ebuild.com). For instructions
>send e-mail to greenbuilding-request@crest.org.
>__________________________________________________________________


O'Brien & Associates
Environmental Building Consultants
Portland General Electric Earth Smart program
obrien@hevanet.com


__________________________________________________________________
This greenbuilding dialogue is sponsored by Oikos (www.oikos.com)
and Environmental Building News (www.ebuild.com). For instructions
send e-mail to greenbuilding-request@crest.org.
__________________________________________________________________


References: