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Is creating vapour resistant barrier on both inside and outside of a cellulose fill wall advisable?

Reading the Fine Building Mag artical 'Designed for Success' (link below) I noticed the 16" double stud wall assembly had a layer of spray foam on the exterior sheeting (couple of inches) with the remaining wall filled with dense pack cellilos and closed with a 6mm poly vapor barrier . With the closed cell spray foam and the poly rap I am assuming moister in the wall is not a concern - why? Is this wall type safe in all climate zones including extream cold - we get 8000 hdd with many days sub -40. Why isn't the due point a consern? Science based feedback would be helpful.

Thanks

Mark

http://www.finehomebuilding.com/pdf/021240050.pdf

Asked by Mark Henry
Posted Sat, 01/18/2014 - 17:21

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2 Answers

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1.
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Mark,
I don't seen any reference whatsoever to an interior polyethylene vapor barrier. Maybe you are just guessing, based on the illustration?

The interior membrane shown in the illustration is probably vapor-permeable -- either netting to hold the cellulose in place, or perhaps a "smart" (variable-permeance) vapor retarder.

Answered by Martin Holladay, GBA Advisor
Posted Sun, 01/19/2014 - 09:23

2.
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The undentified and unlabeled thin flexible layer between the wallboard and studs/insulation could be interpreted to be any number of things, including blowing-mesh.

Also, spray polyurethane foam comes in many densities & vapor retardencies. At 2lbs density most polyurethane runs about 1.2 perms @ 1", and would still be as vapor open as a kraft facer on a batt a 3" (R18-R20), and thus not a true vapor barrier. Toward the end of the text on the third page the author mentions a "flash coat of spray foam", which we can reasonably presume would be closed cell, between 1 & 2 perms.

At the end of the text (bottom of the 3rd page) they note that the author "Steve Baczek is an architect in Reading Mass.". Massachusetts is in US climate zone 5, so the limit on foam/fiber ratio that would work without an interior side vapor barrier tighter than 3 perms would be 25% foam to 75% fiber if going for dew-point control at the foam/fiber boundary, which he clearly isn't doing. But the combined vapor retardency of 3-5 perm interior paint + 1-2 perm foam is more than sufficient protection for OSB or plywood sheathing in climate zone 5. (If the siding is back-ventilated to enhance exterior drying and the it's good enough for US zone 6, even if though doesn't quite meet the letter of IRC code.)

While that stackup wouldn't cut it in muche of the Canadian midwest, northern Ontario, or most of Quebec- the flexible layer would have to be less than 1-perm (poly would be fine- so would a smart vapor retarder), it would still work with a flash-foam of closed cell on the interior side of the sheathing with cellulose fill. Air tight gypsum and "vapor barrier latex" (about 0.5 perms) can also work just fine with flash-foamed sheathing.

Answered by Dana Dorsett
Posted Mon, 01/20/2014 - 14:54

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