Comments on: Vapor Retarder? Vapor Barrier? Perms? What the Heck?!

By: Eric Koleda

Eric Koleda — Thu, 24 Jul 2014 19:58:17 +0000

Insulating basement poured 8
Insulating basement poured 8" concrete walls to finish my basement. Looking at John Mansfield AP Foil Faced Polyiso Foam Sheathing 2"'x4'x8' boards to cover insulation (R13) plus vapor retarder and radiant heat. Will the Polyiso board installation directly to the concrete wall create any water or vapor issues once the framed wall is covered with dry wall?

By: carole davenport

carole davenport — Mon, 21 Jul 2014 10:38:03 +0000

This is a thorny problem.
This is a thorny problem. Would someone address the issue of roof membranes and whether an ice water/shield over the entire rrof deck makes good sense, or does the use of a vapor permeable membrane, v.z., DeckGuard in combo with the ice shield on eaves make sense? Is not an asphalt shingle vapor impermeable? What about the wood or plywood deck?

By: Michael Anschel

Michael Anschel — Tue, 24 Apr 2012 20:48:37 +0000

I love this subject! One of
I love this subject! One of the greatest failings of the ICC was to properly develop a code with any kind of understanding of material properties as it relates to vapor drive, diffusion, adsorption, etc.

The barrier concept, was a tragedy. However when the change was made to retarder, no requirement to remove the barrier materials (poly) was made.

Even worse, there is still no discussion about adsorption at all, and that is a fundamental part of the equation. It is of particular concern when we talk about remodeling and start swapping out a portion of a component in an assembly. 150 SF of plaster and lathe is removed and replaced with gyp-board, somewhere else some redwood sheathing is replaced with plywood. Pretty soon the performance characteristics of the home have changed enough that the tolerance of the system is weakened, and failures begin to occur. Was it a vapor retarder problem or a materials properties limit?

Nothing is ever simple, not in nature at any rate.

;-)

By: Allison Bailes

Allison Bailes — Tue, 24 Apr 2012 17:54:13 +0000

Greg LV:
Greg LV: Thanks for the tip on the variable perm materials. We have pretty much the same problem as you here in my part of climate zone 3. Also, thanks for elucidating the other side of vapor & air barriers. House wraps and taped sheathing have high perm ratings (15-50) and are definitely not vapor retarders. The OSB or plywood that they cover has much lower perm ratings, with 1/2" OSB being a class II vapor retarder and not too far from being class I.

By: Greg La Vardera

Greg La Vardera — Tue, 24 Apr 2012 16:53:13 +0000

Paul,

Paul,

An air barrier will not necessarily be a vapor retarder/barrier. But a vapor retarder/barrier will almost always be an air barrier as well.

Hence the futility of faced batts, and the confusion wrought by house wraps, and now taped sheathing...

By: Greg La Vardera

Greg La Vardera — Tue, 24 Apr 2012 16:48:54 +0000

Its worth at least mentioning
Its worth at least mentioning variable permeability membranes that are now coming on to the market - clearly the way forward, especially for zones like 4 which has such a wide range of conditions, including humid summers and cold winters in the same locations.

By: Allison Bailes

Allison Bailes — Tue, 24 Apr 2012 05:14:02 +0000

Paul P.:
Paul P.: Well, I'm glad I wasn't able to respond more quickly then. It sounds like you went and got a much more detailed answer than I would've given you.

For those who don't want to go to the detailed source, the quick answer is that infiltration can allow a lot more water vapor through than does diffusion, so air sealing is the first priority in keeping water vapor out of building assemblies. And yes, most vapor barrier materials are also good air barriers, when sealed at the edges and penetrations. Diffusion and infiltration are different processes, though.

By: Paul Price

Paul Price — Tue, 24 Apr 2012 02:55:20 +0000

Having just read the
Having just read the reference in Lstiburek by RL Quirouette (ignoring the Canadian VB on warm side preference) I am now much clearer.
http://www.nrc-cnrc.gc.ca/obj/irc/doc/pubs/bpn/54_e.pdf 

Thanks for spurring this reading!

By: Paul Price

Paul Price — Tue, 24 Apr 2012 01:23:16 +0000

Is permeability related to
Is permeability related to 'air barrierness' too? Or can very vapour permeable materials be air barriers?

I am confused on this. Maybe it is obvious, just not to me.

By: Allison Bailes

Allison Bailes — Mon, 23 Apr 2012 20:15:54 +0000

John P.:
John P.: Thanks! No, the photo at the top is a house I did some consulting on a few years ago. It's a modular home in Marietta that was, I believe, certified as LEED Platinum. The Green Curmudgeon (aka Carl Seville) did the HERS rating and LEED certification.