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JohnPeters Posted: Feb 02, 2011 03:18 PM	Poly sheet over OC in climate zone 5 On a fairly regular basis I am having to tend with architects who spec poly sheeting over OC in exterior wall details. I can typically sway them away from going down this road, but it is typically a hard fought battle. Does anyone have any documentation that I can fling in their face the next time this topic comes up? Manufacturers have a hands off approach with this topic - "observe local regulations in your municipality". Well, local regulations are designed with fiberglass in mind and as such a vapor retarder is spec'd for the warm side of the wall. That is great for FG but not for foam. And why architects spec a vapor barrier sheet of poly instead of a vapor permeable membrane is beyond me. Code actually calls for a class II vapor retarder in climate zone 5, not a VB. Further more, by the time the blueboard is plastered, primed and painted a class II to III vapor retarder is already in place. All the more reason a sheet of poly is not necessary. Looking forward to hearing some feedback on this.
mason Posted: Feb 02, 2011 03:55 PM	Check out moisture control articles at buildingscience.com. Many of them were written by Joe Lstiburek who wrote the code change identifying the class 1, II and III vapor retarders in assemblies. They specifically note what type of materials are suitable for a vapor retarder over class III vapor retarders such as open cell foam.
Roger Morrison Posted: Feb 06, 2011 02:45 PM	I have an article on my website discussing this issue. Check out "Cold Climate Recommendations" on the "articles" page of deer-ridge-consulting dot com. Roger
John Shockney Posted: Feb 06, 2011 03:36 PM	I know that the big “D” had an ICC ESS report that said they didn’t need a vapor retarder over 3.5 inches of open cell foam up to I believe zone 7. Airpro
JohnPeters Posted: Feb 06, 2011 04:37 PM	By big "D", I am assuming you mean Demilec. On a side note, as anyone ever heard of recommendations to use spray applied vapor retarders instead of poly sheeting? I have never priced out this process, but I would assume by the time its all said and done closed cell would be the price competitive and better performing option. Thanks for the feedback everyone. JP

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Comments

JohnPeters

Posted: Feb 02, 2011 03:18 PM

Poly sheet over OC in climate zone 5

On a fairly regular basis I am having to tend with architects who spec poly sheeting over OC in exterior wall details. I can typically sway them away from going down this road, but it is typically a hard fought battle. Does anyone have any documentation that I can fling in their face the next time this topic comes up?

Manufacturers have a hands off approach with this topic - "observe local regulations in your municipality". Well, local regulations are designed with fiberglass in mind and as such a vapor retarder is spec'd for the warm side of the wall. That is great for FG but not for foam. And why architects spec a vapor barrier sheet of poly instead of a vapor permeable membrane is beyond me. Code actually calls for a class II vapor retarder in climate zone 5, not a VB.

Further more, by the time the blueboard is plastered, primed and painted a class II to III vapor retarder is already in place. All the more reason a sheet of poly is not necessary.

Looking forward to hearing some feedback on this.

mason

Posted: Feb 02, 2011 03:55 PM

Check out moisture control articles at buildingscience.com. Many of them were written by Joe Lstiburek who wrote the code change identifying the class 1, II and III vapor retarders in assemblies. They specifically note what type of materials are suitable for a vapor retarder over class III vapor retarders such as open cell foam.

Roger Morrison

Posted: Feb 06, 2011 02:45 PM

I have an article on my website discussing this issue. Check out "Cold Climate Recommendations" on the "articles" page of deer-ridge-consulting dot com.

Roger

John Shockney

Posted: Feb 06, 2011 03:36 PM

I know that the big “D” had an ICC ESS report that said they didn’t need a vapor retarder over 3.5 inches of open cell foam up to I believe zone 7.

Airpro

JohnPeters

Posted: Feb 06, 2011 04:37 PM

By big "D", I am assuming you mean Demilec.

On a side note, as anyone ever heard of recommendations to use spray applied vapor retarders instead of poly sheeting? I have never priced out this process, but I would assume by the time its all said and done closed cell would be the price competitive and better performing option.

Thanks for the feedback everyone.

JP

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