# Vapor barrier and rigid foam



## dweeter (Dec 5, 2009)

I am planning to install a new drywall ceiling over strapping over an old, badly cracked plaster ceiling, in part because tearing down the plaster and lathe would release eight inches of vermiculite into the room below.

I was thinking I would use us some leftover 3/4 pink (unfaced) rigid insulation by putting it right against the plaster before nailing in the strapping.

Is it worth adding a layer of 4 or 6 mil plastic to the bottom of that pink board before adding the strapping as a vapor barrier, or does the pink board have enough vapor retarding potential on its own? We live in a cold climate.

I don't mind adding the plastic, but if it is redundant I won't bother.


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## Windows on Wash (Aug 30, 2011)

Wheres the home? 

If the ceiling is airtight, additional vapor retarder level beyond the foam is likely unnecessary.


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## dweeter (Dec 5, 2009)

Thanks very much.

The house is in Vermont. Some significant cracking present. You can't see lathe, but you could pretty easily after a few gentle taps with the hammer.


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## Windows on Wash (Aug 30, 2011)

I would still get that vermiculite out of there if it was me. I used to be an abatement contractor so it isn't that difficult if you use common sense protocols but putting up the rigid foam and then drywall should make it plenty airtight.


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## Gary in WA (Mar 11, 2009)

Bad news: if vapor barrier was required, it would be maximum 1 Perm rating. 1" XPS has a perm rating of 1.1, so your 3/4" board (0.825) would not make it.

Good news: most attics, with 12" min. air space over the insulation do not require any additional vapor barrier. In a cathedral for your climate (Vermont); yes. 1" XPS or asphalt-faced paper FG (1 perm.) would then work. 

Gary


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## Windows on Wash (Aug 30, 2011)

I wonder if he/she already did the job. 

If the attic is vented, I don't ever see a scenario would a vapor barrier would be required. As demonstrated in several of the studies, air movement is the moisture moving vehicle that creates issues in an overwhelming majority of the cases. 

Between the rigid foam and the effectively 3 air barriers, I gotta guess there isn't any air moving.


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## Gary in WA (Mar 11, 2009)

Lol, how did I answer that old post... must have been an old bookmark I wanted to answer because no mention was made/asked of air-space above the insulation- which is the vapor barrier or not requirement. Probably because it sounds like an air barrier is sufficient to stop/slow water vapor when a vapor barrier might be code-required. Sounds like they have a vented attic, no vapor control required. You may have known that from an earlier post I missed. At least readers know some still need a vapor barrier for their attic/insulation/climate, and an air barrier will not substitute.

"As demonstrated in several of the studies, air movement is the moisture moving vehicle that creates issues in an overwhelming majority of the cases. "---- I agree and have been pushing air barriers on this forum since 2009, even before you joined..lol.; http://publicecodes.cyberregs.com/icod/iecc/2009/icod_iecc_2009_4_par026.htm

Air permeable insulation requires an air barrier (surrounding all) in walls but code is lax on ceilings and allows air movement to remove moisture- fine for moisture, lousy for heat. But rated R-value is on a six-sided box, unlike an attic, hence my push for a housewrap over it all. Get closer-to-rated R-value and prevent wind-washing, convective loops and radiant gain. 

Gary


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