# Vapour barrier



## iminaquagmire (Jul 10, 2010)

Rigid insulation acts as the vapor barrier.


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## ccarlisle (Jul 2, 2008)

Does not...ONLY if it is firmly attached to the basement wall and is taped and sealed along the joints top and bottom will it act as a "vapour barrier", otherwise it doesn't.

The purpose of a vapour barrier is to prevent vapour from leaving a warm airspace and reaching a cold airspace, where it will condense; even if you fully cover the wall with rigid insulation - but leave a gap or don't bother to seal the joints - it is useless as a vapour barrier.

You say you have rigid insulation between the studs...that system is useless although it does provide heat insulation. Now whether or not you need a 'vapour barrier' depends a lot on where you are.


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## AGWhitehouse (Jul 1, 2011)

ccarlisle said:


> ONLY if it is ... taped and sealed along the joints top and bottom


...and is of appropriate thickness. XPS at 1" (1.5 perms) is considered a class III (1.0<=10 perms) retarder. Class II (0.1<=1.0 perms) is required in CT. Don't know where the OP is...


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## AGWhitehouse (Jul 1, 2011)

ccarlisle said:


> even if you fully cover the wall with rigid insulation - but leave a gap or don't bother to seal the joints - it is useless as a vapour barrier.


This is not entirely true as a barrier's effectiveness is area weighted. If you had a 100 sq. ft. "barrier" layer with 10 sq. ft. of holes in it then it would still block 90% of the moisture drive (hypothetically speaking as permeability of the barrier layer and air pressure would change that % slightly).

So while the OP installation isn't a 100% effective barrier, it's not totally useless.

Referenced from a GBR in WA post on 8-29-11: "The insulation manufacturer claims it is area weighed--- look for “Should the joints be taped?”, almost ½ way down the page; http://www.insulating-products.com/pdfs/Installation_FAQ.pdf"


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## federer (Aug 20, 2010)

what about vapor barrier in terms of spray foam to roof decking and drywall ceiling


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## AGWhitehouse (Jul 1, 2011)

federer said:


> what about vapor barrier in terms of spray foam to roof decking and drywall ceiling


Depending on the foam cell structure (open or closed), denisty (lbs.), and thickness spray foam can be installed to be both an insulative barrier and a vapor barrier.

Open celled foams are not a vapor retarder, no matter the thickness or density. Closed cell, 2.0lbs, sprayed at 2" thickness is considered a class II vapor retarder.


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## federer (Aug 20, 2010)

AGWhitehouse said:


> Depending on the foam cell structure (open or closed), denisty (lbs.), and thickness spray foam can be installed to be both an insulative barrier and a vapor barrier.
> 
> Open celled foams are not a vapor retarder, no matter the thickness or density. Closed cell, 2.0lbs, sprayed at 2" thickness is considered a class II vapor retarder.


thanks for the info. so for someone in my region (mid atlantic) do i need to put in a vapor barrier if i am spraying open cell foam to underneath roof decking, followed by drywall ceiling? there will be a slight gap of empty space between drywall and the foam


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## Windows on Wash (Aug 30, 2011)

federer said:


> thanks for the info. so for someone in my region (mid atlantic) do i need to put in a vapor barrier if i am spraying open cell foam to underneath roof decking, followed by drywall ceiling? there will be a slight gap of empty space between drywall and the foam


Not needed in our area. Painted drywall that is air tight is more than enough for this region.


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## federer (Aug 20, 2010)

Windows on Wash said:


> Not needed in our area. Painted drywall that is air tight is more than enough for this region.


thanks for reiterating the info. can you please explain the reasoning behind this?


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## Windows on Wash (Aug 30, 2011)

We don't require vapor retarder in this region. We are a pretty blended climate and vapor barrier are more necessary as you get colder and head North.

We don't have the extremes of temperature that would create condensation issues as it pertains to vapor diffusion under normal circumstances. 

Well sealed (i.e. air tight) drywall with a layer of primer and paint is quite sufficient for this region.


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## federer (Aug 20, 2010)

thanks for the detailed info!


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