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Old 09-29-2011, 08:30 PM   #1
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Vapour barrier


we cut out the bottom 2 feet of drywall due to flooding and found rigid insulation in between the studs but no vapour barrier. Should there not be vapour barrier?

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Old 09-29-2011, 11:08 PM   #2
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Vapour barrier


Rigid insulation acts as the vapor barrier.

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Old 09-30-2011, 09:08 AM   #3
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Vapour barrier


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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Old 09-30-2011, 03:39 PM   #4
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Vapour barrier


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Originally Posted by ccarlisle View Post
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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Old 09-30-2011, 04:08 PM   #5
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Vapour barrier


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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 ˝ way down the page; http://www.insulating-products.com/pdfs/Installation_FAQ.pdf"
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Old 10-12-2011, 10:01 PM   #6
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Vapour barrier


what about vapor barrier in terms of spray foam to roof decking and drywall ceiling
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Old 10-14-2011, 10:41 AM   #7
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Vapour barrier


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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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Old 10-16-2011, 11:19 PM   #8
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Vapour barrier


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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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Old 10-16-2011, 11:43 PM   #9
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Vapour barrier


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Originally Posted by federer View Post
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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Old 10-18-2011, 12:36 AM   #10
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Vapour barrier


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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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Old 10-18-2011, 08:37 AM   #11
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Vapour barrier


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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Old 10-18-2011, 09:49 PM   #12
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Vapour barrier


thanks for the detailed info!

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