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Old 03-27-2013, 05:09 PM   #1
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vapor barrier question


Hi,

I keep getting different answers and I see people do this differently on tv so Im going to ask here. When installing a vapor barrier in the basement do you install it against the foundation wall before framing or after?

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Old 03-27-2013, 05:47 PM   #2
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vapor barrier question


Moved to Building and Construction forum.

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Old 03-28-2013, 06:33 AM   #3
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if you edit your profile to include your location you'll get better responses to your question
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Old 03-28-2013, 06:46 AM   #4
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vapor barrier question


As GB suggests, it depends.

In a cool, temperate climate, the v.b. should normally be on the warm side of the insulation; this is to stop internal water vapour getting to the outer layers of insulation, then condensing, and wetting it.

In areas with high humidity, it's usually the other way round, ie v.b. on the cool side of the insulation.

Maybe consult your local Code/Building Inspector - they will have local knowledge of what works best.
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Old 03-28-2013, 06:58 AM   #5
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if you edit your profile to include your location you'll get better responses to your question
how do I edit my profile?
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Old 03-28-2013, 06:59 AM   #6
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As GB suggests, it depends.

In a cool, temperate climate, the v.b. should normally be on the warm side of the insulation; this is to stop internal water vapour getting to the outer layers of insulation, then condensing, and wetting it.

In areas with high humidity, it's usually the other way round, ie v.b. on the cool side of the insulation.

Maybe consult your local Code/Building Inspector - they will have local knowledge of what works best.
I understand that but how come I see some people putting the vapor barrier right up against the foundation walls? The point is to stop vapor from reaching your insulation and framing.
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Old 03-28-2013, 12:38 PM   #7
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vapor barrier question


if you see your name in the upper right of your screen click on it and it will take you to where you can edit your location

just because you see something done doesn't mean it was done correctly
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Old 03-28-2013, 12:57 PM   #8
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vapor barrier question


The rule of thumb with vapor barriers and vapor retarders is "When in Doubt, Leave it Out."

There is a difference between vapor pressure, and just plain ole moisture migration due to capillary action. In basements, the best approach is usually to Thoroseal the block walls if you have a problem. The slab floor should already have a Viscreen (Poly) barrier below, to limit moisture migration.

I would do the above and forget about a separate barrier on the framed walls, because moisture in vapor form is going to travel throughout the basement anyway, as there is no reasonable way to stop it. You can use kraft paper faced batts between the studs to limit heat transfer, but that's all I would do.

Any question that relates to vapor drive has an "It Depends" answer, just like your questions regarding foundations and footings. There is no simple, one size fits all answer. You do the math, and go with what the figures tell you. Liars figure, but figures don't lie.
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Old 03-28-2013, 01:05 PM   #9
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The rule of thumb with vapor barriers and vapor retarders is "When in Doubt, Leave it Out."

There is a difference between vapor pressure, and just plain ole moisture migration due to capillary action. In basements, the best approach is usually to Thoroseal the block walls if you have a problem. The slab floor should already have a Viscreen (Poly) barrier below, to limit moisture migration.

I would do the above and forget about a separate barrier on the framed walls, because moisture in vapor form is going to travel throughout the basement anyway, as there is no reasonable way to stop it. You can use kraft paper faced batts between the studs to limit heat transfer, but that's all I would do.

Any question that relates to vapor drive has an "It Depends" answer, just like your questions regarding foundations and footings. There is no simple, one size fits all answer. You do the math, and go with what the figures tell you. Liars figure, but figures don't lie.
Its a straight forward question. In the basements. Im in Michigan. Does the vapor barrier go on the cold side against the foundation wall or after the insulation and framing?
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Old 03-28-2013, 01:10 PM   #10
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Originally Posted by jagans View Post
The rule of thumb with vapor barriers and vapor retarders is "When in Doubt, Leave it Out."

There is a difference between vapor pressure, and just plain ole moisture migration due to capillary action. In basements, the best approach is usually to Thoroseal the block walls if you have a problem. The slab floor should already have a Viscreen (Poly) barrier below, to limit moisture migration.

I would do the above and forget about a separate barrier on the framed walls, because moisture in vapor form is going to travel throughout the basement anyway, as there is no reasonable way to stop it. You can use kraft paper faced batts between the studs to limit heat transfer, but that's all I would do.

Any question that relates to vapor drive has an "It Depends" answer, just like your questions regarding foundations and footings. There is no simple, one size fits all answer. You do the math, and go with what the figures tell you. Liars figure, but figures don't lie.
Also, my house was built in 1964. I noticed when I looked at the soffits that there are not perforations in them so im assuming there not vented. There are gable vents and square top ridge vents and there has never been any issue of moisture in the attic. Is this normal for an older house?
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Old 03-28-2013, 01:12 PM   #11
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how come I see some people putting the vapor barrier right up against the foundation walls? .
Because they're probably doing it wrong.
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Old 03-28-2013, 01:14 PM   #12
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Because they're probably doing it wrong.
exactly. You get my point? How would putting the poly up against the foundation wall work? The point is to keep the moisture from getting into the insulation and framing. If you put it up against the foundation your exposing the framing and insulation.
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Old 03-28-2013, 01:32 PM   #13
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vapor barrier question


If it's a foundation wall to a conditioned basement, I presume people would put the membrane directly on the inner surface of the blockwork, on the assumption that it will keep any ground-water out.

But they may be confusing the issue. To keep groundwater out, the wall would need tanking, either external or - if in a new fit-out - internal tanking, with a proper drainage sump and pump.

They would still need a v.b. on the room-side of the studs to prevent water vapour condensing within the insulation.
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Old 03-28-2013, 01:38 PM   #14
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If it's a foundation wall to a conditioned basement, I presume people would put the membrane directly on the inner surface of the blockwork, on the assumption that it will keep any ground-water out.

But they may be confusing the issue. To keep groundwater out, the wall would need tanking, either external or - if in a new fit-out - internal tanking, with a proper drainage sump and pump.

They would still need a v.b. on the room-side of the studs to prevent water vapour condensing within the insulation.
tanking? You mean weeping tile system to keep ground water out?
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Old 03-28-2013, 02:04 PM   #15
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vapor barrier question


There's no one answer to that; what you do, or your neighbour does, may be completly different from what your friend down the road may do - and all may have done the 'scientifically correct thing'...that's because local site conditions (viz. your soil conditions, your type of wall etc) have the most to do with which way the whole thing goes down, correctly. In other words, it depends.

That's why you see different methods. Tell us: what's your humidity level down in the basement now? and can you control that level with a dehumidifier without too much problem? There's a whole lot of scientific work out there that'll convince you you've got it right their way - until the next piece of work comes along to convince you of something else. Hence even inspectors aren't in agreement on this.

It's easier to get a right answer in the more pronounced geographical/climate regions - like up here in Canada, or down south; but any region that is intermediate to either of those and you'll find different opinions on it.

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