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Old 10-09-2009, 04:34 AM   #1
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vapor barrier


Hello Everyon,

My question is about vapor barrier. In basement I have a concrete walls and new construction. I have a black tarr paper installed by my builder on outside of concrete walls as well. I am framing and insulating. Should my vapor barrier be on both sides or only outside.
I am from hamilton , canada

Thanks
Ranjodh

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Old 10-09-2009, 07:35 AM   #2
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Up here vapour barriers go on the inside of your basement wall, right underneath the drywall. Ideally, the basement wall will have 2" of pink polystyrene glued to it and taped, then a stud wall, then vapour barrier, then drywall.

Outside, the tar paper is just part of a waterproofing system, and channels physical drops of water down to the weeling tile and is not a vapour barrier.

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Old 10-09-2009, 12:07 PM   #3
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http://www.buildingscience.com/docum...gs?full_view=1
http://www.buildingscience.com/docum...hterm=basement
Be safe, Gary
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Old 10-09-2009, 04:06 PM   #4
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I was just checking joist areas have insulation but vapor barrier done by builder have holes in it and several places have obstructions of wires, ducts. I am thinking if I am just over-concerned about them.
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Old 10-10-2009, 07:54 AM   #5
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All it takes is a few rips, a few holes, for the vapour barrrier to be useless; if the vb is useless, then the effectiveness of the insulation is seriously compromised, almost to the point where it is also useless.

The whole purpose of vapour barriers is to prevent air movement i.e vapour from travelling from warm to cold and depositing the water it holds onto something like the outside wall.

If that happens, then you get mould and mildew problems behind the drywall. So, really the vb is perhaps the most important element of the entire wall/insulation/vb/drywall system.

Just because plastic 6mil vb is the cheapest, doesn't mean it is the least important...
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Old 10-10-2009, 01:14 PM   #6
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The barrier on the outside of the concrete walls is a moisture barrier (foundation damp-proofing), not a vapor barrier. A vapor barrier on the interior side of the wall is still a good idea if done correctly.

I think a lot of people go wrong by installing plastic as a vapor/moisture barrier on the interior side of a wall. That prevents the wall from breathing at all, and in my opinion that leads to nothing but problems down the road. You don't want to create a moist environment, or install materials that allow one to exist. If water permeates your foundation walls, your basement isn't a good canidate for finishing until the foundation is properly damp-proofed (felt paper alone is not sufficient and should never be allowed). Putting plastic on the inside will lead to mold and even materials degradation. The only vapor barrier you should need on the interior side is the kraft facing on the insulation in the wall.
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Old 10-10-2009, 03:46 PM   #7
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"Putting plastic on the inside will lead to mold and even materials degradation."
Hmmm. Partially true, IMO. Not sure, tho'

It is quite acceptable to have air movement behind a wall of the first and/or second floor of a house - and here batt insulation that the air can pass through is fine. This is because the temperature gradient i.e the temperature of the wall at different heights is more or less uniform - not from inside to outside but from top to bottom...

Not so in basements; here a portion of the wall is below grade and in severe climates like ours that temperature gradient can be quite a few degrees. Once you have a gradient, then you get a chimney effect as the warmer air at the bottom of the wall rises. Once the warmer air hits the cold wall above grade, then condensation occurs.

6mil plastic sheeting as a vb is minimum code up here and a good reason not to use it. Acceptable but not right. Spray-foam, or rigid polystyrene are modern solutions to that problem. If done in this manner, a plastic vb becomes redundant.

Of course you should mitigate moisture infiltration from the outside; but from the inside you want to stop air movement. No movement = no condensation = no mould - even with wood studs.

Hope we agree, kc!
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Old 10-10-2009, 04:28 PM   #8
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"Up here vapour barriers go on the inside of your basement wall, right underneath the drywall. Ideally, the basement wall will have 2" of pink polystyrene glued to it and taped, then a stud wall, then vapour barrier, then drywall." ----- I don't agree. I base my judgment on experience and reading Building Science, especially articles as on page 8, 12, and 16, here: http://www.buildingscienceseminars.c...w&Retrofit.pdf
Be safe, Gary
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Old 10-11-2009, 05:01 AM   #9
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Thank-you very much everyone

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