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Old 03-06-2013, 09:47 AM   #1
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Vapor barrier


Did I need to put a vapor barrier over my insulation prior to putting up the Sheetrock? I have never installed a plastic before but now I am seeing this in different places and I am wondering what everybody thinks? There is no evidence of water in this basement. What is the point?

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Old 03-06-2013, 10:07 AM   #2
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#1 Go back and add your location to your profile, very important that you do.
Just go up to quick links and edit info.
The point is your going to have warm moist air in the room hitting a cold moist wall and condencing without one.

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Old 03-06-2013, 08:23 PM   #3
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As Joe said, where are you located?

http://www.buildingscience.com/docum...ong-from-start

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Old 03-06-2013, 08:32 PM   #4
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Yes, you need a vapor barrior. Warm, moist air will travel through your wall where it cools and condenses. Check the codes in your area for thickness requirements, I.E. 4mill, 6 mill, etc.
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Old 03-06-2013, 09:10 PM   #5
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I think its really funny when people say yes you need a vapor barrier, and tell someone where to put it without even knowing the parameters.

I learned a long time ago in the commercial roofing business that

1. There is no perfect vapor barrier, only vapor retarders.

2. When in doubt, leave it out.

Vapor pressure moves from warm moist to cold, and the rule of thumb is to have the dew point fall in the middle of the insulation, so you dont have hot moist air hitting a cold surface, like a glass of ice water on a hot humid day.

Putting a vapor retarder in the wrong place can result in "Pumping Up" the system in question with water because it cant escape when conditions reverse, like when you turn on the air conditioning. Thats why the rule of thumb is No. 2 Above. Unless tthe conditions remain the same at all times.
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