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Old 08-06-2011, 07:06 PM   #1
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Vapor barrier


IM replacing my vapor barrier. I went to buy some plastic and the sales person told me putting plastic on the ground has been superseded. He told me that the the preferred method is now to staple the plastic to the floor joists putting a barrier between the air in the crawlspace and the air in the house.

I like the sound of this (have allergies) but cant find any info supporting or contradicting.

Any input?

Thanks

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Old 08-06-2011, 09:31 PM   #2
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No plastic on joist bottoms, only the ground (dirt). Housewrap on joist bottoms.

Where are you located?

And, welcome to the forum!

Gary

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Old 08-06-2011, 09:31 PM   #3
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Putting VB on the "outside" wall contradicts the physics of the VB itself. The VB is supposed to act as a barrier to moisture as it leaves the "insulated" or "warm" side of the structure.

WINTER SCENARIO:

As this moisture escapes the interior, (warm side) it comes into contact with exceedingly cooler air (cold side) until the dewpoint is reached and the moisture condenses into liquid water and causes the usual damage that excessive moisture causes! Its the exact same principal as warm air rising in the atmosphere and condensing into a cloud at altitude until the point where water vapor content becomes heavy enough to fall back to earth.

Installing the VB to the floor joist will trap moisture between the floor and the VB. Effectively negating the R-value of the insulation, depending on the moisture content.

SUMMER SCENARIO:

Warm moist air from the crawlspace permeates through the insulation to the floor, but the VB (installed on the floor side of the joist) prevents direct contact with the relatively cooler air (via the A/C) of the space on the interior side of the floor.

Although moisture content would be present and measurable with humidity indicators, it would likely not be enough to condense into liquid moisture. The problem is usually more pronounced in the winter scenario.

Again, the whole idea is to keep the insulation dry and void of moisture so as to retain its rated R-value. Damp moisture laden insulation no longer functions as insulation!

The purpose of the Vapor Retarder on insulation is it's own self defense mechanism against too high a moisture content in the insulation. It "retards" ~ reduces the available moisture content leaving the interior so as to reduce the possibility of that moisture content from condensing into liquid water.

The best thing anyone with a crawlspace can do is to button up and completely close off the crawlspace from the outside air. In the "old days", it was thought that crawlspace venting was a good way to "let the moist air out"! Scientist have now researched and verified that in fact it makes the matter worse and simply lets more and more moisture in. I have went through extensive consultation with a leading foundation/water proofing company (Woods Basement Systems) about this topic, along with my own self education through various organizations online.

I'll attach a couple of PDFs on VBs and crawlspace venting. There are more websites too on the PDFs.

One bit of proof of this is ~

My ductwork use to drip profusely from the cold metal duct in contact with the warm humid air in the crawlspace. Since I closed off the vents, the condensation has been reduced to at least half of what it was. I still need to get the ducts off the joist and insulate them to fully alleviate the problem.

In any event, the concept or a VB is primarily driven by climate in a particular geography. Areas with milder less humid winters suggest a VB is not needed.



Nothing inherently against salesman but their generally there to sell, NOT educate.
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Old 08-06-2011, 09:48 PM   #4
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Thanks very much on the input. I live in Atlanta where the humidity is above 90% more often than not. Lots of Radon in the area too.

I think I will seal off the crawl space. I also like the house wrap idea. Would really like something between the house and the crawl space. Too many afternoons I come in and the house smell musty.

Thanks again
Tom
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Old 08-06-2011, 10:00 PM   #5
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I tried to post some PDFs but they were too big! Ahhh ~ Technology!

Anyway, here's a link that's probably better than the PDFs.

http://www.inspectapedia.com/Energy/...r_Location.htm

BTW, I would also get the dirt floor of the crawlspace covered ASAP as well.

This is all a hot topic on my end here as I'm in the process of doing this as well. Covering the crawlspace floor and sealing the vents etc will make a big difference in the air quality!
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Old 08-06-2011, 10:05 PM   #6
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Thanks again. Very helpful info.
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Old 08-09-2011, 02:56 PM   #7
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We had that musty smell in a cabin of ours with a dirt floor crawlspace. We covered the dirt floor with heavy black plastic and sealed it around the edges (concrete block) and it made a huge difference. We live in PA so it's not quite so humid.

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