vapor barrier

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Old 03-13-02, 02:31 PM
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vapor barrier

I've read reference to a vapor barrier on numerous occassions when finishing a basement. Not to sound ignorant but what exactly is it & what's its purpose? Does one need it? I recently framed two basement walls(Interior non-load bearing partition walls) & assume I don't need it for them but I'm considering covering an outside poured concrete wall & from what I've read in the forums it might be needed there.
 
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Old 03-13-02, 03:53 PM
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You can go to http://www.eren.doe.gov/consumerinfo/refbriefs/bd4.html this is U.S.Dept. of Energy brief on vapor barriers. In the first paragraph they rename vapor barriers to Vapor Diffusion Retardents (VDR). This brief was actually commissioned by the DOE and ASHREA was the one who actually wrote it. For your information, this is in part reaction by ASHREA concerning Sick Building Syndrome (SBS). That's a long story.

What VDR's or vapor barriers do is diffuse moisture before going into the insulation. Insulation that have a vapor barrier usually have a rating labled on it, namely SP-15. Basically what it does is reduce the moisture content in heat by diffusion to around 15%. At that percentage, it is highly unlikely that the heat could condense when dew point temperature is reached inside the insulation. This is especially important when using fiberglass insulation. This is because glass has a very low absorbancy when it comes to moisture. So there is a much higher probability of condensation with fiberglass insulation when a vapor barrier is not used concerning heat transfer.

Another good point is vapor barriers are only used with insulation. A moisture barrier is something some one uses to stop water going through. Used a lot in basement water proofing. Vapor bariers came about after insulation was introduced in homes.

There is great debate over vapor barriers at the highest levels in this country. For an example, the DOE had a Fact Sheet on Vapor barriers a few months ago. This sheet was written by OakRidge National Laboratory. When it comes to Engineering and scientific research, you can't go much higher than them. Yet the DOE pulled the fact sheet and replaced it with this brief. In response to this OakRidge is now preparing a handbook on moisture in the home which the DOE site will support.

What all this means to the average Joe's, like you and I, don't be surprised if we hear conflicting theories on vapor barriers, considering the brightest people in this country can't work in conjunction with each other. Instead it appears, they rather point the finger at each other and others.

The thing to remember with vapor barriers is why it came about and it's sole purpose. That is, all heat has moisture in it. As the heat travels through insulation, dew point temperature will be reached. The vapor barrier reduces the moisture content in the heat to a percentage that makes it highly unlikely that the heat will condense when dew point temp. is reached. In other words, if there is insufficient moisture in the heat, it cannot condense. Hence, the vapor barrier is always applied to the warm side of the insulation. For example, if you live in an area where heat and humidity is present most of the year, like Florida, the vapor barrier is installed between the outside and insulation. Whereas, if you live in an area where you use a lot of heat during the winter, like Boston, Ma., the vapor barrier goes between the inside of the home and insulation.
 
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Old 03-13-02, 05:49 PM
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Thanks for a very detailed answer.
 
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