Vapor Barrier with polystyrene under slab?


Old 04-30-04, 06:15 PM
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Vapor Barrier with polystyrene under slab?

Just wondering what the general consensus here is regarding using a vapor barrier under slab with 2" polystrene insulation.

The insulation manufacturer and Wirsbo (This is a radiant slab) recommend vapor barrier first and then the insulation on top.

I've had others tell me the insulation is its own vapor barrier and another is not needed.
I also read the recommendations at They recommend no vapor barrier with XPS. Though, elsewhere on the site they recommend that if you do use a vapor barrier, it should be _on top_ of the insulation.

Also, when laying down the insulation, are most of you taping seams? Are there any problem with voids laying the insulation over 2b gravel? Or, should I use some crushed stone as a top layer for a more even surface?

Thanks for your thoughts and opinions,

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Old 04-30-04, 10:03 PM
Join Date: Nov 2001
Location: USA
Posts: 1,873
The terms vapor barrier and moisture barrier appear to be the same and people have a tendancy to use one for the other. A moisture barrier is used to prevent water (liquid) from contacting and/or penetrating. A vapor barrier is used to prevent heat or air transported moisture (vapor) from condensing into a liquid.

The reason why the manufacturer recommends installing the Moisture barrier first and putting the insulation on top is because if the insulation has water contact over a period of time, the insulation will degrade. Taping the seams of the moisture barrier reduces the probability that water will get in contact with the insulation.

This apparently contradicts what many have stated concerning vapor barriers, even myself. There are several characteristics of heat and how the moisture in heat reacts. For example, heat rises and a warm object radiates heat in all directions. The most common cause for moisture related problems concerning insulation and vapor barriers is air transported moisture. The radiant floor system you intend to install is air tight. The likelihood that heated air would cause a moisture problem in a radiant floor system is near zero. The insulation you intend to install will probably have an aluminum foil sheathing, which will act as a radiant barrier. So the radiant heat would be directed towards the house and not the ground or the moisture barrier.

Considering the aforementioned, I would have to agree with the manufacturer and lay the moisture barrier first, tape the seams and lay the insulation on top of it.

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