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Fiberglass Insulation in an offset 2x4 Stud Cavity (Basement)

MKM's Avatar

Join Date: Apr 2001
Posts: 74

08-31-02, 05:47 PM   #1  
Fiberglass Insulation in an offset 2x4 Stud Cavity (Basement)

I'm finishing my basement. It has a poured concrete foundation. I framed the walls with 2x4s. The framing is set off from the concrete by about an inch. (in one place, as much as two inches). When I place my R-13 Kraft faced insulation, there is a gap between the pink side of the insulation and the poured concrete. Is this good, bad, or does it not matter?


Well, I just read about face stapling and inset stapling. I never realized inset was an option. That being said, I guess I could just back the insulation all the way up to the wall and inset staple it, although face stapling would seem easier to do and would seem to provide a better vapor barrier. Is one method any better that the other?

Last edited by MKM; 08-31-02 at 06:06 PM.
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resercon's Avatar

Join Date: Nov 2001
Posts: 1,873

09-05-02, 12:13 PM   #2  
I would recommend leaving the one inch gap between the insulation and the basement wall. It will act as a drainage plane. Insulation does not stop heat, it resists heat flow through it. Which explicitly implies there is a constant flow of heat through the insulation, when you are heating the basement. All heat has moisture in it, what a vapor barrier does is reduce the % of humidity in the heat flowing through the insulation to a point where, when dew point temperature is reached there is insufficient humidity in the heat for condensation to occur. But you must remember this heat flow with a low % of humidity is constant.

The problem is actually the 2 different material present and their characteristics. Fiberglass has a very high expulsion rate with moisture and a very low absorbancy rate. Concrete has a much slower expulsion rate and a much higher absorbancy rate compared to fiberglass. When the 2 touch each other in this application, the probability for condensation is high because of the conditions and the material characteristics. The dead air space will absorb the humidity which will allow time for the concrete to absorb it. This is known as a drainage plane.

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