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Do you recommend blue 2" + 1" Dow foam board between studs for a cinder block basement with stud wall?

by LiveModern Webmaster last modified Jan 04, 2012 02:05 AM
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by last modified Oct 20, 2010

The wall is part below-grade and part above-grade. I am replacing a bottom joist which has dry rot from water infiltration. Can I use Blue foam, and do I want an air space between the block and the framed wall?



The use of extruded polystyrene for insulation is great for a basement application. The product is less susceptible to moisture than fiberglass batts in losing insulating power or to the possibility of the growth of mold due to condensation; therefore, no air space would be required.

The first thing you have to do is to determine the level of moisture coming through the wall, and correct it by whatever means necessary to keep it as dry as possible. U.S. Department of Energy recommends a minimum level of R-10 or greater in your area. See the U.S. D.O.E. website for additional information.

Extruded polystyrene attains an R-value of approximately 5 per inch. I would recommend a level of about R-15, or about 3” of insulation. Different manufacturers produce different thicknesses with different R-values; you can choose the manufacturer you wish and apply the thicknesses as described below.

  1. I would recommend that you install one layer of continuous insulation board between 1½” to 2” thick, giving you an uninterrupted layer of insulation against the block of at least R-7. (The studs for wall framing have an R-value of approximately 5, creating a bridge through the insulation barrier if installed directly against the foundation wall.)
  2. Install the stud framing against the continuous insulation layer and install an additional layer of insulation between 1” to 1½” thick to produce a total R-value of 15 between the studs.
  3. To complete the assembly, install a layer of gypsum board to encase the entire assembly to produce a fire resistant layer around the insulation. This is required by building code and because even though the material has a high resistance to ignition, once it does burn it gives off a thick black smoke that contains many toxic elements.




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