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« Only 4% of Americans Know that Buildings Represent 48% of CO2 Emissions | Main | Foundation Slab Floating on Reclaimed Insulation is Foundation of Energy Efficiency »

November 23, 2008


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johnny question asker...

So, what is the difference in R Value and cost between say, 3 inches vs. 6 inches vs 12 inches of XPS? Seems like might be excessive, i.e., diminishing return.

Carol Gulyas

Dear Johnny: I posed your question to Marko Spiegel, the engineer who has done the energy modeling for the house; here is his answer:

"This argument is common and could be argued in regard to the walls as well, which have 12" of XPS. The argument might hold when referring to the last two or 3 inches. The last few in. of insulation have diminishing some degree that is true. What this argument does not recognize is that it is not the individual component which needs justification, it is the sum of the whole: our goal here is to make a complete super-insulated envelope which allows you to throw out your furnace. To meet the goal, we need R40 under the slab. With 10.5 in and reused PS you will have around R50. It will keep your feet nice and warm in winter." Marko's website is here:


reclaimed XPS, how did you go about finding that? any suggestions for finding in southern illinois? also what was the cost/effect comparison to new? Thanks.

Carol Gulyas

We found it at Plywood King in Spencer, Indiana, but you could also ask around at home supply stores in your area. The number of Plywood King in Spencer, IN is 812-876-4341. It cost about half, and there was no great difference in quality. It depends on what you're using it for, which in our case was under the foundation. In our case the XPS had come from the roof of a church that was deconstructed with the intention of recycling the materials.

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