Jim Wright’s house in western Arkansas has a pier foundation that elevates floor framing about 40 inches off the ground. Unlike a house with a basement, crawl space, or slab foundation, there is no enclosure at the bottom of the house, so the floor is more or less like another exterior wall.
How, Wright wonders, should this be insulated?
“I am considering two methods of insulation,” he writes in a Q&A post at GreenBuildingAdvisor. “The first method is 7.25-inch-thick Roxul batts (R-30) that are covered with 7/16-inch OSB sheets. The second method is 1.5-inch (R-10) of sprayed closed-cell foam.”
Not counting his labor to install the mineral wool batts, the cost of either of those options is about $2,000. Because of its thickness, the Roxul would have three times the insulating value of the foam. But it would be harder to install because the 2×8 batts are installed on 19.2-inch centers and all of the batts would have to be cut to fit.
“A big advantage of the closed-cell foam is that I would not be doing any of the labor,” Wright adds. “Also, it appears that the foam would seal better against air infiltration. The only apparent disadvantage of the foam is the lesser R-value. However, I’m not sure that I really need more than R-10 in the floor.”
Overall, the 68-year-old Wright is leaning toward the spray foam. “What say ye?” he asks.
The first job is to meet minimum code requirements
To meet the requirements of the 2009 International Residential Code, floor insulation in Wright’s climate zone should have a minimum R-value of 19, says GBA senior editor Martin Holladay. While the mineral wool batts easily get the job done, the proposed thin layer of spray polyurethane foam would not.
“Your spray foam contractor is suggesting an…
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