FDBF specifically designed to be installed under crawl spaces to cut the heat loss from radiant heating systems. Actual field testing has shown this product to be superior to fiber glass insulation in crawl spaces. 2 INCHES of high density foam is the best insulation available for sealing the rim joist.
FDBF is the best insulation we have found to install in the crawl space. It reflects 75% more heat than fiber glass. It is very easy to install and when you tape the seams you eliminate the need for a vapor barrier. The manufacturer rates this as R-15.5 when used in a crawl space. Our testing has shown that each additional inch of dead air space adds an R-3.
EACH INCH OF DEAD AIR SPACE BETWEEN THE FOIL AND THE TUBING IS WORTH AN R-3 WHEN THE FDBF IS PLACED BELOW THE TUBING
We do require FDBF in crawl spaces, fiberglass can be combined with FDBF to increase R-value in a crawlspace. In areas that are heated below and have sheetrock installed on the ceiling below no insulation is needed or required except in areas with vaulted ceilings above.