John writes from Newfoundland, New Jersey: "I have a crawl space a floor to be insulated but the spacing is a full 16" and not 16" on center. I cut 24" bats of insulation but there are still some spaces. I would like to apply a layer of foam insulation under the rafters to not only improve the R-value but help seal up the air gaps and help hold up the 3.5" insulation presently there. The crawl space floor is earth. There are pipes but they are all insulated. Will there be a problem with moisture buildup in the fiberglass insulation in the space between the foam and the floor if I do this?"

Answer: If you use one-inch thick foam it is unlikely you will have a moisture problem in the floor between the foam and the fiber glass. I would also make sure you have plastic on the soil at the bottom of the crawl space.