Charles writes again from Greenland, New Hampshire: "I ask how we could pick-up the additional R-4 to attain an R-25 in a 2x6 wall. Your advice was to apply a layer of Formular over the sheathing. I shared that with our chief engineer who questioned if we would be trapping moisture between the vapor barrier and the surface if the foam. He feels that the foam should have a lower permeability rating than the vapor barrier to allow any moisture to escape from the outside. Will this assembly be bothered by this?"

Answer: FOAMULAR® insulation at one inch has a perm rating of 1.1 and Kraft facing has a perm rating of 1.0, so FOAMULAR insulation does have a lower perm rating. FOAMULAR insulation is not considered to be a vapor retarder. If the engineer would like to discuss this with someone, he can call the Owens Corning customer service department at 1-800-Get Pink™ (438-7465).