Theresa writes from Sedalia, Missouri: "I have a deep space on my sunporch I have enclosed with wood siding on the outside. The space is deeper than my insulation. Does this cause a problem with the empty air space behind the insulation and between the new exterior siding? About 3.5 inches of air space left. Also should I cut slits in the paper face if I install plastic sheeting behind the sheetrock for a vapor barier to prevent trapped air and mold? What mil of plastic should I use for the vapor barier?"

Answer: No, there is no need to slash the facing. Since both vapor retarders are on the interior side of the insulation and will stay warm in the winter you shouldn't get condensation on either surface. Besides, any water vapor that passes through the poly will most certainly get through the Kraft paper. Six mil poly is about 10 times stronger than Kraft. The thickness recommendation on the poly is 4-6 mil. Having an air space on the back side of fiberglas insulation is not a concern. However, covering the insulation on the back side with a solid material will improve the energy efficiency of the wall.