Jim writes from Naperville, Illinois: "I would like to insulate a 15X25 boathouse for infrequent heating needs in the winter and finish the ceiling. The flat roof has a vinyl Duradek roof covering over 3/4 inch tongue and groove plywood. The flat roof is 2X12 construction (12 inches on center) that slopes downward 1/4 inch per foot for 15 feet from the south side of the roof to the north side. There is an overhang and eave vents on the north side (low side) of the roof and no eaves or vents on the north side (high side) of the roof. I am concerned that all the heat will collect on the north side (high side) of the roof during the summer months if I install insulation between the 2X12's given the 1/4 inch slope. Is heat build-up or condensation a concern with such a small slope if I only intend to infrequently heat the boathouse? What type of insulation would you recommend for this roof application and how much of a gap is required between the roof deck and the insulation? If I need to create ventilation on the north side do you have any recommendations?"

Answer: For 2"x12" construction we recommend Kraft-faced R-38C with the facing installed toward the warm-in-winter side. A minimum one inch of air space is required between the insulation and the roof deck. I don't have a special recommendation for ventilating the north side. Owens Corning markets VentSure® ridge vents that are installed with new roofing shingles but there are other products on the market that may be appropriate for your project.