David writes from Portland, Oregon: "I have a stand alone woodshop that has an uninsulated ceiling. The roof is metal on top of plywood and the joists are open and 2x8. I'm curious what I can add into the bays without having to create air void space and then having to add some kind of ventilation at the rim. Can I just add insulation and not have to worry about ventilation if the insulation is still exposed?"

Answer: Owens Corning manufactures asphalt shingles. With this type of roof, ventilation is required under the deck. Typically, if insulating under the deck you are insulating a living space and the insulation has a Kraft-paper vapor retarder on it that will face the warm-in-winter side. The vapor retarder must be covered with a 15-minute thermal barrier like gypsum wall board.