Don writes from Ravensdale, Washington: "Which product do I use for a flat roof (ceiling) made of 2"x10'"s on 24" centers? I'm concerned that if I use R-30 there won't be any space left for the venting to work properly and I can't find an R-25 product with a vapor barrier."

Answer: If you do have an air space, how will it be vented? Simply opening it on the ends won't really vent the space unless some type of forced ventilation is in place like a fan. In most cases flat ceilings cannot be vented so filling the cavity is acceptable. You should install a vapor retarder over the insulation.