My son in Vancouver is considering the purchase of a home which has a low rise roof and no interior access to the attic. The building inspector gave the house a good report except for limited insulation in the attic and some blistering on the roof surface. He suggested that when the time came for repairing/replacing the roofing, it might be a good idea to add some foam insulation to the inside surface of the roof to make up for the small amount of fibre-glass insulation between the ceiling joists. I am wondering if this process would result in a vapour barrier on the attic floor and another one on the inside of the roof with space between them and thus resulting in condensation between the two? Can you help?
Answer:
The suggestion you got from the building inspector is about as bad as it gets. An air gap between two layers of insulation (attic floor/roof deck) is an open invitation to all kinds of things; condensation being only the worst. Two vapour barriers separated by insulation and an air gap in any situation just adds to an already long and disasterous list. When the time comes to replace shingles, some of the roof decking cana be temporarily removed and insulation blown in on top of what is there. Make sure you don't compromise soffit ventilation while you're at it.
