Warren writes from Winnipeg, Manitoba: "My cabin is 18" above grade on piers, no skirting. The joist cavity is uninsulated and there is no vapor barrier under the floor sheathing. If I were to insulate the floor cavity, could I put vapor barrier over the sheathing before I install the sub flooring? Putting vapor barrier around the joist cavities from below sounds like a chore."

Answer: Yes. You want the vapor retarder closest to the floor towards the warm side of the living space, not facing down below towards the ground.