Dan writes from Buffalo, New York: "I have a one-time access to an area above an attached garage where a knee wall from an overhead room meets the cold attic space from the roof. Currently, the knee wall (2 x 4) is insulated with fiberglass and I want to increase the R-value. I have read that foam backer board could be installed against the studs but wouldn't an additional layer of fiberglass be more efficient than the foam backer? What should I use and what thickness should I go with."

Answer: We recommend adding unfaced fiberglass to the back of the existing insulation. Be sure not to block any ventilation. You may need insulation supports since unfaced insulation cannot be stapled.