How do I add additional layers of attic insulation to a joist cavity that is only half full?
If joist cavity is only half full and the recommended insulation level in your area is R-38, fill the joist cavity flush with the top with the appropriate thickness of fiber glass insulation. Then lay a second layer of unfaced insulation perpendicular to the existing layer to give a total of 12″ (or an R-value of 38) in the joist cavity. Covering the attic joists with the second layer will reduce heat loss through the wood.
