How to Insulate a Wall

For narrow stud bays, you have to trim down the batt's width. To do that, lay a batt on the floor in front of the cavity, as shown, and line up one edge with the inside face of the stud bay. Then lay a straight 2x4 in line with the inside face of the neighboring stud and press down on it with one knee. Run a utility knife alongside the 2x4, trimming the batt with a series of shallow cuts.