To install fiberglass insulation, put batts into each stud space, making sure there are no voids or gaps left. Fit the top part first, and cut the length in place, adding an extra 1/2 inch. Split a batt in half along the depth, placing one half behind and the other in front of electrical cables. Place one-half of the split batt behind an electrical box, and cut the front half to fit the batt around the electrical box.
Fiberglass batts are friction-fit into stud spaces, requiring accurate measurements to avoid voids and gaps.
Fold the batt in half, and hold it against a stud edge. Hold the batt against the previous stud face, and cut it along the length. Make sure the width of the batt segment is 1/2 inch wider than the stud space. Alternatively, measure the stud space, and add 1/2 inch to the width. Put the batt on the floor, and use a utility knife and a straightedge to make a straight cut along the length.
Position the top edge inside the stud space, and use a sharp utility knife to cut the batt to length against the bottom plate. Add 1/2 inch to the length of the batt for a snug fit. Fit batts into stud spaces correctly by pushing the batt all the way into the stud space and pulling out the front edges, making sure the batt is flush with the stud faces.
Insulate behind plumbing pipes, but leave the front uncovered. Cut thin batt strips to fill shim spaces around windows and doors with a wide putty knife, to prevent air leaks.