Step 3: Use your thumbnail to crease around the edges so that it lies flat against the undercarriage and the joists. This provides the maximum space for insulation. Use a few staples to hold the edge flaps in place. Be sure to put the staples near the top edge so that they will be covered by the tape. The corners will buckle but that’s dealt with in the next step. Note that it’s good to put some boards across so you have somewhere to put your knees and toes as you’re working on the interior bays. That way you’re not damaging your already-wrapped bays or your undercarriage.