Step 2: In most homes, the exterior walls are covered with drywall. We install the drywall on the outside of the exterior walls before raising the wall panels. This provides the cleanest possible look. Some companies prefer to infill the drywall between timbers after the SIPS are up. This is very difficult to do well, and even if extreme care is taken, and the initial result is good, the timbers WILL move and shrink (even kiln-dried), inexorably leaving gaps between drywall and timber and a nasty mess all-in-all.