A fiberglass exterior entry door should be caulked to weatherproof the frame and prevent cold air from getting into the house. The caulk can be applied to the subsill and the doorsill before installing the new door.
Caulk should not be applied directly to the new fiberglass door. Instead, caulking should be done around the subsill and the doorsill. Before caulking, each surface should be wiped clean of dirt and debris and be allowed to dry completely.
To create a tight seal, a caulking gun should be used to apply a line of caulk along the subsill, approximately 1 inch from the front edge. A parallel line should run about 1 inch from the back edge. Between the two lines, adding a wavy line of caulk provides extra coverage. The caulk lines can also extend 1 or 2 inches up either side of the door opening. The same caulk pattern should be repeated on the bottom of the doorsill.
When choosing caulk, homeowners should look for a type that is compatible with fiberglass surfaces. The caulk should be made specifically for weatherproofing. For homes in very cold climates, it is important to choose a caulk that can withstand temperature extremes without cracking.