The modern truck cap is made with fiberglass, a suitable material because it can be molded into many shapes and is strong. One of its weaknesses, however, is that it doesn't have a very high R-value, which is a rating of a material's ability to resist heat transfer. To protect your cargo against temperature extremes, you can insulate the cap with some basic tools.
Section off the interior space of the truck cap in to individual flat sections. The sections should be as large as possible taking into account each section must lie flatly against the cap's shell for proper adhesion.
Measure the length and diameter of the truck cap's ceiling. Transfer these dimensions over to the 1-inch polystyrene insulation and mark the shape with a black marker. You can use another piece of insulation as your straight edge.
Cut along the lines you've drawn with the razor knife.
Position the cut insulation on the ceiling and check that it fits properly. You can trim off any excess insulation with the razor knife.
Lay the insulation flat on the ground and put a continuous bead of construction adhesive around its perimeter. Then make a snaking pattern across its center, covering it liberally.
Reposition the insulation on the ceiling of the cap with the glue side facing the fiberglass. Press and hold the insulation in place until it adheres. The tighter you fit the individual sections the better the insulation will perform. It may be helpful to have a second person assist with this step to apply an even pressure.
Tape all of the joints in the insulation with Tyvek tape.
