Dormers extend out from a roof's sloped surface and often look like a second- or third-story room's window from the outside. Some dormers provide attic areas with a natural light source. Insulating a home's dormers reduces the amount of energy used for heating or cooling the house's interior spaces. The dormer's ceiling insulation creates a thermal barrier that separates the conditioned interior air from the superheated or cooled attic air. The dormer's wall insulation reduces heat transfer into the dormer area from the home's exterior.
Check local building codes to determine the insulation's required R-rating. Local building codes calculate the needed insulation valve, the R-rating, based on the normal local climate. Insulation with a higher R-rating insulates better than insulation with a lower value.
Put on a long-sleeved shirt, safety glasses and a dust mask.
Measure the dormer's walls from the top plate to the floor with a tape measure and transfer the measurement to a sheet of batt insulation. Batt insulation consists of woven strands of fibreglass glued to a paper backing.
Cut the dormer wall's insulation to size with a knife. Compress the batt insulation and cut through the insulation from the paper side. Do not rip or pull the fibreglass.
Hold the cut insulation against the dormer walls with the paper side facing the attic or room and the fibreglass facing the exterior wall. Secure the flaps on the sides of the batt insulation to the dormer wall studs with an insulation stapler. Place a staple every 4 to 6 inches along each wall stud.
Measure the dormer's ceilings for insulation with the tape measure, if needed. Do not insulate the dormer's ceiling when it connects to an uninsulated attic space. Cut the ceiling's batt insulation to size with the knife.
Staple the dormer's ceiling insulation's flaps to the roof trusses with the insulation's paper backing facing down. Start at the attic end of the dormer and work toward the dormer's wall. Place a staple every 4 to 6 inches along each rafter.