A dormer is an architectural feature that extends horizontally from a sloped roof. Dormers can provide additional living space in an attic or a loft, or they can be strictly for decoration. If you're finishing out an attic, or if you're trying to conserve energy by insulating the roof, it's important to insulate the dormer as well. Dormers come in all sizes but you can insulate big one or little ones with fibreglass batt insulation.
Purchase fibreglass batt insulation in widths to match the distance between framing rafters in the dormer. Standard rafter spacing in new residential construction is 16 inches, but if the dormer is in a garage or outbuilding, the rafters may be 24 inches apart. Insulation batts sell in 15- and 23-inch widths to accommodate both sizes.
Wear long sleeves, long trousers, a hat, protective eyewear, gloves and a respirator mask when installing fibreglass insulation. Tiny glass fibres can irritate skin and may become airborne when cut.
Measure and cut the batt insulation to fit the spaces between the rafters in the dormer. Insulate the upper-sloped sections first and the sidewalls last.
Position the cut batts into the space between the rafters, with the paper side facing you. Don't push and cram the batts in; it must fit loosely to maintain optimal insulation factor.
Fold the paper nailing strips out from each edge of the batt, and position them over the rafters on each side.
Staple the paper nailing strips to the rafters, using a staple gun. Insert one staple about every 8 inches. Add an extra staple in the top and bottom inch of the paper strip.
Continue measuring each rafter space (they will differ), and install a cut piece of batt insulation in each one.
Cut additional narrow strips of batt insulation to tuck around dormer windows, if applicable. The idea is to fill every air space loosely with insulation.
The higher the R-value of the insulation, the greater the energy savings. For round-top dormers, insulate in the same manner, fitting the batts into the curved spaces before stapling to the curved rafters.