Roof rafters should always be insulated, especially if the area directly beneath the rafters is usable space. Failure to properly insulate your roof rafters will result in hot and cold air exchange between the exterior and interior of your home, hence an increase in heating and cooling costs. If you are planning on insulating your roof rafters, then you should know about the materials and techniques available for a successful insulation project.
Planning for materials
Measure the distance between the rafters in order to determine the width of insulation that you need to install. You will buy the batt insulation in 40 cm or 60 cm (16 or 24 inch) wide rolls that will have 8 to 10 pieces of 2.4 m (8 foot) insulation. Also, the thickness of the rafters will determine the R-value of the insulation that you will use: 5 by 15 cm (2 by 6 inch) rafters will accept R-19 insulation, and 5 by 20 cm (2 by 8 inch) or 5 by 25 cm (2 by 10 inch) rafters will accept R-30 insulation. You will also need a heavy duty hammer stapler in order to fasten the insulation, and a razor knife and a 5 by 10 cm (2 by 4 inch) beam to cut the insulation when necessary.
Make sure that you have proper ventilation and lighting when insulating your roof rafters. If you are working in an attic, use an extension light for visibility and a box fan for ventilation. Wear long sleeves, eye goggles and a face mask to protect your body from the fibreglass particles of the insulation. Measure the length of the rafter into which you will be insulating. If it is more than 20 cm (8 feet) long, then install a whole piece of insulation. Stuff the fibreglass side of the insulation into the area between the rafters, with the paper facing out. Fold the paper tabs over the rafters and staple the paper to the rafters. If you need to cut your insulation, measure and mark the insulation, and press a short 5 by 10 cm (2 by 4 inch) beam down onto the paper side of the insulation while it rests on the floor. With the insulation compressed, cut the insulation using a razor knife. After all of the insulation has been installed in between the rafters, cover the entire ceiling with polyurethane plastic. Have a helper hold the plastic onto the ceiling while you staple the plastic into place. Pull the plastic tight, and after you have stapled it into place, poke small holes in the plastic to allow the insulation to breath. The holes should be no larger than a small 51 mm (6 penny) nail.