Renovations or repairs provide an opportunity to ensure a well-insulated attic ceiling and walls with an effective air and vapour barrier.

A popular renovation activity is to convert an existing unfinished attic into a new living space. Depending on the depth of the rafter space, it may be difficult to achieve the recommended high attic RSI values. If you choose to finish your attic, check for local code requirements and then consider the following steps:

• Install collar ties between every pair of rafters (see Figure 5-23). These ties will provide structural support for both the roof and the new ceiling.
• Ensure that the insulation installed in the rafter spaces fits snugly against the top of the walls to form a continuous thermal envelope. There must be no gaps around the perimeter of the attic floor, otherwise heat will escape and ice damming may occur near the eaves. In most cases, it should be possible to insulate each rafter space all the way down to the eaves and over the top of the exterior wall. Leave a clear space between the top of the insulation and the underside of the roof sheathing to allow for roof ventilation. Seal any roof vents into the parts of the attic that are not heated.
• Staple a continuous polyethylene air and vapour barrier to the rafters and end-wall studs; then carefully seal the edges and seams with acoustical sealant.
• If your attic is not too cramped, consider adding strapping onto the rafter bottoms and wall studs to allow for more insulation (up to the recommended minimum level for attic insulation, if possible). After installing insulation in the existing cavities, nail or screw strapping (2 x 2 or 2 x 3 on-edge) perpendicular to the rafters and studs. Space the strapping to suit the width of the insulation you will use. However, the strapping should be no more than 610 mm (24 in.) apart - measured on centre - or you will have difficulty attaching the interior finish. Fit insulation snugly between the strapping, covering all the rafters and studs. Finally, staple a continuous, sealed polyethylene air and vapour barrier to the strapping. Alternatively, secure rigid board insulation 25 to 50 mm (1 to 2 in.) to the rafters and studs, seal or install a vapour barrier and then cover with an approved finish (e.g. drywall).
• Insulation between the collar ties is applied from below in much the same way, with a continuous polyethylene air and vapour barrier applied last. If the collar ties have already been insulated and if there is access to the upper portion, then more insulation may be added as in a normal attic.