Truss or Rafter-Type Roof Ceilings

Before the 1970s, conventional roof trusses were usually about 150 mm (6 in.) deep where they rested on exterior walls, which didn’t leave much space for attic insulation. Since the oil crisis of the 1970s, roof trusses have a “raised heel” design at the plane of the exterior wall, which makes them 250 mm (10 in.) deep or more at this location and provides more space for insulation. The NBC requires that the level of insulation where the roof meets an exterior wall must be equal to or greater than the insulation level in the wall.

Thick batts are made for insulating roofs in widths equal to the full centre-to-centre distance of standard roof framing. The lower portion is slightly compressed when installed between the framing, but the upper portion retains its width and covers the tops of the framing, thus reducing thermal bridging (heat loss) through the framing.

Loose fill insulation can also be used to cover the framing. Use insulation baffles to prevent loose fill insulation from spilling onto eave vents (Figure 24) and from being displaced by wind entering the vents. Where loose fill insulation is installed in a roof space over a sloped ceiling, the slope of the surface must not exceed 4.5:12 for mineral fibre or cellulose fibre insulation, and 2.5:12 for other types of loose fill insulation.

Figure 24

Eave details to avoid blocking ventilation

Source : Canada Mortgage and Housing Corporation (CMHC)