Joist-Type Roof Ceilings

When roofing is attached directly to the top of ceiling joists as is the case for cathedral or sloped ceilings, the space above the insulation must be vented to prevent the accumulation of moisture, or the space must be filled with insulation (such as sprayed closed-cell foam) that prevents air vapour from condensing within the insulation or on other surfaces in the roof (Figures 25 and 26).

Figure 25

Figure 26

Source : Canada Mortgage and Housing Corporation (CMHC)