How to insulate a vaulted cathedral ceiling

Updated February 21, 2017

Ceiling insulation is required in conditioned homes and buildings. If insulation is not installed, not only will energy loss be significant but condensation will occur where the warm air meets the cold air. This condensation can cause considerable damage from rot and mould. Vaulted cathedral ceilings are becoming popular in newer homes and buildings. Because a lot of these buildings lack an attic, insulation must be installed in the roof joist space between the plywood sheeting and the finished drywall below. Installation is fairly simple.

Install vent chutes in the ceiling joist space between the top plate of the wall and the ridge line of the roof. The vent chute is a polystyrene foam chute that maintains at least 2.5 cm (1 inch) between the plywood sheeting and the insulation. This allows roof cooling to occur and provides a place for water vapour to flow out.

Measure the length of the ceiling joist, using a tape measure. Roll out the insulation on the floor and cut it to the needed length with your utility knife.

Press the insulation into the joist space with the paper facing toward the interior of the room. Staple the paper facing to the joists every 15 to 20 cm (6 to 8 inches) along the entire joist space. Allow 7.5 cm (3 inches) of clearance space around all can lights that are not IC-rated, from metal chimneys, flues and other heat-generating sources.

Measure each successive joist space and install the insulation in the same manner until your cathedral ceiling is insulated.


You may also install unfaced insulation into the joist spaces. Friction will hold the insulation in place once it is pressed into the joist space. However, you may need to install a plastic vapour barrier on the bottom of the joists. Consult your local building code to see if that is the case.

Things You'll Need

  • Tape measure
  • Polystyrene foam vent chutes
  • Staples
  • Staple gun
  • Utility knife
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