Insulating a vaulted ceiling from the interior requires removal of the interior finish. The joist space must be accessed to ensure adequate ventilation against the underside of the existing sheathing. Blown-in cellulose or fibreglass insulation should not be installed blind from the inside into vaulted ceilings or flat roofs. Ventilation is required to inhibit condensation and allow the wood structure to dry, and is particularly important in a retrofit application. Since the ceiling finish must be removed anyway, it is advisable to add interior insulation to increase the R-value of the roof assembly. However, this is not required. Additional insulation can be installed on the underside of the existing roof joists, or extra framing can be added to increase the roof joist depth.

Conceptual Insulating Procedure: Interior Retrofit

1. Remove the interior finish and insulation (if any) and expose the existing roof sheathing. Inspect and repair framing and sheathing as needed.
2. Use insulation baffles such as 1/4" plywood or perforated baffles to create a vent on the underside of the sheathing.
3. Install batt insulation to the depth of the roof joist against the baffles.
4. Install rigid foam insulation against the underside of the roof joists. Tape joints to ensure airtightness. Fasten insulation to the underside of the roof joists with wood strapping.
5. Install interior finish.

Option: Use gypsum board with gaskets to create airtight drywall. If no rigid foam is installed or if rigid mineral wool is used in place of rigid foam, an air barrier/ vapour retarder such as polyethylene or vapour retarder paint on drywall must be installed at the interior side of the insulation. Alternatively, use closed-cell spray foam as the air barrier and vapour retarder.