Insulating a sloped roof from the exterior requires gaining access to the existing roof sheathing and re-roofing. Where exterior roofing work is already required, or if there is no interior access, insulation should be added to the exterior of the roof. The amount of insulation to add depends on local requirements (maximum height restrictions) and cost effectiveness. Note that the addition of exterior insulation to a roof that is already insulated on the interior is acceptable. Ventilation is recommended between the new insulation and the new sheathing to allow moisture to dry out and reduce ice-damming issues in snowy regions. Adding exterior insulation often requires overhangs to be re-constructed and may change the appearance of the house.

Conceptual Insulating Procedure: Exterior Retrofit

1. Remove the existing roofing and underlay. Inspect and repair framing and sheathing as needed.
2. Install a new self-adhered airtight membrane over the existing sheathing. Use vapour impermeable air barrier if no vapour retarder is present in the existing roof assembly.
3. Attach rigid mineral wool or foam insulation over the air barrier with strapping to allow for cross ventilation over the insulation.
4. Install new sheathing over the strapping.
5. Install new underlay and roofing.

Note: A ventilation path must be present for the cross strapping to allow ventilation over the insulation and at the underside of the new roof sheathing.

Note: Continuity in the air barrier must be present between the underside of the roof and the top of the exterior wall. Spray foam or other sealing methods should be used.