Re-siding is the time to insulate walls from the outside

Replacing siding is expensive, and it is the one moment when insulating walls from the outside costs least extra. Missing that moment means living with the same wall performance for decades.

Why walls matter

NRCan says walls can account for about 20 percent of a house's heat loss, and that insulating walls should go together with air sealing.

Combine it with re-siding

NRCan notes that exterior insulation can be combined with re-siding. A new air barrier can be installed on the exterior while the existing interior vapour barrier stays in place.

Keep it continuous

The insulation and the air barrier should run continuously, without breaks or thermal bridges. NRCan's illustration shows insulating sheathing added over a 2x6 wall.

Interior retrofits

If insulation is added from the inside, NRCan advises placing the vapour barrier on the warm side, within the first third of the wall's total insulation value. Interior work is more disruptive, which is one reason exterior insulation is often preferred.

What to ask the contractor

Ask how thick the exterior insulation will be, how the air barrier will be lapped and sealed at windows, doors and corners, how windows will be extended or flashed for the thicker wall, and how the work meets the building code in your area.

Prevention Checklist

• Decide on exterior insulation before ordering new siding.
• Ask for a continuous air barrier on the outside.
• Ask how windows and doors will be flashed for the thicker wall.
• Avoid gaps and thermal bridges in the insulation.
• Check permit and code requirements for the wall assembly.

References

1. Natural Resources Canada. Keeping the Heat In - Section 7: Insulating walls, Web page, 20 percent; exterior with re-siding; continuity; vapour barrier. Accessed 2026-11-01.
2. Natural Resources Canada. Keeping the Heat In - 7 Insulating walls (PDF), Web page, Same. Accessed 2026-11-01.

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