Exterior wall insulation: the big one, and the one with conditions

Walls lose fifteen to twenty-five per cent of the heat in a typical older house and are the hardest part of the envelope to improve. They are also where the largest rebates sit, and where the most damage gets done by well-meant work.

Three routes, three different risks

Cavity fill in a frame wall is the cheapest and works well where the cavity is genuinely empty and the wall can dry. In a wall with no air barrier it can also move the condensation plane into the insulation.

Exterior insulation under new cladding is the best outcome technically — it covers the thermal bridges, keeps the structure warm and dry, and is the only route that meaningfully improves a solid masonry wall. It is also the most expensive and usually only sensible when the cladding is being replaced anyway.

Interior insulation is the one to be careful with. On a solid masonry wall it makes the masonry colder and wetter through the winter, and done without understanding the drying assembly it is how a sound century house quietly fails over a decade.

What it pays

Ontario's insulation envelope reaches $7,700 across attic, wall, foundation and exposed floor on the assessment path. Efficiency Manitoba's per-gigajoule payment rewards wall work heavily because the savings are large. Both require evaluations.

Why the assessment comes first here, not last

Wall work is the measure where an energy model most often changes the plan — by showing the wall is not the biggest loss, by showing the cavity is not empty, or by showing the assembly cannot safely take insulation on the inside. It is also the measure where getting it wrong is most expensive to undo.