Filling a wall cavity changes more about a house than its heating bill.
Lower heat loss through the wall. True, and on an empty 2x4 cavity, substantial. But it is not what customers phone us about afterwards.
An uninsulated exterior wall in February sits at maybe 8 °C on its inside surface. You do not feel air temperature alone; you radiate heat to cold surfaces, and a cold wall pulls heat out of you from across the room. Fill the cavity and that surface comes up ten degrees or more. People describe the room as warmer even when the thermostat has not moved, and they are not imagining it.
A cold wall surface sets up a convection loop: air chills against it, falls, runs across the floor, and you feel it at your ankles as a draught from nowhere. That is why rooms with big exterior walls feel draughty even with the windows sealed. Warm the surface and the loop stops.
Injection foam in a cavity is a mass and damping layer. It will not make a house on Highway 7 silent, but the difference on a busy street is audible and it is the benefit customers mention unprompted more than any other.
Mould in the corner of an exterior wall behind furniture is a surface-temperature problem. Warm the surface above the dew point of the room air and it stops forming. Wiping it off treats the symptom; insulating the wall treats the cause.
Walls are not on the fast rebate stream. They pay only through the assessment pathway, which needs an energy advisor before and after and at least two upgrades together. If you are doing the attic anyway, adding the walls is what moves you onto that pathway — the arithmetic is here and it is usually favourable.
And one honest limit: if your walls are solid masonry with no cavity, as many Guelph, Hamilton and old Toronto houses are, there is nothing to fill. More on that here.
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