Insulating a Century Home Without Wrecking It

Why a century house is different

A house built in 1910 was designed to dry out. Solid masonry walls, no vapour barrier, generous air leakage, and a chimney pulling air through the whole thing. It was uncomfortable and expensive to heat, and it survived a hundred Ontario winters because moisture that got into the assembly could get out again.

Insulate it as though it were a 1990s house and you can take away the drying without taking away the moisture. That is how you end up with rot inside a wall that was fine for a century.

The four things that decide the outcome

1. Knob-and-tube wiring comes first

Old cloth-and-porcelain wiring was designed to shed its heat into open air. Burying it in insulation is a fire risk and generally an insurance problem. If we find it in your attic, we stop. It is an electrician's job before it is ours, and we will not blow over it whatever the schedule looks like.

2. Solid masonry walls have no cavity

Double-brick and stone walls — common in Guelph, Hamilton, old Toronto and Galt — cannot be drill-and-filled, because there is nothing to fill. Insulating them means adding a layer inside or outside, which is a renovation with real consequences for the wall's ability to dry. On those houses the honest advice is usually: do the attic, do the rim joist, do the air sealing, and leave the walls alone unless you are already opening them up.

3. The attic is nearly always the right answer anyway

Whatever the walls are doing, the ceiling plane of a century house is normally the biggest and cheapest win, and it does not interfere with how the walls dry. Seal it, ventilate it, and blow it to R–50.

4. Ventilation is usually the missing piece

Many century houses have no soffit vents at all, because the roof was never built for them. Adding deep insulation to an unventilated attic is exactly the mistake that causes frost and rot. Sometimes vents have to be cut. Sometimes the roof geometry means a different strategy. Either way it gets decided before the insulation goes in, not after.

What we will refuse to do

Blow over knob-and-tube. Seal a cathedral or flat-roof assembly with no ventilation path and no survey. And spray foam the underside of a plank roof deck on an old house without a proper assessment of how that assembly will dry — it is the single most expensive mistake made on heritage houses in Ontario.