Attic Insulation Is a Summer Measure Too

What is happening above your ceiling in July

Dark shingles in direct Ontario sun reach 70 °C or more. The attic beneath them sits somewhere between 50 and 65 °C through the afternoon. That heat radiates downward onto the top of your bedroom ceiling, and then the ceiling radiates it into the room — which is why upstairs rooms keep getting hotter into the evening, long after the sun is off the west wall.

Your air conditioner is fighting that, continuously, through the whole of the afternoon and well into the night.

What insulation does about it

The same thing it does in winter, in the opposite direction. R–50 between a 60 °C attic and a 23 °C bedroom slows the transfer dramatically. In practice, customers report the upstairs becoming usable in the afternoon and the air conditioner cycling off rather than running continuously.

What ventilation does about it

Arguably more, in summer. Soffit-to-ridge airflow carries the hot air out of the attic before it has time to radiate downwards. An attic with clear intake at the eaves and exhaust at the ridge runs significantly cooler than a sealed one. This is the same airflow that keeps the assembly dry in winter, which is why it is not an optional extra on our jobs.

What does not work

Powered attic fans. With insufficient soffit intake, a powered fan pulls its make-up air out of your air-conditioned house through every ceiling gap. You end up paying to cool your attic. Fix the intake instead.

Radiant barrier alone, retrofitted over existing insulation. In the Ontario climate the case for it is weak compared with simply adding depth and clearing the vents, and it is sold hard.

The scheduling point

Attics are done all year. Summer work is easier to schedule and, frankly, more unpleasant for the crew — which is our problem, not yours. The real argument for summer is not a programme deadline — there isn't one — it is that you are buying next January's heating bill now, and the diary is empty. The figures are here.