The Intake
The soffit is the intake for the whole attic, and on most Toronto houses it is either blocked or was never actually opened.
Short answer
Attic ventilation needs air coming in low at the soffit and leaving high at the ridge. Without intake, exhaust vents pull their air out of the house instead. The two universal defects: insulation stuffed into the eaves, and a perforated soffit panel with no opening cut through the sheathing behind it.
The two defects
Insulation pushed into the eave, blocking the airway between the soffit and the attic. Universal on houses where insulation was topped up without baffles.
A perforated aluminum or vinyl soffit panel installed over solid wood soffit or sheathing with no opening cut through it. It looks exactly like a vent and admits nothing.
Both of these are invisible from the ground and both are easy to confirm from inside the attic.
Paint, which on an old wood soffit with slot vents can close them completely.
Insect screening clogged with dust and paint.
And soffit that was capped over during a renovation with no vents provided at all.
How to check
Go into the attic on a bright day and look toward the eave. You should see daylight at the soffit. If you cannot, the intake is blocked — by insulation, by an uncut panel, or by both. That one look answers the question and takes a minute.
Why it matters so much in Toronto
An attic with exhaust and no intake pulls its replacement air from inside the house, which brings moisture with it.
That moisture condenses on the cold sheathing, which is the frost people find in January and the staining they find in April.
The attic also stays warmer, which melts the snow on the roof, which is the ice dam.
Which means a blocked soffit is simultaneously the attic mould problem, the ice dam problem and the frozen eavestrough problem.
And in summer it is the reason the attic bakes and the shingles age faster.
One defect, four consequences, and it is the cheapest of all of them to fix.
Fixing it
Clear the insulation out of the eave and fit baffles so it cannot slump back.
Cut openings through the sheathing behind perforated panels where they were never cut.
Add continuous vented soffit where there is none, which is usually done during soffit and fascia work.
Clean or replace clogged screening.
Balance the intake against the exhaust rather than just adding more of one.
And do it at the same time as the eavestrough, fascia or roof work, because the access is already there and it costs a fraction of a separate visit.
Common questions
How do I know if my soffit vents work?
Look toward the eave from inside the attic on a bright day. You should see daylight.
Why does a perforated panel not count?
Because frequently there is no opening cut through the sheathing behind it.
What does a blocked soffit cause?
Attic frost and mould, ice dams, frozen eavestrough, and faster shingle ageing.