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Ventilation
Balancing attic intake and exhaust
Attic ventilation is not about moving as much air as possible. It works best when intake at the eaves and exhaust near the ridge are matched, so that outdoor air enters low, washes the underside of the roof deck, and leaves high.
Intake and exhaust as a pair
A vented attic relies on two openings working together. Intake vents, usually in the soffits at the eaves, let cool outdoor air enter. Exhaust vents near the ridge let warmer attic air escape. The natural rise of warmer air, helped by wind, drives a slow, continuous flow between them.
When intake and exhaust are roughly balanced, air follows the intended path along the roof deck. When they are not, the system can short-circuit or even reverse.
What happens when the balance is off
- Too much exhaust, too little intake: ridge vents can pull air from other ridge openings or from the house itself, drawing conditioned indoor air into the attic instead of fresh outdoor air.
- Blocked soffits: insulation pushed into the eaves chokes intake, so the upper vents cannot do their job and dead air sits against the deck.
- Mixed vent types: combining several exhaust styles on one roof can let one opening feed another rather than venting the attic.
Why a cold, washed deck matters in winter
In a cold climate the goal is a roof deck that stays close to outdoor temperature. Moving outdoor air along the underside of the deck carries away the small amount of heat that still escapes from below and removes water vapour before it can condense or frost. A deck kept cold melts less snow, and less melt means fewer ice dams at the eaves.
| Symptom | Likely cause |
|---|---|
| Frost on attic nails or sheathing | Indoor moisture entering and not being carried away. |
| Stuffy, warm attic in winter | Restricted intake or blocked soffit vents. |
| Ice forming mainly at the eaves | Warm roof deck melting snow that refreezes lower down. |
A note on sealed-roof assemblies
Not every roof is vented. Some assemblies are designed as unvented, with insulation applied directly against the deck and no airflow path. These are legitimate but follow different rules. Mixing a vented design with unvented details, or partially blocking a vented attic, tends to cause the moisture problems that balanced ventilation is meant to avoid. Confirm which approach your roof was built for before changing vents.