Attic Ventilation Calculator

Two things, and they pull in different directions across the seasons. In winter a vented attic is meant to stay cold, so that snow on the roof does not melt from beneath and refreeze at the eaves as an ice dam, and so that moisture drifting up from the house is carried away before it condenses on the sheathing. In summer it is meant to dump heat, so the roof deck runs cooler and the ceiling below it gains less.

Both jobs are done by air moving from low to high - in at the eaves, out at the ridge - driven by the stack effect and by wind. It is an unglamorous, entirely passive system, and the code specifies it as a hole size rather than an airflow rate because that is all it can usefully specify for something with no fan in it.

The winter job is the one that matters more, and it is the one people underestimate. An attic that runs even a few degrees above freezing while there is snow on the roof will melt that snow at the top and refreeze it at the overhang, and the resulting dam backs water up under the shingles. Ventilation is one of three tools against that, and on its own it is the weakest of them.

Air that leaves an attic has to be replaced. If the exhaust openings are larger than the intake openings, the attic is at negative pressure relative to whatever it can most easily draw from - and what it can most easily draw from is the heated house below, through every gap in the ceiling plane.

That is a bad outcome twice over. It pulls heated air out of the living space, which costs money directly. And that air is full of household moisture - from cooking, washing, breathing - which then meets a cold roof deck and condenses on it. Attics that frost up in January and drip in February are very often attics with a generous ridge vent and blocked soffits.

The code handles this by capping the upper portion at 50% of the required area rather than by prescribing a split. In practice the rule to work to is simpler: intake at least equals exhaust, and if in doubt err towards more intake. There is no failure mode for an attic with too much soffit vent.

The single most common fault in an otherwise correctly built attic is that the soffit vents are buried. Loose-fill insulation blown to a modern depth will slump, drift and creep towards the eaves over a decade, and the eave is exactly where the insulation is thinnest and most needs to be deep. Somebody tops up the insulation, pushes it out to the edge to avoid a cold strip along the ceiling, and closes the intake completely.

The fix is a baffle - a rigid channel stapled between the rafters at each bay that holds insulation back and keeps a clear air path from the soffit up over the top plate. They cost very little, they are straightforward to fit from inside the attic, and retrofitting them is one of the highest-value hours available in an older house.

The second most common fault is soffit that was never vented in the first place. Perforated vinyl soffit looks like ventilation and often is, but plenty of houses have solid soffit panels with a decorative pattern pressed into them and no openings at all behind. If air is meant to be coming in there, it is worth confirming that it can.