Home Ventilation Rate Calculator

For most of the history of house building, ventilation was accidental. Air leaked in around window frames, through unsealed floorboards, down chimneys and through the gaps where the building met itself, and the resulting air change was enough — wastefully so — to keep indoor pollutants diluted. Nobody designed it; it happened because the buildings were loose.

Energy codes changed that. A house built to a modern envelope standard leaks a small fraction of what a 1950s house did, which is exactly the point: air that leaks out in January is air you paid to heat. But the pollutants did not go away. Cooking, cleaning products, off-gassing from furniture and finishes, moisture from showers and breathing, radon from the ground, and carbon dioxide from the occupants all still accumulate — and now nothing is diluting them.

That is the trade the standard exists to manage. ASHRAE 62.2 is the American residential answer: build tight, then ventilate deliberately, at a known rate, through a path you chose rather than one the building happened to have. The slogan in the building-science world — "build tight, ventilate right" — is glib but exactly correct, and the failure mode it warns against is a house sealed to a high standard with no ventilation strategy at all.

The requirement is 0.03 cfm per square foot of floor area plus 7.5 cfm per bedroom-plus-one, and the split is not arbitrary.

The floor-area term addresses the building. Paint, adhesives, engineered wood, carpet, upholstery and cabinetry emit volatile organic compounds continuously, at a rate roughly proportional to how much of them there is — which scales with area. A large empty house still needs ventilation.

The occupancy term addresses the people. Carbon dioxide, water vapour, and the various compounds collectively called bioeffluents are produced per person, not per square foot. The standard uses bedrooms plus one rather than an actual headcount because occupancy changes over a building’s life and cannot be inspected at design stage, while the number of sleeping rooms is fixed and countable. A three-bedroom house is assumed to hold four people whether it currently does or not.

Local exhaust sits on top of both, and is separate for a good reason: kitchens and bathrooms generate their pollutants and moisture in concentrated bursts at a known location, and capturing them at source is far more effective than diluting them across the whole house afterwards.

The three strategies differ in what they do to the pressure inside the house, and that difference matters more than it sounds.

Ventilation is not free. Every cubic foot of outdoor air brought inside has to be heated or cooled to indoor conditions, and the shorthand for that load is 1.08 times the airflow in cfm times the temperature difference in Fahrenheit, giving BTU per hour.

For the 90 cfm example above, on a 20°F day with the house at 70°F, that is 1.08 × 90 × 50, or about 4,860 BTU/hr — a continuous load roughly equivalent to leaving a small space heater running all winter. Over a heating season in a cold climate it is a meaningful fraction of the heating bill.

This is what makes heat recovery worth its cost, and also what makes it not worth its cost everywhere. In climate zones 5 and above, where that temperature difference persists for months, an HRV recovering 75% pays back its premium over a simple exhaust fan in a reasonable number of years. In climate zones 1 through 3, where the difference is small for most of the year, the same unit may never recover its cost, and a well-controlled exhaust fan is the sensible engineering answer.

Meeting the number on paper and delivering the air are different achievements, and the gap between them is wide in practice.