Hecena — Indoor Air Quality at Home | hecena.com
A comparison of ventilation classes with winter ACH ranges of 0.50 to 1.47 and rates expressed in CFM per person.
Ventilation is classified as natural, mechanical or mixed-mode. Natural ventilation relies on wind pressure and the stack effect; mechanical ventilation uses fans. Mixed-mode combines both, switching depending on outdoor conditions.
Ventilation rates may be expressed as volumetric flow, airflow per person or per floor area, or air changes per hour (ACH). Rates can also be written as CFM per person or CFM per square foot. For a home, ACH is usually the easiest to reason about: it is how many times per hour the room's air volume is replaced.
In winter, air changes per hour may range from about 0.50 in a tightly air-sealed house to about 1.47 in a loosely air-sealed house. Natural ventilation in a tight house without deliberate opening sits near the low end; a loose house with leaky windows sits near the high end and trades heat for freshness.
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Mechanical ventilation gives a predictable rate regardless of wind or temperature difference, but adds energy use and filter or duct maintenance. Natural ventilation costs nothing to run but gives you whatever the weather provides — which may be below 0.50 ACH on a still, cold day.
Mixed-mode is the practical middle for most homes with cold winters: natural ventilation when conditions allow, mechanical when they do not. The decision criterion is simple — keep the house inside the 0.50–1.47 ACH bracket without overshooting into heat loss.
Whichever class you choose, remember what ventilation is for: the intentional introduction of outdoor air into a space to dilute and displace indoor pollutants. Filtration can complement it, but only ventilation moves fresh air in.
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