Home ventilation & ASHRAE 62.2
A tighter house is more efficient — and needs deliberate fresh air. ASHRAE 62.2 is the standard that says how much.
Older houses were so leaky they ventilated themselves through gaps and cracks. Modern air-sealed homes are far more efficient, but they no longer breathe on their own, which can let humidity, cooking odors, and indoor pollutants build up. ASHRAE Standard 62.2 defines how much mechanical fresh air a home needs. This guide explains the requirement, the formula, and how to sanity-check it with air changes per hour.
Why mechanical ventilation matters
"Build tight, ventilate right" is the guiding principle of modern home design. Sealing a house cuts the heating and cooling energy wasted on uncontrolled air leakage, but it also removes the accidental fresh-air exchange that leakage used to provide. Deliberate, controlled ventilation replaces that exchange on purpose — bringing in a measured amount of outdoor air and, ideally, exhausting stale air — so you get the efficiency of a tight envelope without the stuffiness or moisture problems.
The ASHRAE 62.2 formula
The standard sets a whole-house continuous ventilation rate based on floor area and the number of bedrooms (a proxy for occupancy):
CFM = 0.03 × floor area (sq ft) + 7.5 × (bedrooms + 1)
The first term scales with the size of the house; the second with how many people are likely to live in it. For a 2,000 sq ft, 3-bedroom home: 0.03 × 2,000 + 7.5 × (3 + 1) = 60 + 30 = 90 CFM of continuous fresh air. The ventilation-CFM calculator runs this for any home. Note this is the whole-house background rate; kitchens and bathrooms need their own local spot ventilation (range hood and bath fans) on top of it.
Checking it with air changes per hour
A useful cross-check is air changes per hour (ACH) — how many times the ventilation system replaces the entire volume of air in the space each hour:
ACH = CFM × 60 ÷ volume (ft³)
Take that 90 CFM in a 2,000 sq ft home with 8-foot ceilings (16,000 ft³): 90 × 60 ÷ 16,000 = 0.34 ACH. That is a typical whole-house background rate — gentle and continuous, not a gale. The air-changes-per-hour calculator converts between CFM and ACH so you can see whether a fan is moving a sensible amount of air for the room.
Ways to deliver the fresh air
There are three common approaches. Exhaust-only ventilation runs a quiet continuous fan (often in a bathroom) that pulls stale air out and lets fresh air seep in through the envelope — simple and cheap. Supply-only pushes filtered outdoor air in. Balanced systems — an HRV (heat recovery ventilator) or ERV (energy recovery ventilator) — exhaust and supply in equal measure while recovering heat (and, in an ERV, some moisture) from the outgoing air, so you get fresh air without throwing away the energy you spent conditioning it. In cold or humid climates an HRV/ERV is often the efficient choice; the ASHRAE 62.2 CFM target is the same regardless of which method you pick.
Does existing leakage count?
A fair question: if my house already leaks some air, do I still need the full 62.2 rate from a fan? The standard’s modern approach is to size the mechanical ventilation to the whole-house target and treat natural infiltration as a bonus rather than a credit you can bank — because infiltration is wildly variable. It depends on wind, indoor-outdoor temperature difference and where the leaks are, so it delivers plenty of fresh air on a cold, windy day and almost none on a still, mild one, exactly when you may want it most. Designing to the mechanical rate means you get reliable, controlled fresh air regardless of the weather.
If you want to know how leaky your house actually is, a blower-door test measures it — it pressurizes the house with a calibrated fan and reports air changes at a standard pressure (ACH50). That is a diagnostic number for the building envelope, distinct from the gentle ventilation ACH you compute from a fan’s CFM. Tighten the envelope to save energy, then ventilate to the 62.2 target on purpose: build tight, ventilate right.
Ventilation and your other systems
Ventilation interacts with sizing, humidity and comfort. Bringing in outdoor air adds a small load your heating and cooling equipment must handle, which a proper Manual J already accounts for. In a humid climate, ventilating with muggy outdoor air raises the moisture your AC or dehumidifier must remove, so an ERV that limits incoming moisture helps. And the same 400-CFM-per-ton airflow logic that sizes your ducts (duct CFM tool) governs how that conditioned air is distributed. Size the fresh-air rate to ASHRAE 62.2, deliver it with a method suited to your climate, and cross-check it with ACH — that is the whole recipe for healthy air in an efficient home.