A modern, tightly built house doesn't leak enough air to stay healthy on its own, so it needs mechanical ventilation — a fan that deliberately brings in fresh air and pushes out stale, moist, polluted air. ASHRAE 62.2 sets the target rate: roughly 0.03 CFM per square foot plus 7.5 CFM per person, which works out to about 60–90 CFM for a typical 3-bedroom home. There are three ways to deliver it — exhaust-only (simplest), supply-only, and balanced HRV/ERV (best, and it recovers heat). Spot fans in the bath and kitchen handle the moisture and odor bursts; whole-house ventilation handles the background air the rest of the time.
Need the rate for your house? The whole-house ventilation calculator computes the ASHRAE 62.2 airflow, subtracts the infiltration credit, and sizes the fan — free, no signup. This guide explains why tight homes need ventilation, what “good” indoor air actually means, and how the three system types differ.
🏠 Why tight houses need ventilation
Older houses were leaky — air sneaked in around windows, outlets, and rim joists fast enough to dilute indoor pollutants without anyone thinking about it. That “natural infiltration” wasted a lot of energy, so modern codes push for a tight, well-sealed envelope. The trade-off is that a tight house no longer breathes on its own, and the same pollutants that used to leak away now build up: moisture from cooking and showering, carbon dioxide from people, VOCs off-gassing from furniture and finishes, combustion byproducts, and radon.
That's the modern rule of thumb: build tight, ventilate right. You seal the envelope so air only moves where you want it, then add a controlled amount of fresh air with a fan. Ventilation isn't a luxury in a new or well-sealed home — it's the mechanism that replaces the leakage you deliberately removed.
The humidity connection. A house with no ventilation traps moisture, and trapped moisture is what feeds mold, dust mites, and condensation on cold windows. Ventilation is the first line of humidity control; a dehumidifier handles what's left in a damp climate or basement.
📏 How much air — ASHRAE 62.2
The industry standard for residential ventilation is ASHRAE 62.2. It sets the whole-house rate from two things: the floor area (bigger house, more air) and the number of bedrooms, used as a stand-in for occupancy. The formula is about 0.03 CFM per square foot of floor area plus 7.5 CFM per (bedrooms + 1). For a 2,000-square-foot, 3-bedroom house that's roughly 90 CFM of continuous fresh air.
The IRC (building code) uses a lower coefficient — about 0.01 CFM per square foot — because it credits more natural infiltration than ASHRAE does. For the same house that lands closer to 50 CFM. Both are “code,” but they answer slightly different questions: the IRC number is a floor, and the ASHRAE number is the health-based target. The calculator shows both so you can see the spread.
One nuance worth knowing: ASHRAE 62.2 lets you subtract an infiltration credit — the small amount of natural leakage a house still has — from the total required rate, so the mechanical fan only has to make up the difference. A leakier house needs a smaller fan; a very tight house gets little or no credit and the fan carries almost the whole load.
🔀 Three ways to ventilate
Once you know the rate, there are three strategies for moving that air, and they differ in cost, comfort, and how they affect the pressure in your house.
- Exhaust-only: a quiet continuous fan (often an upgraded bath fan) pulls stale air out, and fresh air leaks back in through the envelope. It's the cheapest and simplest, but it slightly depressurizes the house, which can pull in radon or backdraft a combustion appliance in the wrong situation, and you don't control where the makeup air comes from.
- Supply-only: a fan pushes filtered fresh air in and stale air leaks out. This slightly pressurizes the house (which keeps radon and outdoor pollutants out) and lets you filter the incoming air, but in a humid climate you're pushing damp air into the walls.
- Balanced (HRV/ERV): equal supply and exhaust fans, so the house stays pressure-neutral. The premium version runs both airstreams through a heat- or energy-recovery core that transfers heat (an HRV) or heat and moisture (an ERV) from the outgoing air to the incoming air — so you ventilate without throwing away your heating or cooling. It costs the most and needs ductwork, but it's the best-performing option and the standard for tight, high-performance homes.
HRV vs ERV. An HRV recovers heat only — good for cold, dry climates. An ERV also transfers moisture, which helps keep humidity out in a hot, humid climate and keeps it in during a dry winter. Match the core to your climate.
🚿 Spot ventilation vs whole-house
Whole-house ventilation handles the steady background air. Spot ventilation handles the bursts: a bathroom exhaust fan clears the moisture from a shower before it condenses on the walls, and a kitchen range hood pulls grease, steam, and combustion byproducts straight off the cooktop. You need both layers — a whole-house fan running at 60 CFM can't keep up with the moisture a hot shower dumps into a small bathroom.
Two rules make spot ventilation actually work. First, the fan has to terminate outdoors (IRC M1501.1) — dumping a bath fan into the attic is a leading cause of attic mold and rotted sheathing. Second, a big kitchen hood (over about 400 CFM, per IRC M1503.4) can depressurize a tight house enough to backdraft a water heater or furnace, so it needs makeup air to replace what it exhausts.
🌫️ What “good” indoor air means
Ventilation targets the pollutants that build up indoors. Moisture is the big one — keep relative humidity roughly in the 30–50% range and you starve mold and dust mites. Carbon dioxide from breathing is a proxy for how stale the air is; it climbs in a closed bedroom overnight and a little fresh air knocks it back down. VOCs off-gas from new furniture, paint, and cabinetry. Radon seeps up from the soil and is the reason pressure balance matters. And ordinary particulates — cooking smoke, dust — are why filtered supply air and a good range hood help.
You don't solve all of these with a single fan. Ventilation dilutes and removes; source control (low-VOC finishes, a radon system, sealing combustion appliances) and filtration do the rest. But adequate fresh air is the baseline every healthy house is built on.
✅ The bottom line
A tight house needs a deliberate supply of fresh air, and ASHRAE 62.2 tells you how much. Size the whole-house rate for your floor area and bedroom count, subtract the infiltration credit, pick the system type that fits your climate and budget (balanced HRV/ERV if you can), and layer spot fans in the bath and kitchen on top. Vent everything outdoors, and give a big range hood the makeup air it needs.
Adopted ventilation codes and the accepted edition of ASHRAE 62.2 vary by jurisdiction, so confirm your local requirements before relying on any figure here. Start with the ventilation calculator to see your target rate and the fan size it implies.
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