Modern homes are sealed tight enough that they need mechanical ventilation. The 1980s and earlier housing stock leaked enough air through cracks that infiltration covered the fresh-air requirement; new construction at 3 ACH50 or tighter does not. ASHRAE 62.2 is the standard the IECC and most state codes reference for "how much fresh air is enough." Here is the math and the practical strategies.

The ASHRAE 62.2 whole-house formula

ASHRAE 62.2-2019 sets the whole-house ventilation rate as:

Q = 0.03 × A_floor + 7.5 × (Nbr + 1)

Where:

  • Q = total whole-house ventilation in CFM
  • A_floor = conditioned floor area in ft²
  • Nbr + 1 = number of bedrooms plus 1 (assumed occupancy)

A 2,000 ft² 3-bedroom home: Q = 0.03 × 2000 + 7.5 × 4 = 60 + 30 = 90 CFM. That's the continuous fresh-air target. Run a bath fan at 50% for 24 hours and you've delivered roughly that.

When mechanical ventilation is required

ASHRAE 62.2 (and the IECC reference) requires mechanical ventilation when the home tests below 5 ACH50on a blower-door test. Practical translation:

  • Pre-2000 home with original windows and no air sealing — usually 8-15 ACH50 — natural infiltration covers it
  • Post-2010 home built to IECC code — usually 3-5 ACH50 — right at the threshold
  • Any home retrofitted with sprayfoam, new windows, and air sealing — often 1-3 ACH50 — definitely needs mechanical

Run the calc with the ventilation CFM calculator.

Local exhaust requirements

ASHRAE 62.2 also sets minimums for local (point-source) exhaust:

  • Kitchen: 100 CFM intermittent (range hood) or 5 ACH continuous at the kitchen volume — usually 25 CFM continuous
  • Bathroom: 50 CFM intermittent or 20 CFM continuous, per bathroom
  • Laundry: 50 CFM intermittent

A bath fan can pull double duty — 50 CFM during shower use, 20 CFM continuous as part of the whole-house ventilation total. Panasonic WhisperGreen and similar dual-speed fans are designed exactly for this.

Three ventilation strategies

Exhaust-only. Run a continuous bath fan at the calculated CFM. Cheapest, simplest, slightly depressurizes the house. Fresh air enters through cracks and passive vents. Works fine in mild climates.

Supply-only. Duct outdoor air to the return side of the air handler. Slightly pressurizes the house, which keeps moisture out of walls in cold climates. Mixing with conditioned air avoids the "cold draft" problem.

Balanced (HRV / ERV). Best by far. A heat recovery ventilator (cold climate) or energy recovery ventilator (humid climate) recovers 60-90% of the heat or moisture from the exhaust stream and transfers it to the incoming fresh air. Costs $1,200-3,000 installed; pays back in energy savings in 5-10 years and gives the cleanest indoor air.

Sizing an HRV or ERV

Match the unit's rated CFM to the ASHRAE 62.2 whole-house number, plus a 25% margin for actual installed flow vs rated flow. A 90 CFM target means a 110-120 CFM rated HRV.

HRV vs ERV:

  • HRV recovers heat only. Good for cold dry climates (zones 6-8) where you want to dump indoor moisture in winter.
  • ERV recovers heat AND moisture. Good for humid climates (zones 1-4) where keeping summer humidity out is the priority. Also good for cold climates if your home runs too dry in winter.

Common ventilation mistakes

Spec'ing a 110-CFM bath fan and assuming it delivers 110. Installed flow is typically 60-70% of rated due to flex duct restrictions, elbows, and roof caps. Verify with a flow hood or oversize the fan.

Skipping ventilation in a tight retrofit.Spray-foaming an attic and replacing windows takes a 12 ACH50 house to 3 ACH50 in a weekend. Indoor air quality complaints follow within months — moisture on windows, lingering cooking smells, off-gassing buildup. Add mechanical ventilation as part of any envelope retrofit.

Pulling fresh air from a garage or attic.The fresh-air intake has to be outdoors, away from the garage, dryer vent, plumbing vent, and bathroom exhaust. Minimum 10 ft of separation per ASHRAE 62.2.

Estimate only. The ventilation CFM calculator uses ASHRAE 62.2-2019 rates. Code adoption varies by jurisdiction. Installed flow must be verified at the grille with a flow hood, and tight new construction requires a blower-door test to confirm the mechanical-ventilation threshold.