Ventilation 4 min read Updated 2026-10-18

Plan Fresh-Air Ventilation for a Tight House

ASHRAE 62.2 for a 2,400 ft² four-bedroom house, what that air costs to heat through a winter, and when an ERV or HRV pays for itself.

Sealing a house is one of the most effective energy upgrades there is, and it creates a new problem: the air that used to leak in now has to be brought in on purpose. Done well, mechanical ventilation gives better air than random leaks ever did. This guide sets the rate for one house with ASHRAE 62.2, works out what that fresh air costs to heat, and compares a simple exhaust fan with heat-recovery ventilation.

The house

A two-storey 2,400 ft² house with four bedrooms has just been air-sealed and insulated. A blower door test after the work found it tight enough that natural leakage alone no longer provides adequate fresh air. Ceilings average 8.5 ft, the climate has about 5,000 heating degree-days (°F) a year, and the house is heated by a 95% efficient gas furnace with gas at $1.40 per therm.

Step 1: the required rate

ASHRAE 62.2-2016 sets the total rate as 0.03 CFM per square foot plus 7.5 CFM for each occupant, counted as bedrooms plus one. The ventilation rate calculator gives 0.03 × 2,400 + 7.5 × 5 = 109.5 CFM, about 0.32 air changes per hour. The standard allows a credit for measured infiltration, which in this tight house would be small, so we will design for the full 110 CFM of continuous fan airflow.

Step 2: what the air costs to heat

Every cubic foot of outdoor air brought in has to be warmed to room temperature. The sensible heat equation gives the rate: 1.08 × CFM × ΔT. On the 65 °F design day that is 1.08 × 109.5 × 65 = 7,687 BTU/h, a substantial part of a tight house's heating load, which the heat loss calculator would show alongside walls and windows. Over a season, multiply by 24 hours and the degree-days: 1.08 × 109.5 × 24 × 5,000 = 14.2 million BTU. At 95% efficiency that is 149 therms of gas, about $209 a year.

Step 3: exhaust fan or heat recovery?

The simplest system is a quiet, continuously running exhaust fan, often an upgraded bathroom fan, with outdoor air entering through the remaining small leaks or dedicated inlets. It costs little to buy and to run, but all of the $209 of heating is lost with the exhaust air. A heat recovery ventilator (HRV) or energy recovery ventilator (ERV) passes the outgoing and incoming air through a heat exchanger. With 70% sensible recovery, it would save about $146 a year in heat in this climate, plus a little in summer cooling. Against an installed cost of perhaps $2,000 to $4,000 more than an exhaust fan, the payback is long on energy alone. The case for an HRV or ERV is stronger in very cold climates, where it also avoids cold draughts from inlets, in houses that are extremely tight, and where filtered, distributed fresh air to each bedroom matters.

Step 4: make the fan deliver its rating

A fan rated at 110 CFM on a short, straight, 6 in. duct may deliver close to its rating; the same fan on 20 ft of crushed 4 in. flex may deliver half. Use the airflow and duct calculators to keep the duct sized for low resistance, choose a fan rated at a realistic static pressure such as 0.25 in. w.c., and measure the delivered airflow once installed. If the fan runs intermittently, for example 50% of each hour, it must deliver twice the continuous rate, 219 CFM, while running.

Common mistakes

  • Counting intermittent bathroom fans. A fan switched on for ten minutes after a shower is local exhaust, not whole-house ventilation.
  • Using the 2010 rate without checking the code. It gives 61.5 CFM for this house, little more than half the 2016 figure.
  • Trusting nameplate airflow. Duct resistance can halve the real figure.
  • Oversizing for safety. Twice the required rate doubles the heating cost and can over-dry winter air.

Ventilation checklist

  1. Calculate the 62.2 rate from floor area and bedrooms, and confirm which edition your code uses.
  2. Apply any infiltration credit from a blower door test.
  3. Estimate the seasonal heating cost of the fresh air from CFM, degree-days and fuel price.
  4. Compare an exhaust-only fan with an HRV or ERV on cost, comfort and climate.
  5. Size the ductwork for low resistance and pick a fan rated at a realistic static pressure.
  6. Measure delivered airflow after installation and adjust the speed or run time.