What Are Air Changes per Hour (ACH)?

What ACH measures

Imagine the air in your house as one big box. One air change means a whole box of outdoor air has come in and a whole box of indoor air has gone out. ACH counts how many times that happens per hour.

Two very different things use the same unit. Infiltration is the uncontrolled air that leaks in through cracks. Ventilation is the planned fresh air from fans or a ventilator. Both can be expressed in ACH.

Because natural leakage changes with wind and temperature, builders test airtightness under a fixed pressure. A blower door fan in an exterior door pulls the house to 50 pascals below outdoors, and the airflow it takes, cfm50, becomes ACH50 ¹.

How ACH50 is measured

1. Close all exterior doors and windows; open interior doors.
2. Turn off combustion appliances and exhaust fans.
3. Fit the blower door into an exterior doorway.
4. Run the fan to hold the house at 50 Pa below outdoors.
5. Read the airflow through the fan: that is cfm50.
6. ACH50 = cfm50 × 60 ÷ house volume (cubic feet).

The tester signs a report for the building official on new homes where the code requires it.

Where ACH fits in HVAC design

1. Blower door test → ACH50 (airtightness)
2. → Manual J infiltration load (leakier house, bigger load) ⁴
3. → ASHRAE 62.2 ventilation rate, minus any infiltration credit ³
4. → Ventilation fan or ERV/HRV sized in CFM
5. → Equipment sized by Manual S

Kinds of air change figures

Worked example: one house, two measures

Assumed house: 2,000 sq ft with 8-foot ceilings (16,000 cu ft), 3 bedrooms, blower door reading of 800 cfm50. Limits from the 2021 and 2024 IECC ^{2 1}; ventilation from ASHRAE 62.2 ³.

The 3.0 is a stress-test number at 50 Pa; on a calm day the house leaks far less. The ventilation rate is the planned fresh air the house should get in normal use. Sealing this house to 2.5 ACH50 would need cfm50 at or below 667.

How a blower door number changes the load calculation

Infiltration is one of the loads a Manual J must account for, alongside walls, windows, ventilation and internal gains ⁴. ACCA describes Manual J as the ANSI-recognized standard for equipment sizing loads ⁵. Without a test, the designer has to pick an assumed tightness level, and a cautious guess pushes the load up.

ACCA training lists "design to the worst case scenario (e.g., very loose house)" as one way loads get inflated ⁴. A measured ACH50 removes that guess.

For a house that has had air sealing, the measured number is often much lower than the default a designer would have used, and the equipment can shrink to match.

Sources: [5] [4]

Where you see ACH

• On a blower door report for a new home or an energy audit.
• In energy code inspection forms ².
• In a Manual J report as the infiltration input ⁴.
• In ventilation design for ERVs and HRVs.
• In air purifier marketing, as "air changes" in a room, a different use.

Low ACH50: benefits and trade-offs

What ACH50 does not tell you

ACH50 does not say where the leaks are. A smoke pencil or infrared camera during the test finds them. It also does not tell you the real leakage on a given day, which depends on wind and indoor-outdoor temperature difference.

Small homes can look leaky on ACH50 because the same size crack is a bigger share of a smaller volume. That is one reason the 2024 IECC allows a per-surface-area metric, 0.22 cfm/ft² of envelope area, on some compliance paths, and 0.27 cfm/ft² for homes of 1,500 sq ft or less ¹.

How airtightness affects cost

Air sealing is often one of the cheaper energy upgrades, and it can let you buy smaller HVAC equipment because the Manual J load drops ⁴. The test itself is usually part of an energy audit or new-home inspection.

HyreHVAC does not list sealing prices; see HVAC energy audits and the energy savings calculator.

How a house gets tighter

1. Blower door test to get a baseline ACH50.
2. Find leaks: attic hatch, top plates, plumbing and wire holes, recessed lights, rim joists.
3. Seal with caulk, foam, gaskets and weatherstripping.
4. Seal ducts too; return leaks in attics behave like house leaks.
5. Re-test; add or check mechanical ventilation ³.
6. Have combustion appliances checked for safe venting after sealing. A tighter house changes how exhaust fans and vented appliances interact, so this check is not optional.

Keeping a tight house healthy

• Run bath and kitchen fans when in use.
• Keep ventilation fans or ERV/HRV running as designed.
• Watch indoor humidity; see relative humidity.
• Test CO alarms monthly if you have gas appliances.

Warning signs

• Very leaky: drafts, dust lines on carpet edges, high bills.
• Very tight without ventilation: stuffy air, window condensation, lingering odors.
• Backdrafting signs at a gas water heater or furnace (soot, CO alarm): get out and call a pro.
• Moisture or mold on cold walls or closets.

Code limits on ACH50

2021 IECC. As printed on the City of Sanger, Texas, test form, air leakage must be no more than 5 ACH50 in Climate Zones 1 and 2 and 3 ACH50 in Zones 3–8; the form notes a North Central Texas regional amendment allowing 5 ACH50 ².

2024 IECC (R402.5.1.3). For buildings following the prescriptive path (R401.2.1): no more than 4.0 ACH50 in Climate Zones 0–2, 3.0 in Zones 3–5 and 2.5 in Zones 6–8.

Other paths allow 4.0 ACH50 or 0.22 cfm/ft² of envelope area, with 0.27 cfm/ft² for some attached units and homes of 1,500 sq ft or less.

A reduced air leakage credit (R408.2.1.4) requires 2.0 to 2.5 ACH50 ¹.

Ventilation. ASHRAE 62.2 sets whole-dwelling ventilation and lets designers take an infiltration credit based on a blower door test ³. Adoption of each edition varies by state.

Sources: [2] [1] [3]

ACH vs related terms

Common misconceptions

Myth A house needs to "breathe" through leaks.

Reality Leaks are random and uncontrolled; planned ventilation is better ³.

Myth ACH50 is how much a house leaks every day.

Reality It is measured at 50 Pa; natural leakage is far lower.

Myth Tighter is always risky.

Reality Tight plus ventilation is the goal; risk comes from tight without ventilation.

Climate zone sets the target

When this matters to you

ACH50 matters when you build, buy a new home, or plan an energy upgrade. A test before replacing HVAC tells the Manual J designer what infiltration to use, which can change the equipment size.

Next step: ask for a blower door test as part of an energy audit, and plan ventilation with our ERV and HRV services page.

Related terms

Ventilation Rate

Ventilation rate is the amount of outdoor air a ventilation system brings into a home, or stale air it exhausts, usually measured in cubic feet per minute (cfm).

Duct Leakage

Duct leakage is the air that escapes from, or is pulled into, a heating and cooling duct system through gaps, holes and loose joints instead of moving between the equipment and the rooms.

Energy Recovery Ventilator (ERV)

An energy recovery ventilator (ERV) is a balanced ventilation unit that exchanges stale indoor air for fresh outdoor air through a core that transfers both heat and water vapor between the two streams, so incoming air arrives closer to indoor temperature and humidity than it would through a plain fan.

Heat Recovery Ventilator (HRV)

A heat recovery ventilator (HRV) is a balanced ventilation unit that exhausts stale indoor air and brings in an equal flow of fresh outdoor air, passing both streams through a heat exchanger so the outgoing air warms or cools the incoming air without mixing them; it transfers heat (sensible energy) only, not moisture.

CFM (Cubic Feet per Minute)

CFM, or cubic feet per minute, is the unit HVAC uses for airflow: the volume of air, in cubic feet, that a blower, duct, register or fan moves each minute.

HVAC Energy Audit

An HVAC energy audit, usually called a home energy audit or assessment, is an inspection and set of measured tests, such as a blower door test and duct leakage test, that shows where a building loses energy through its envelope, ducts and heating and cooling equipment, and ranks the fixes worth making.

Manual J Load Calculation

A Manual J load calculation is the ACCA standard method for working out how many BTU per hour a home loses in winter and gains in summer at local design temperatures, so heating and cooling equipment can be sized to the house.

Indoor Air Quality

Indoor air quality (IAQ) is the condition of the air inside and around a building, especially how it affects the health and comfort of the people in it.

Also referenced from: CFM (Cubic Feet per Minute), Duct Leakage, Ventilation Rate, Heat Recovery Ventilator (HRV), Demand-Controlled Ventilation, HVAC Energy Audit