Zone 1 has the lowest N, meaning the most infiltration for a given ACH50, and Zone 4 the highest. Choose the zone for your site from the wind zone map used with the LBL n-factor table.
📊 ACH50
🌀 Natural ACH (Estimated)
Compliance Check
Full Calculation Table
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How to Use the Blower Door Calculator
Enter the airflow reading from the blower door fan at 50 pascals, and the total conditioned building volume so the calculator can produce ACH50.
Conditioned floor area is optional. It produces a secondary benchmark, CFM50 per 100 square feet of floor area, alongside the ACH50 result.
Pick the LBL wind zone for the site, the number of storeys, and how sheltered the house is. Together they set the divide-by-N factor used to estimate natural, unpressurized infiltration from your ACH50 result.
Pick the code or program target for your project, then click Calculate Airtightness to see ACH50, equivalent leakage area, estimated natural ACH, and a pass or fail result.
Understanding Blower Door Testing
A blower door test is the standard way to measure how airtight a building envelope actually is. Rather than guessing from visible gaps around windows and doors, the test puts a number on total air leakage through every crack, penetration, and seam in the envelope, from top plates to rim joists to electrical boxes.
How the Test Works
A calibrated fan mounts in an exterior doorway with a flexible panel sealing the rest of the opening. The fan depressurizes, or in some protocols pressurizes, the building to a standard test pressure of 50 pascals, roughly equivalent to a 32 km/h wind hitting every side of the house at once. The airflow the fan needs to maintain that pressure difference is the CFM50 reading, and more leakage means the fan has to move more air to hold the same pressure.
Why ACH50 Instead of Raw CFM50
A large house naturally needs more airflow to reach 50 pascals than a small one, even if both are built equally tight. Dividing CFM50 by building volume and normalizing to an hourly rate produces ACH50, air changes per hour at 50 pascals, which lets you compare airtightness across buildings of any size on a level basis. This is the number that Canadian codes and energy programs actually set targets against. ENERGY STAR for New Homes, for example, requires its homes to meet airtightness thresholds that depend on the version of the standard, the province, and the climate zone, with on-site verification by an energy advisor.
Equivalent Leakage Area
Equivalent leakage area, often abbreviated ELA or EqLA, converts the CFM50 reading into the size of a single, sharp-edged hole that would leak the same amount of air at a reference pressure. The Canadian definition uses 10 pascals, and this calculator applies the common field approximation of CFM50 divided by 10 to get square inches, then converts to square centimetres. It gives a more intuitive physical picture than a ratio, and the R-2000 Standard uses a related measure, normalized leakage area at 10 pascals, as an alternative to ACH50.
From ACH50 to Natural Infiltration
ACH50 describes leakage at an artificial, pressurized test condition, not what actually happens under normal wind and stack effect pressures day to day. A widely used simplified method, developed at Lawrence Berkeley Laboratory, divides ACH50 by a factor called N to estimate natural air changes per hour under typical weather. In the full method N depends on climate, building height, and wind shielding, and taller buildings use a lower N because stack effect pushes more air through the envelope. This calculator uses the commonly published n-factor table: a base value for the wind zone, from 15.5 in Zone 1 to 24.5 in Zone 4 for a one-storey house with normal exposure, multiplied by 1.0, 0.9, 0.8, or 0.7 for 1, 1.5, 2, or 3 storeys, and by 1.2 for a well-shielded site or 0.9 for an exposed one. Windier and colder sites get a lower N, so the same ACH50 gives a higher natural rate there. Treat the natural ACH figure as a rough guide. To turn a natural air change rate into a heating or cooling load, the infiltration load calculator picks up where this estimate leaves off.
Canadian Airtightness Targets
There is no single national pass mark. Under NBC 2020 Section 9.36, airtightness is credited through energy tiers, and the tiered performance path treats 3.2 ACH50 as the untested default, so this calculator offers 3.2 as a reference point rather than a code limit. The R-2000 Standard published by Natural Resources Canada requires no more than 1.5 ACH50, or a normalized leakage area of 0.7 cm² per m² at 10 pascals. The CHBA Net Zero program uses a 1.5 ACH50 maximum for detached homes and 2.0 for attached homes, and Passive House certification asks for 0.6. A tighter envelope reduces heat loss but raises the importance of proper mechanical ventilation, since the building can no longer rely on uncontrolled leakage for fresh air.
Pairing Blower Door Results With Ventilation Design
A very tight result is only a benefit if mechanical ventilation is sized to compensate for the reduced natural air exchange. Use the ASHRAE 62.2 calculator to confirm the home still gets adequate fresh air after air sealing work brings ACH50 down.
Frequently Asked Questions
A blower door test uses a calibrated fan mounted in an exterior doorway to depressurize or pressurize a building to a standard test pressure, usually 50 pascals, while measuring the airflow needed to maintain that pressure. The reading, called CFM50, quantifies the total air leakage through the building envelope and feeds directly into this calculator to produce ACH50.
ACH50 is the number of times the entire volume of air in a building would be exchanged in one hour at the 50 pascal test pressure. It normalizes the CFM50 reading for building size, which makes it possible to compare airtightness between buildings of different sizes and to check compliance against code and program targets like NBC 2020 or R-2000.
There is no single national requirement. NBC 2020 credits airtightness through its energy tiers, and its tiered performance path treats 3.2 ACH50 as the untested default, so a lower value has to be shown with a blower door test. The R-2000 Standard requires 1.5 ACH50 or better, CHBA's Net Zero program uses 1.5 for detached and 2.0 for attached homes, and Passive House uses 0.6. Always confirm the specific requirement for your jurisdiction and program before treating a result as pass or fail.
Not for a certified result. A calibrated blower door fan and manometer are required to produce a CFM50 reading that codes and energy programs will accept for permit or certification purposes. A smoke test can help locate individual leaks visually, but it does not quantify total envelope leakage the way a proper blower door test does.
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