The bands are round numbers. For your own town enter the January 2.5% or 1% design temperature from NBC Appendix C as a custom value.
📊 Natural ACH
🔥 Heating Load
Calculation Breakdown
Full Calculation Table
| Parameter | Value | Result |
|---|
How to Use the Infiltration Calculator
Use the blower door method if you have an ACH50 test result, or the construction-era method for a rough estimate without a test.
Enter the conditioned volume of the house and, for the blower door method, the ACH50 result and an N-factor that matches the height and wind exposure.
Pick a climate band or enter the January design temperature for your town, used to convert infiltration airflow into a heating load.
Click Calculate Infiltration to see the natural air change rate, infiltration airflow and equivalent heating load in metric or imperial units.
Turning a Blower Door Result Into a Real Number
A blower door test depressurizes a house to a standard 50 Pascals and measures how much air the fan needs to move to hold that pressure, giving a result called ACH50. That number is useful for comparing houses and tracking air sealing progress, but 50 Pa is a much bigger pressure difference than what drives infiltration day to day, which comes from wind and the stack effect of warm air rising and escaping through upper leaks while cold air enters lower ones.
The N-Factor Method
The divide-by-N method from Lawrence Berkeley National Laboratory estimates the average natural air change rate as ACH50 divided by N. N is the product of a climate factor, a height factor, a shielding factor and a leakiness factor. Height runs from 1.0 for one storey down to 0.7 for three, and shielding from 1.2 for a sheltered lot down to 0.9 for an exposed one. Both go the way the physics says: a taller house has more stack effect and an exposed one sees more wind, so for the same blower door result it leaks more in ordinary weather, and N gets smaller. The old rule of thumb, ACH50 divided by 20, is the one-storey, normal-exposure case. With a climate factor of 20, the buttons above give N of 24 for a sheltered one-storey house, 20 for a typical one-storey, 16 for a typical two-storey and about 13 for an exposed three-storey. Take a 350 m³ two-storey house that tests at 4.5 ACH50. At N of 16 it leaks 0.28 air changes per hour, which is 350 × 0.28 ÷ 3.6 = 27.3 L/s (58 CFM).
Estimating Without a Test
If no blower door test is available, a rough estimate based on construction era can stand in, though it carries much more uncertainty than an actual test. The presets here are working figures: 0.8, 0.4, 0.2 and 0.09 natural air changes, which at an N of 16 correspond to roughly 13, 6, 3 and 1.5 ACH50. The last one matches the R-2000 standard from Natural Resources Canada, which requires an air change rate of no more than 1.5 at 50 Pa. Passive House goes further, to 0.6 ACH50. Pre-1980s construction without a dedicated air barrier often runs far above the first figure, so treat any era estimate as a placeholder until a test replaces it.
From Air Changes to a Heating Load
Airflow in L/s is the volume in m³ times the air change rate, divided by 3.6. Warming that air takes about 1.2 W per L/s for each degree, so the load is 1.2 × airflow × (21 °C minus the design temperature). For the example house at -25 °C the temperature difference is 46 K, and 1.2 × 27.3 × 46 comes to 1,509 W, or about 5,150 BTU/h. The design temperature should come from climatic data for your own town, and the design temperature lookup is the quickest way to find it. The air change rate from ACH50 divided by N is an average-weather figure, so on the coldest, windiest night leakage runs higher than this. Read the load as a reasonable estimate rather than a ceiling.
Does Leakage Count as Ventilation?
In new Canadian houses it does not. The National Building Code asks for a mechanical ventilation system, since leakage swings with weather and cannot guarantee a steady supply of fresh air. ASHRAE 62.2 treats it differently. As this Lawrence Berkeley National Laboratory review of infiltration in ASHRAE residential standards explains, the standard builds a fixed default amount of infiltration into its totals and, for existing buildings, lets measured leakage above that default count in part. Either way the credit rests on a measured blower door result, which is another reason an era estimate is a poor basis for a ventilation decision.
Why Tighter Isn't Automatically Better Without a Plan
Reducing infiltration cuts the uncontrolled heating and cooling load calculated here, which is almost always good for energy use. The catch is that leakage was also providing some incidental fresh air, and a tight house can no longer count on that. Once a home gets tight enough, mechanical ventilation through an HRV or ERV does all the work. Check the whole-house ventilation calculator for the continuous rate a tight home needs, and size the recovery equipment with the HRV calculator or ERV calculator. Before you pay for sealing work, the air sealing savings calculator puts a dollar figure on the heat you would stop losing, which tells you how much the job is worth.
Leaks, Exhaust Fans and Pressure Balance
Infiltration is also the make-up path for any exhaust fan that is not matched by supply air. A leaky shell hides a supply and exhaust mismatch because outdoor air finds its way in, while a tight one shows the mismatch as pressure, which can pull combustion gases back down a flue or starve a fan. The balanced ventilation calculator checks whether supply and exhaust match, and the ventilation heat load calculator adds the load of the mechanical airflow to the infiltration load found here.
Frequently Asked Questions
Divide ACH50 by an N-factor from the Lawrence Berkeley National Laboratory (LBL) method. N combines climate, building height, wind shielding and leakiness, and a common rule of thumb is 20 for a one-storey house in normal exposure. Taller or more exposed houses get a lower N, because they leak more in ordinary weather, so this calculator uses 24 for a sheltered one-storey house, 20 for a typical one-storey, 16 for a typical two-storey and 13 for an exposed three-storey. A result of 4.5 ACH50 at an N of 16 works out to 0.28 natural air changes per hour.
It depends on age and build quality, which is why a test beats any estimate. Older houses without a continuous air barrier often test at 7 to 15 ACH50 or worse, and newer code-built houses typically land in the low single digits. The R-2000 standard requires 1.5 ACH50 or less, and Passive House targets 0.6 ACH50.
In a new Canadian house it does not. The National Building Code calls for a mechanical ventilation system, because leakage swings with wind, temperature and equipment operation and cannot guarantee a steady rate. ASHRAE 62.2 does allow a limited credit based on measured leakage, mostly for existing homes. A tighter home needs its ventilation system more, not less. Check the whole-house ventilation calculator for the continuous rate.
The load is about 1.2 W for every L/s of leakage for each degree of temperature difference. The example house (350 m³, 4.5 ACH50, N of 16) leaks 27.3 L/s, which is a 1,509 W load at -25 °C. Over a season of 4,000 degree days that air takes about 3,150 kWh of heat. Air sealing is often one of the cheapest upgrades before adding or upsizing equipment, since the heat you stop losing also shrinks the furnace you need.