Blower Door ACH50 & Natural Infiltration Calculator
Convert a blower door test result to air changes per hour at 50 Pa (ACH50), CFM50 per square foot of envelope, equivalent leakage area, estimated natural infiltration with the LBL N-factor, compare with code tightness targets (IECC 3 or 5 ACH50, Passive House 0.6) and see when mechanical ventilation is required.
How it works
ACH50 = CFM50 × 60 ÷ volume; CFM50/ft² = CFM50 ÷ envelope area; natural CFM ≈ CFM50 ÷ N (LBL N-factor 15–30 by climate, height, shielding); ELA (in²) ≈ 0.055 × CFM50
A blower door depressurises a house to 50 pascals — roughly a 20 mph wind on all sides at once — and measures the airflow needed to hold it there, which is a repeatable proxy for how leaky the envelope is. Dividing by the house volume gives ACH50, the number building codes regulate, while dividing by envelope area is fairer when comparing houses of different shapes. Real infiltration under natural wind and stack pressures is far smaller and weather-dependent; the Lawrence Berkeley Lab "N-factor" divides CFM50 by 15–30 depending on climate, height and shielding to estimate the annual average. Once a house is tight enough that natural leakage falls below the ventilation people need, mechanical ventilation becomes mandatory — build tight, ventilate right.
Worked example
A 2,200 ft² two-storey house with 9 ft ceilings (19,800 ft³) testing at 1,800 CFM50 has 5.45 ACH50 — a typical newer existing home, missing the 3 ACH50 code target for cold climates — and about 0.30 CFM50 per ft² of envelope, equivalent to a 10″ square hole. With N ≈ 16.7 the natural infiltration averages 108 CFM, above the ≈ 96 CFM ASHRAE 62.2 rate but delivered only when the weather cooperates.
Frequently asked questions
What ACH50 should I aim for in a retrofit?
Every point helps; getting an old house from 12 to 6 ACH50 typically cuts heating bills 10–20%. Below about 5 add mechanical ventilation, and below 3 make sure combustion appliances are sealed-combustion or tested for backdrafting.
Why does the N-factor vary so much?
It captures how strongly typical weather pushes air through the leaks: cold windy climates and tall exposed houses have large driving pressures (small N, more infiltration), while a sheltered bungalow in a mild climate barely breathes (large N).
Related calculators
Standard HVAC field formulas (sensible heat equation, fan laws, ASHRAE 62.2 and blower-door conventions) at sea-level air density unless noted. Use manufacturer data, a Manual J/D/S load calculation and a licensed HVAC technician for design and commissioning decisions.