Home Energy Audit Calculator
Convert a blower door result into air changes per hour and an airtightness rating.
About Home Energy Audit Calculator
The blower door measures leakage at an artificial 50 pascal pressure, and dividing by volume gives a figure comparable between houses of different sizes. Natural infiltration is roughly a twentieth of the ACH50 result. Air sealing is normally the cheapest efficiency measure per unit of energy saved, and it must come before insulation because leaks bypass insulation entirely.
Formula
Home Energy Audit
ACH50 = blower door flow at 50 Pa ÷ building volume
- ACH50
- Air changes per hour at 50 pascals of pressure difference
- rule of 20
- Natural air change is roughly ACH50 divided by 20
Worked example
With the default values below, Home Energy Audit Calculator returns Air Changes at 50 Pa of 8.738. Change any input above and every figure here is recalculated in your browser.
Inputs used
- Blower door resultminimum 1 cfm50
- 1,800 cfm50
- Floor areaminimum 1 m2
- 140 m2
- Average ceiling heightminimum 1.5 m
- 2.5 m
- Target air changes at 50 Paminimum 0.1
- 3
Results
- Air Changes at 50 Pa
- 8.738 ACH50
- Building Volume
- 350.00 m3
- Airtightness Rating
- Leaky
- Leakage Reduction Needed
- 1,182.0 cfm50
- Estimated Natural Air Change Rate
- 0.4369 ACH
Step by step
- 1Values used
- 2Home Energy Audit
- 3Air Changes at 50 Pa= 8.738 ACH50
- 4Building Volume= 350.00 m3
- 5Airtightness Rating= Leaky
- 6Leakage Reduction Needed= 1,182.0 cfm50
- 7Estimated Natural Air Change Rate= 0.4369 ACH
Assumptions and limits
- The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
- In particular, this calculator does not take inflation, tax, fees or charges or compounding frequency into account.
- Figures are rounded for display to 4 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.
How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.
Frequently Asked Questions
How is Home Energy Audit calculated?
ACH50 = blower door flow at 50 Pa ÷ building volume The blower door measures leakage at an artificial 50 pascal pressure, and dividing by volume gives a figure comparable between houses of different sizes. Natural infiltration is roughly a twentieth of the ACH50 result.
Why does Home Energy Audit matter?
Air sealing is normally the cheapest efficiency measure per unit of energy saved, and it must come before insulation because leaks bypass insulation entirely.
What values do I need to enter?
This calculator takes 4 inputs: Blower door result, Floor area, Average ceiling height, Target air changes at 50 Pa. The pre-filled defaults are a realistic starting point — replace them with figures from your own environment for a result you can act on.