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Airflow the blower door had to move to hold the envelope at its reference pressure.
Take it straight off the test report for the whole envelope being pressurised, with the building in the same configuration the worst-case test will use. A result from a single unit in a multi-family building describes that unit's envelope, which includes its party walls, and is not the figure for the block.
Pressure difference the leakage figure above was measured at.
Most residential protocols report at 50 Pa. Some report at 75 Pa, and a few extrapolate to lower pressures. Enter the pressure the number beside it actually belongs to, because the whole extrapolation hangs on that pairing being right.
Exponent in the leakage power law, from a multi-point test report where one exists.
It describes the character of the leaks: a value near 0.5 behaves like large sharp-edged openings, near 1.0 like long narrow cracks. Extrapolating from 50 Pa down to single-digit pascals is a long way, and the exponent is what decides how far the answer moves over that range — which is why it is worth finding the measured one.
Everything that can exhaust at once: range hood, dryer, bathroom fans, central vacuum, and the exhaust side of the recovery unit.
The worst case is a real operating state, not a hypothetical one — every exhaust appliance on, interior doors in their least helpful positions, and the envelope closed. A solid-fuel appliance drawing on its own flue belongs in this total as well.
Supply air the ventilation system delivers while the worst case is running.
Enter what the supply fan actually delivers during the worst case, not its rating. A recovery unit in defrost may have stopped or reversed its supply fan for the duration of the cycle, and in a cold-climate house with atmospheric appliances that detail decides the answer.
Dedicated makeup air that is interlocked to open or run whenever the large exhaust appliance does.
Only interlocked provision counts. A passive vent with no damper control, or a note asking the occupants to open a window, is not makeup air and should be entered as zero — that assumption is the difference between a compliant design and a hazard.
The cap the governing code or standard applies to this building.
Where naturally aspirated combustion appliances share the envelope the cap is a small number of pascals, and both the number and the exhaust capacity that triggers mandatory makeup air are jurisdictional. Confirm the figure locally rather than carrying one over from the last province or state you worked in.
Worst-case envelope depressurisation
17 Pa
This configuration exceeds the limit on the power-law estimate, and the remedies are interlocked makeup air, a sealed-combustion appliance, or a smaller hood — never a note in the handover pack asking the occupants to crack a window. Confirm with a blower door and a manometer before changing the design.
- Net exhaust flow across the envelope
- 529.72 CFM
- Permitted depressurisation
- 5 Pa
- Margin to the limit
- -12.21 Pa
- Net exhaust the envelope carries at the limit
- 237.18 CFM
They open the calculator with your figures already in it
Worst-Case Envelope Depressurisation Calculator: 17.21 Pa — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- Envelope leakage follows the power law Q = C x dP^n, so a single-point blower-door result at a stated reference pressure gives dP = P_ref x (Q_net / Q_ref)^(1/n) for any other flow
- CSA F326, Residential Mechanical Ventilation Systems, together with the applicable mechanical and fuel-gas codes, sets the depressurisation limit and the exhaust capacity at which dedicated makeup air becomes mandatory — both are jurisdictional, which is why the limit is entered here rather than published by this page
- A flow exponent of 0.65 is the conventional assumption for a house envelope when the test report gives no measured exponent; it is a rule of thumb, not a property of your building, and a measured exponent from a multi-point test should replace it
Inputs used
- Envelope Leakage at the Test Pressure
- 1059.44 CFM
- Blower-Door Reference Pressure (Pa)
- 50
- Envelope Flow Exponent
- 0.65
- Total Simultaneous Exhaust Flow
- 529.72 CFM
- Balanced Mechanical Supply Flow
- 0 CFM
- Interlocked Makeup Air
- 0 CFM
- Permitted Depressurisation Limit (Pa)
- 5
Intermediate steps
- Net exhaust flow across the envelope
- 529.72 CFM
- Permitted depressurisation
- 5 Pa
- Margin to the limit
- -12.21 Pa
- Net exhaust the envelope carries at the limit
- 237.18 CFM
Confidence note: This configuration exceeds the limit on the power-law estimate, and the remedies are interlocked makeup air, a sealed-combustion appliance, or a smaller hood — never a note in the handover pack asking the occupants to crack a window. Confirm with a blower door and a manometer before changing the design.
What this calculation does not cover
- Ignores stack effect and wind, both of which add to or subtract from the mechanical pressure and can dominate on a cold or exposed day.
- Treats the envelope as one zone. A closed interior door between the exhaust and the leakage it draws through creates a worse local pressure than this whole-building figure.
Add the equipment this sizes
This result is a specification — 17 Pa — not a quantity. Put the thing it sizes into your project: how many, what you call it, and your supplier’s price.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-08-30 · in the site-wide review of 2026-09-06 · v1.0.0
Regulatory standards & verification citations3
- Envelope leakage follows the power law Q = C x dP^n, so a single-point blower-door result at a stated reference pressure gives dP = P_ref x (Q_net / Q_ref)^(1/n) for any other flow
- CSA F326, Residential Mechanical Ventilation Systems, together with the applicable mechanical and fuel-gas codes, sets the depressurisation limit and the exhaust capacity at which dedicated makeup air becomes mandatory — both are jurisdictional, which is why the limit is entered here rather than published by this page
- A flow exponent of 0.65 is the conventional assumption for a house envelope when the test report gives no measured exponent; it is a rule of thumb, not a property of your building, and a measured exponent from a multi-point test should replace it
Which documents these citations point at
Standards referenced: CSA F326 (CSA Group, Canada).
Cite this page
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