Glossary · Testing & air leaks
Pascal (Pa) and "50 Pa"
Also called: pascals · 50 Pa · 50 pascals · inches of water column · inch of water · in. w.c.
A small unit of air pressure; airtightness tests hold a house at 50 pascals, about 1/2,000 of normal air pressure, so results can be compared.
What it means
A pascal is a very small amount of pressure, so building pressures are read with a sensitive gauge called a manometer. Testing every house at the same 50 Pa difference makes leakage results (CFM50 and ACH50) comparable from home to home. Duct leakage tests use 25 Pa, and combustion safety checks look at pressure changes of just a few pascals.
Why it matters in an audit
Pressure readings show the auditor how hard air is being pushed through leaks and whether exhaust fans could pull combustion gases back down a chimney (see depressurization).
Typical numbers
Reference values with their sources. Your auditor’s report gives the figure for your own house.
| What | Value | Source |
|---|---|---|
| 50 Pa in US units | About 0.20 inch of water column (1 in. w.c. at 60°F = 248.84 Pa) | nist.gov ↗ |
| Duct leakage test pressure | 25 Pa (0.10 in. w.c.) | basc.pnnl.gov ↗ |
| For scale | Standard air pressure is 101,325 Pa, so 50 Pa is about 1/2,000 of it | nist.gov ↗ |
In the United States
US gauges and HVAC specifications often use inches of water column (in. w.c.) instead of pascals.