CFM50

When someone comes to your house and uses a blower door to diagnose the condition of the thermal envelope, a number of different tests and procedures may be performed. In the end, however, you will always be given a number expressed as CFM50. What does it mean?

It is the airflow (cubic feet per minute) that must be exhausted from a house to maintain a pressure of 50 pascals on the shell. So what?

Well, a pressure of 50 pascals is roughly equivalent to the force of a 20 mile an hour wind. It is a nice round number that is really arbitrary. This has become an industry standard because it is a strong enough pressure to learn about the shell without damaging it. More importantly, so many people use this standard that it is easier to compare one house to another. One contractor may boast: "We brought this house from 5200 CFM50 down to 3750 in two days of caulking and foaming." And another may answer: "That's nothing. Our project was blowing 6850 CFM50 and we after we spent the afternoon packing the side attics, it came in under 2800 ."

A leakier house will have more or larger bypasses through it's pressure boundary. We have to run the blower door fan faster to push more air out to get the pressure up to 50 Pa. As the pressure barrier is improved or rebuilt, the house shell gets tighter and the pressure boundary is more resistant to air flow. Now fewer CFM are needed to reach 50 Pa.

CFM50 can be thought of as a unit of effective leakage area. A higher number indicates a bigger aggregate hole in the house. As a rule of thumb, it takes roughly 1 square foot of leakage in the pressure barrier to produce 1000 CFM50. So a house that tests at 4500 has about four and a half square feet of holes letting air pass through it.

None of this says that a house with a lower CFM50 will perform better than one with a higher CFM50. If the shell of a building is very tight except for large cellar leaks, it may have a high CFM50, but be very habitable. More often, the bypasses through a thermal envelope will be randomly distributed. Some architectural styles lead to large holes into the attic. These buildings can be better judged by this pressure measurement.

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