CFM50 And ACH50 – How They Work (And How They Don’t)

by Erik North on November 12, 2012

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CFM50 and ACH50 are two blower door measures that I bandy around all the time but have never defined. If I were trying to play it mysterious I would say, hey, we have to keep some of this mysterious.

A more ‘actual’ reason would be that, despite my sincere belief otherwise, talking about technical building science is juuuuuust a bit boring. But don’t let that stop you from reading this post. Besides, the science of your house is pretty technical so it may behoove you to dive right in.

CFM50 And ACH50 – Getting Into it

A blower door is one of the most valuable auditor tools with the unfortunately side effect of making it look like the house is being bug-bombed. When passersby ask what the blower door is, it’d be miles easier to say you’re fumigating the house.

The blower door is a vinyl sheathed aluminum frame with a giant fan. The frame is adjustable and can be fit into any standard door frame. The fan blows out, depressurizing the building envelope. This simulates the effect of wind blowing on every surface of the house’s exterior.

This is incredibly useful for finding big air leaks (and big sources of heat loss) in the building envelope. It also measures how much air is moving through the building enclosure. This is where CFM50 and ACH50 come into the picture. CFM50 is the cubic feet of air moving through fan at 50 pascals of negative pressure. OK…what the heck did that last stretch of Latin mean and why is it important?

CFM50 (and an embarrassing admission) – Up front: CFM50 by itself is not terribly useful. It tells you the approximate magnitude of the house’s air leakage under pressure and that’s it. You’re measuring a single data point and forcing an abnormal condition to obtain it.

CFM50 tells you how much air is moving through the envelope at a forced pressure. And as tempting as it may be to extrapolate other things, CFM50 leaves a lot out. It doesn’t tell you exactly how the portion of air leakage from each house section, what the air leakage would be under natural conditions, how much ventilation you need, or whether light beer tastes great or is less filling?

ACH50 – Gosh, CFM50 is useless! Can’t it be used for anything? Well, it is used to directly calculate ACH50 which is the most commonly used modern yardstick for building envelope tightness.

ACH50 is the number of time the air volume in a building changes per hour at 50 pascals of pressure. It is the CFM50 * 60 minutes (to convert to hours) divided by the house’s volume. While there are quibbles about its appropriateness for all buildings, ACH50 is a straightforward way to make apple-ish to apple-ish comparisons of different house’s air tightness.

The limitations of ACH50 is one that’s been noted at GBA and Energy Vanguard (and a lot of other places but those are the ones I read). ACH50 measures the volume of air moving through the house. But what we’re actually testing is the building shell (aka the exterior walls and surfaces). I mean, who cares how much air is moving between your bedroom and living room? We care about how much heated air is escaping to the outside.

But that is for another post where we’ll talk about why measuring the surface air leakage is more important and how to measure it. CFM50 and ACH50 remain the basic language of interpreting blower doors. Other building measures use these as their foundation. So…if you want to know about blower doors, you need to know CFM50 and ACH50.

{ 2 comments… read them below or add one }

Eric November 16, 2012 at 11:16 pm

For volume when calculating ACH50, does that include the entire volume of the house – basement too?


Erik North November 16, 2012 at 11:26 pm


Hey, great name! Here’s a good overview of all things blower door.

You are testing the volume of the air barrier. If that includes the basement, then yes. And since the interior wall is usually the air barrier, you would use the interior measurements.


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