In last week’s blog, I suggested that talking about infiltration rates in terms of air changes per hour isn’t an accurate way to portray air leakage. The problem is that you’re dividing by volume but the leaks happen at the surface. I don’t think ACH₅₀ is going away anytime soon, and I use it myself because everyone else does, even though it’s biased toward larger houses.

The quantity that drives me bonkers, though, is ACH_nat, which stands for Air Changes per Hour under “natural” conditions. Every HERS rater and BPI class teaches people how to calculate this thing and what it means: A house with 1 ACH_nat exchanges all it air with outside air once each hour. The only problem is that it isn’t true. Let’s start from the beginning.

Air leakage and blower door basics

Air leakage happens when you have both of the following:

• a pressure difference, and
• a pathway.

If the pressure outside a building is the same as the pressure inside, you can open all the windows, and little air will move through them. Likewise, you can have a huge pressure difference across the building enclosure, but if there are no holes, there’s no air leakage. So, if you eliminate one or the other of those two conditions, you eliminate the air leakage.

Now, the blower door is really good at telling us about the pathway, that is, the hole in the house. The more air flow a blower door measures, the bigger the hole in the house. (When I say “hole,” singular, I’m referring to the sum of all the leakage sites.) It doesn’t tell us anything about the distribution of the holes or their individual sizes and shapes, although we can walk around with the blower door running and locate the larger leaks. Joe Lstiburek wrote a great article about this last year: Just Right and Airtight.

The problem with extrapolation

What the blower door cannot do, however, is tell us anything about the pressure differences that the home experiences. When the blower door is operating, it puts the house under a controlled pressure difference. When the blower door is not running, the pressure in the house varies season to season and moment to moment. The main driving forces for those changes are wind, the stack effect (warm air rising in the house), and mechanical systems.

Calculating the ACH_nat based on a single point test attempts to factor in the effects of wind and stack effect, but at great cost to the precision. You go from ~3% uncertainty to 20%, 30%, possibly even 50% or more. I don’t think it’s helpful to tell a homeowner, “Your home leaks at the rate of 1 air change per hour — but it could be as low as 0.5 or as high as 1.5, and if you’ve got problems with the mechanical systems, it could even be 2 or more air changes per hour.”

Pressure differences created by mechanical systems can eclipse those created by wind and stack effect. The disconnected supply duct, the leaky panned return, the 1200 cfm commercial range hood with no makeup air — all these things can dwarf the effects we’re trying to capture in ACH_nat.

Blower doors help us estimate how big the holes are

I believe that the pathways for air leakage are really all we need to be concerned with when we’re talking about blower door results. If a house has a hole, we want to know how big it is, if it’s small enough to meet the code or program requirements, and what we can do to make it smaller. That’s what the blower door is good at. We can still help builders and homeowners without extrapolating to the point of meaninglessness if we just focus on the hole.

My point about normalizing the (almost) raw number, cfm₅₀, to the surface area rather than the volume makes sense when you see it this way. If you’ve got a hole in the building enclosure, you compare it to the size of the building enclosure, not the volume.

Yes, pressure differences are important, but let’s understand the limitations of the blower door. It’s great at telling us about the pathways for air leakage through the building enclosure. It can’t tell us anything about the “natural” pressure differences a building will experience.

If we have this tool — the blower door — that gives us numbers with a precision of +/-3%, why would anyone want to manipulate the results into something with uncertainties of 20%, 30%, possibly even 50% or more?!

It’s time to quit using ACH_nat to talk about infiltration.

An alternative way to explain air leakage to homeowners

Defenders of the ACH_nat make the seemingly valid point that it’s conceptually easier to explain to homeowners. When you extrapolate from a house under test pressure to a house under “natural” pressure differences, the ACH_nat tells you how quickly the air in your house is exchanging with outside air.

If, for example, a house comes in at 0.5 ACH_nat, the energy auditor will tell the homeowner that all the air in their home is exchanged with outside air every two hours. Yes, it’s an easy concept for homeowners to grasp, but, as I’ve shown above, it’s not true.

There’s another option here that has far less uncertainty. Blower door results can be turned into leakage areas. The two main ones are Effective Leakage Area (ELA) and Equivalent Leakage Area (EqLA). Both attempt to quantify the size of the hole based on the cfm₅₀.

Another relation that I use also turns cfm₅₀ into hole size: Each 1000 cfm₅₀ is approximately equal to 1 square foot of hole. Don’t believe it? Set up your blower door and try it. Run your test as normal and find your result. Then go to the other side of the house and add a 1 square foot hole. You can do this most precisely with saws, but homeowners and builders probably won’t appreciate your leaving a permanent hole. I usually open a window and measure it to get one square foot. Run the test again, and you’ll find that it’s approximately 1,000 cfm₅₀ higher.

So if you’ve done a blower door test and the result is 6,240 cfm₅₀, the house has a hole that’s about 6 square feet. Point to that 2030 window in the kitchen and tell them that they’ve got a window that size completely open to the outside, 24/7/365. I’ve found that to be just as effective as telling them how many times the air in their house exchanges… and I feel a lot better about my truthfulness when I do so.

Allison Bailes of Decatur, Georgia, is a speaker, writer, energy consultant, RESNET-certified trainer, and the author of the Energy Vanguard Blog. You can follow him on Twitter at @EnergyVanguard.