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Building Science

Air Leaks Happen at the Surface, Not in the Volume

So why do we still report blower-door results in ‘air changes per hour’?

Use a blower door to determine your home’s air leakage rate. Diane Milliken of Horizon Residential Energy Services Maine uses a blower door to measure the amount of air leakage coming in through the surface of the building enclosure.
Image Credit: Energy Vanguard

During the Westford Symposium on Building Science in 2010,* I was watching the tweets from the people who were there. At one point, I saw this one: “@EFL_Guy: ‘Air leaks through surfaces, not volume’ Joe Lstiburek.” I’d been meaning to blog about this issue for a while, so I wrote an article about it. Now, a couple of years later, it’s time for a little update. Also, between my original article and this update, Lstiburek wrote an excellent article on what blower doors are good at: Just Right and Airtight. You’ll want to read that article, too.

If you know nothing about the field of building science, Joe’s statement above probably seems so obvious that you wonder why anyone would even utter it. Had it only been so obvious to a few more people back in the early days of testing buildings for infiltration, we wouldn’t be in the mess we’re in now.

How do we compare air leakage in homes of different sizes?

The reason we’re talking about this, of course, is that we want to be able to compare the blower door test results for houses of different sizes. If I told you that I have two friends who both weigh 200 pounds, you’d have no idea if either is overweight. If I then tell you that one is 6’4″ and the other is 5’2″, you now have a better picture of the situation. The former is at a normal weight for his height, while the latter is obese. We can “normalize” the results by taking their heights into consideration.

We generally talk about the size of houses in terms of the square footage of conditioned floor area (CFA). Fortunately, no one decided to institutionalize a blower door metric using that quantity because, although leakage would scale with CFA, it doesn’t yield an accurate picture. As you already know from the title of this article and from Joe’s statement, air leakage occurs at the surface of the building enclosure, which doesn’t keep the same proportion to the CFA for different houses. A 3,000-square-foot ranch house, for example, will have more enclosure area than a 3,000-square-foot colonial, because more of its floor and ceiling areas are part of the enclosure.

These days, everyone who learns how to use a blower door gets indoctrinated into the cult of the ACH (air changes per hour). You take the raw number from your test — cubic feet per minute (cfm) of air flow with a specified pressure difference between the house and the outside (usually 50 Pascals), also called the cfm50 — and you divide it by the volume. (You also multiply by 60 to change minutes to hours.)

So we normalize our blower door results using the home’s volume for this metric.

Volume is the wrong quantity to normalize blower door results

Why volume?! It’s perfectly obvious that if you want to normalize a number, you divide by something that makes sense. Would it make sense to measure the efficiency of your car in miles per square foot of gasoline? That’s a similarly absurd ratio, because you pay for gallons of gasoline, and a given volume of gasoline can have different surface areas depending on the shape of the container. (That reminds me of the joke about the spherical cow.)

It’s the same with houses. Just as two homes with the same CFA can have different surface areas, two houses with the same volume can have different surface areas. And, as Joe said, air leaks through the surface, not through the volume, so not only do we not care about volume when we’re talking about air infiltration, it’s misleading.

Normalizing to volume also builds in a bias toward larger homes. Since surface area is proportional to the square of the radius and volume is proportional to the cube of the radius, the volume increases faster than the surface area as a house grows in size. So, large houses benefit when dividing by volume instead of surface area.

You want a fair comparison

The whole reason to divide the cfm50 by anything is so that you can compare infiltration rates in different size houses. If you can’t get a fair comparison between small houses and large houses if we use volume, why are we using volume?

Here’s another example for you. Let’s say you want to paint your house, so you go to, where they tell you that it takes about one gallon per 350 square feet of surface area you’re going to paint. What if over at, however, they told you that you’ll need one gallon for each thousand cubic feet of volume? Whose advice are you going to take? Right! Dummies are better than El Stupidos. You’re not painting the volume; you’re painting the surface.

Air leaks into and out of the house through all the surfaces that make up the building enclosure: floors, ceilings, and walls. If you wanted to reduce your home’s air leakage, you wouldn’t seal the volume. You’d seal the leaks through the surfaces. (Just don’t seal those weep holes at the bottom of your brick walls!)

An alternative to air changes per hour

As I commented in Martin Holladay’s musing on blower doors a while back, there’s an easy solution to this problem. Let’s use the quantity that Southface uses in the EarthCraft House program: what they call the Envelope Leakage Ratio, or ELR (although I’d call it the Enclosure Leakage Ratio). Divide the cfm50 by the surface area of the building enclosure. It’s simple. It makes sense.

Despite my protestations here, I do use ACH50 all the time. It’s pretty well established in energy codes and programs, and it’s probably not going away. Just understand that when you use ACH50, it’s not the best metric and it’s biased toward larger houses.

The quantity that I really dislike, however, is ACHnat, which supposedly takes your blower door result and then tells you how leaky the house will be under “natural” conditions, when the blower door is turned off. Next week I’ll tackle that one.

* Yes, that was the one that inspired one of my most popular articles, I Don’t Need No Stinkin’ Building Science Summer Camp.

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


  1. stuccofirst | | #1

    We would still need ACH50 when calculating the BAS, or can we get that from the surface area calc also?

  2. GBA Editor
    Allison A. Bailes III, PhD | | #2

    Response to shane claflin
    Ah, yes, ventilation and the BAS - Building Airflow Standard. As I said at the end of the article, I still use ACH50 when I have to for programs and energy codes. For the real answer to your question, though, Joe Lstiburek wrote about this issue better than I have here. See his article, Just Right and Airtight. You'll find there that he refers to the old saying, Build tight, ventilate right. Then he writes, "Blower doors can help measure the 'tight' part. They cannot measure the 'ventilate right' part."

    If you're not constrained by program or code requirements, you can follow his advice for ventilation: "To me, the ventilate right part is easy: put in a ventilation system and pick a rate." In footnote 4, he explains how to do that:

    Our approach is to design and install a controlled ventilation system that is capable of ventilating at 1.5 times the current ASHRAE Standard 62.2 recommended rate, commissioning the system at 60% of the current ASHRAE Standard 62.2 recommended rate and giving control of the ventilation system to the occupant and telling them they can turn it up, turn it down or turn it off. They are responsible for their own environment. This personal responsibility thing is big with us. Of course, there are folks who want to control your thermostat, and folks who are going to want to control your ventilation stat. I say, we hunt these people down and get them out of the gene pool. Just my opinion, mind you. Apparently, elections have consequences. Before I get a zillion comments, I know it is not quite the same. Most of us know that there is a safe range for temperature: don’t turn off the heat completely as things will freeze. Also, most of us can sense temperature; we know when it is too hot and when it is too cold. Ventilation is a little bit different. It can be argued that most people don’t know when they have enough ventilation or when they have too little. I like ranges. A range of ventilation rates that fit between the bookends of too much and too little. We can set the “bookends” from a policy perspective and then get out of folks’ way.

  3. user-1119494 | | #3

    Preaching to the choir, mate!

    I agree that leakage/volume is foolish, but can see good arguments for either 1) leakage/surface area (how well you have built) or 2) leakage/floor area (how leaky the house is compared to livable space). If we use just one we will get problems: eg: #1 could tell you that two houses are the same even if one has a small floor space and 12' ceilings while #2 encourages huge houses. How about also using (leakage)/(occupancy rating) to encourage efficient spaces?

  4. Joseph Rice | | #4

    House shapes
    Wouldn't using (leakage/surface area) reward houses with complicated shapes?

    Designing houses with simple shapes in order to minimize surface area seems like one of the biggest "bang for the buck" ways to save energy and I'd hate to see that penalized.

    (Leakage/floor area) seems like an improvement over (leakage/volume) insofar as it doesn't reward high ceilings. (Leakage/floor area) also rewards more efficient shapes, like the 2 story colonial vs the 1 story ranch with the same floor area, since presumably the colonial - with less surface area - would have less leakage.

    No metric will be perfect but (leakage/floor area) seems like a nice middle ground.

  5. Expert Member
    Dana Dorsett | | #5

    Not really (response to Joseph Rice)
    While a house with complicated shape may have the same surface area as some other house (with subsequent conducted heat transfer upticks), they're notoriously fraught with air-sealing difficulties. The more corners and angles you have, the more difficult it is to achieve and maintain the air seal, so in practice it's not really going to reward complex shaped houses due to the extra detailing required to make them tight at any standard.

    Whatever the metric is, the fact that codes are requiring ANY air-leakage limits is a huge step forward from prior decades. Whether it's by surface area or volume, there's now a stake in the ground, and air-sealing is one of the cheapest energy use upgrades that can be made, and VERY cheap during new construction.

  6. Jesse Thompson | | #6

    CFM50 / Shell SF vs ACH50
    Another variation on the theme:

    The focus on ACH50 instead of CFM50 / shell area is one of the odder decisions of the Passivhaus standard (link bait!). Why make an air tightness standard that is harder to hit on efficient, small compact buildings and easier on large buildings?

    If you go to PH Conferences, you see a fair bit of excitement about Passivhaus schools and gymnasiums in Europe as a leading edge of innovation, but air sealing a gym to 0.6 ACH50 is easy compared to a small house because there is so little enclosure wrapping all that air!

    For example, we got to 0.6 ACH50 on a tiny Net-Zero project of ours (BrightBuilt Barn, 800SF, 3,400 sf shell). That was less than 100 CFM50 going through the blower door fan, and the builders worked extremely hard to get to that number back in 2008. That would be 0.029 CFM50 / SF Shell for reference.

    In contrast, our local State Housing Authority sets a standard of 0.25 CFM50 / SF of shell across their new construction (10x leakier than we hit on the BrightBuilt project!). It's a reasonably tough standard to hit for commercial contractors. On the BrightBuilt Barn that would have been 5.75 ACH50!

    To compare with a larger scale building, we have a 3 story, 14 unit multi-family building we're working on: 26,000 SF of living area, 31,000 SF of shell area (almost 10x the shell as the BBB project).

    0.6 ACH50 on this project would be 2,650 CFM50 through the fan, or 0.085 CFM50 / SF Shell. That's only 3x as hard as our state's CFM50 / shell target to get the glory of building a multi-family Passivhaus in a cold climate. At the same time, to hit the State's 0.25 CFM50 / SF shell number would be 1.75 ACH50, usually considered a "tight" building, but nothing world-beating.

    I'm with this blog. We should be specifying a consistent shell construction quality based on CFM50 / shell area, and that number should be set at a level that provides good security against moisture damage in wood framed buildings and gets us the energy savings / climate damage reductions we need.

    Next topic, what should that number be? Certainly less than 0.1 CFM50 / SF shell. 0.075? 0.05?

    Jesse Thompson

  7. BobRVt | | #7

    Volume Thoughts
    I'm an energy auditor, and when trained by Efficiency Vermont, we learned ACH CFM50 and ACH natural. We have to report ACH CFM50 for the incentive structure in Vermont, and we use the conversion for natural to compare homes.

    I get the idea that air leaks out of flat surface area, not volume. That being said, there is some sense in using Volume which no one has mentioned. The term is Air Changes per Hour. What is an air change? The volume of the interior of the house that has to be heated from outside to inside temperature for human creatures to survive in winter climes.

    When I explain this to customers, they get the volume concept immediately. I often multiply it by 24 to drive home how many times in a single day they have to heat the air in their homes. 1.5 ACH natural means 36 times per day. If we can improve that by 25%, they will be down to 27 times per day. They understand it, and so it has meaning.

    How would you make as clear & simple an explanation to a customer if you were using CFM50/SF Shell on a remediation job?

    Bias toward a big house or a little house here is largely irrelevant. The customer is seldom comparing his house to anyone else's. For the customer, they just want to know how much less heat they are going to use, and ACH creates a very easily visualized standard.

    Comparing between houses may be important when competing contractors are trying to justify their approaches and their results in new construction. In remediation, where people already know how much fuel they use and how uncomfortable they are, the visual example of the number of volumes per day is quite powerful.

    Some people convert the ACH number to a square footage of leakage. I don't know the math behind that, but it results in an image of a hole in the side of the house. If you take that hole and make it 25% smaller, you get the same result, but it does not convert so easily in visual terms into how much heat you are using or saving from said hole as the ACH number does.

    The kind of simplification and inaccuracy happens all the time when converting from the scientific community to the general public. Take miles per gallon. People religiously compare those. But these numbers are provided by the manufacturer, and not based on a driven test, but tail pipe emissions. Manufacturers sometimes get caught exaggerating or making mistakes, have to revise their numbers, and pay customers a balancing amount to make up for their error. Despite all of that inaccuracy, mpg is what the public looks at, because it is measurable at the pump. Meanwhile in the background, scientists, policy advocates and engineers debate the efficiency of standard internal combustion motors vs those with super chargers, turbo chargers, diesel, hybrids, the cost of added weight for safety desires, batteries, battery disposal, lithium wars in Africa, etc. All that most of the public wants is good mpg.

    I'm not advocating the exclusive use of ACH nor the avoidance of CFM50/SF Shell. But we do need a clear and simple way to explain it to the public. ACH is a useful metric for bringing people along the path to doing the right thing. Same as mpg.

    Bob Rueter

  8. stuccofirst | | #8

    Thanks Allison!
    So yeah, what I gather from Joe L's comments is; In order to get federal and state incentive you must follow their minimum ventilation requirement, which may, or may not be a scientifically proven number that will ensure the health and safety of the occupants. In a nutshell, the house could be tighter and still be safe to live in. State airflow requirements are riddled with corporate backing and so the standard is lax. Managed airflow into the enclosure(hehe) is the future.

  9. Expert Member
    Dana Dorsett | | #9

    The way Joe L. tells it...
    IIRC in some of his ramblings Lstiburek points out that the Canadian R2000 spec became 1.5ACH/50 only because it was twice as tight as the ~3ACH/50 average that most decent quality builders were already building, not because of any economic or scientific rationale.

    For most houses built to the IRC 2012 spec of 3ACH/50 won't present a significant heating/cooling increment over 1ACH/50 or 0.6ACH/50 or any other such thing. For VERY high-R homes it'll make a difference, but there's not necessarily an economic rationale to support it.

    WHERE the air leakage is makes a difference too. At the same ACH/50 or cfm/sq-ft numbers the infiltration drives of a house where most of that leakage is at the foundation sill and attic will be several times that where most of the leakage is concentrated near the neutral plane of the stack.

    It's pretty easy to get sucked into the minutae, but as crude as it is, an ACH/50 number is a reasonable indicator of the actual air infiltration. Sure, it's barely better than an order-of-magnitude estimate, a pH, if you like, but that's actually pretty good.

    Would changing the standards to a cfm/sq-ft provide a more accurate picture of the actual natural infiltration?

    I think not- since it doesn't tell you where the leakage is occurring, and the location of the leakage does actually matter. Either standard will tell you the degree to which air-sealing was attended to during construction, but neither will tell you how much affect that has on the heating & cooling performance compared to other houses that scored the same cfm/50 numbers.

    ACHnatural models are a joke, and a BAD joke at that- barely better than a WAG, IMHO. There are many assumptions about leak locations and average wind speeds etc to derive a realistic number from any CFM/50 measurement.

  10. user-1135248 | | #10

    This is precisely why I asked "why is all this such a mess" in
    Q&A a couple of months ago. In the meantime my small Cape
    retrofit here tested out around 0.9 ACH50, which isn't quite
    passiv level but we were all pretty happy about it.

    ACH does seem to make a bit of sense to me because it implies
    what you might expect as a heat loss rate. Not that buildings
    spend much time under pressure differentials like that, but if
    a given fixed CFM means 1 ACH for one house and 5 ACH for
    another one, obviously the latter is going to start feeling
    cold earlier in the game.

    Leakage expressed as the hole in a *percentage* of surface area
    would completely make sense to me -- it would best express
    overall building quality pretty much independent of size.


  11. monokote | | #11

    Good 'Ole Days
    Ah yes - the 'good ole days' when Allison lived vicariously through our/my Tweets at BSC.... ;)

    Great article, as usual!

    --Stephen Davis
    aka @EFL_Guy

  12. greenophilic | | #12

    IAQ and ACH
    Certainly another reason to report ACH value has to do with air quality and not exclusively energy. The relation between ventilation rate and indoor pollutant concentrations is determined by the volume, not the surface area. The ACH at any given natural pressure difference will be proportional to the ACH50 number. Just a thought. I think the CFM/ft2 or leakage area per ft2 metrics are useful for assessing construction quality, but not necessarily air quality or energy use.

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