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Musings of an Energy Nerd

Is it Time to Move Away From ACH @50 Pascals?

Momentum is building to abandon ach50 in favor of cfm50 per square foot of shell area

These days, lots of residential builders are focusing their attention on air sealing details. This focus is due in part to the increasing stringency of airtightness requirements in many residential building codes. In Vermont, code developers are now debating the question of which airtightness metric to use in the residential code: air changes per hour at 50 Pascals, or cfm50 per square foot of building shell. [Photo credit: Eric Whetzel]

A residential blower door test result is usually performed at a pressure difference of 50 Pascals, and the results are reported as an airflow rate in cfm. A report might read, for example, that the result is 320 cfm @50 Pascals.

If you have this result, and you know that your home has a volume of 19,200 cubic feet, you can calculate the air changes per hour (ACH) @50 Pascals. Here’s how: You multiply 320 by 60 (to convert cfm to cubic feet per hour), and then you divide the product by the home’s volume. The result: Your home is rated at 1.0 ACH @ 50 Pascals (or 1.0 ach50). Most blower door contractors include ach50 in their reports.

Compact homes with few bump-outs are easier to air seal than convoluted or stretched-out homes with bay windows and dormers. Given the same air sealing approach, a compact home is likely to have a lower ach50 result than a stretched-out, convoluted home. It’s also much easier to hit a certain ach50 target with a larger home than a smaller home. Because of these facts, some blower-door specialists note that it makes sense to focus less on ACH @ 50 Pascals, and to adopt a different metric — namely cfm50 per square foot of building envelope (that is, cfm50 per square foot of shell). This latter approach provides a better snapshot of the tightness of a building’s shell than ach50.

For example, in his 2012 blog titled  “How Much Air Leakage In Your Home is Too Much?”, Allison Bailes wrote, “The most common unit used by blower door operators is ACH50… I prefer cfm50 per square foot of building envelope … The two reasons for that choice are that (i) air leakage happens at the surface, not in the volume, and (ii) it’s the best unit, in my opinion, to express what…

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28 Comments

  1. Randy Williams | | #1

    Nice Discussion. I conduct blower door testing in Northern Minnesota which has had testing as part of the code since 2015. The builders I work with have grown accustom to the ACH50 number, most have a basic understanding of the test results. Changing to the CFM50/sq. ft. now, for us, would create confusion among some builders. I agree that CFM50/sq. ft. is a better metric and wish we used that for our test result from the beginning. As a side note, I primarily conduct testing in rural communities. Most new homes being constructed in my area fall a outside code enforcement area, very few are tested. Most homeowners are unaware of the testing requirement. Education, or at least awareness is important. Good luck Vermont.

  2. Rick Evans | | #2

    Great article! Thank you Martin. As usual, little Vermont is leading the way...

  3. User avater
    John Semmelhack | | #3

    Worth noting: PHIUS (Passive House Institute U.S.) moved from 0.6 ACH50 to 0.05cfm50/sf as its air-tightness threshold in 2015 in order to provide a more level field for the large range of building sizes participating in the program.

  4. User avater
    David Goodyear | | #4

    Martin, Just a note, the photo is credited to me....I didn't take that picture. I believe that is from Eric Whetzel. Cant say for sure but looks like the house he has been blogging about.

    Rick is right. This is great article. I think that CFM50/sqft makes sense. Defining where the square feet are measured though could probably be up for debate. PHIUS uses the area of the thermal envelope which I am not sure is the best metric. Seems that this could lead to artificially better CFM50/sqft since the thermal envelope is typically outside the air barrier of the building. I haven't done a whole lot of digging on the matter but it seems that normalizing CFM50 by the area of the air tight barrier makes more sense since this is the source plane of the leakage. I guess that you could argue that the path of leakage is from the exterior so the exterior of the thermal envelope makes sense. Any thoughts

    1. Trevor Lambert | | #6

      David,

      I don't think it's relevant at what plane the air is actually leaking out. The surface area measurement is purely a means to compare house sizes. If you have two houses exactly the same internal living space, but one has an external air barrier and the other has an internal one, does it really make any sense to allow the house with the external air barrier more leakage? I don't think so. I think the only metric that makes sense is the internal shell surface area. I think that is the same as the thermal envelope, but maybe not exactly in all cases(?).

      1. User avater
        David Goodyear | | #8

        The internal shell surface makes sense. however, I was surprised that for passive house that the surface area is not the interior of the thermal envelope its the exterior of the thermal envelope. With the errors in estimation of energy use, maybe it doesn't matter a whole lot.

  5. Trevor Lambert | | #5

    “And actually these [calculations] are conservative because they use the same wall-to-floor area coefficient, and that should really be higher for large houses that are almost always more complex than small ones (thus have even more shell area for the same floor area than my examples).”

    If I'm understanding this quote correctly, I think he's got it backwards. A larger house gets the benefit of larger volume per surface area. A more complex layout adds surface area for the same size, so this actually reduces the volume to area ratio. In his parenthetical, he references a shell area to floor area comparison, but the examples he is referring to were floor area to volume comparisons.

  6. User avater
    Armando Cobo | | #7

    Nerd arguments are really frustrating. For the last 10-15 years the residential industry and Codes have done a pretty good job educating Builders and Consumer alike about what ACH50 air-tightness looks like and created some expectations for comparison about “quality construction”.
    We all know that the R-value for insulation is just a simple guide to let consumers know how “well” insulated their home is… yet we all know the real R-value comes from a testing done in control environments, not in real life, and in a house, depends on framing factors and quality of installation.
    We all know that MPG in a particular cars is just a guide, but actually depends on how fast you drive, whether you drive in city or highway, or maintenance detail; yet we all look the gas economy in MPG before we purchase a car.
    So what if ACH50 in a big house is different than a small house. Let’s be happy that many of us use ACH50 to great success with our clients, without having to explain the nerdy details. It’s really easy for Codes and HERS Raters to include both numbers in the reports, which are computer driven, and no one has to do any calculations by omitting one number or another.
    Dumb argument… include both numbers and everyone will be happy!

    1. User avater
      John Semmelhack | | #9

      Armando - the question isn't which metric to include in a report...it's which metric to use for codes/standards. The I-codes and state codes currently use the same metric and threshold for 600sf apartments and >10,000sf single-family homes, and everything in between. The difference in quality of the air barrier between the 600sf apartment at 3ACH50 and the 10,000sf home at 3ACH50 is significant enough, in my opinion, to warrant changing the metric to cfm50/sf to level the playing field.

    2. Trevor Lambert | | #11

      The analogies made to fuel economy and R-value aren't very valid. While the theoretical and real world values of R-value and fuel economy are different, there is no systemic bias.

      Imagine that instead of MPG fuel economy, the metric was simply average gallons of fuel used per yer. So huge RVs look like they get better fuel economy than a small, commuter car. That would be a valid analogy.

      If the metric was changed from ACH50 to CFM/sq foot surface area, it wouldn't require explaining nerdy details to your clients. You'd just be telling them a different number.

    3. User avater
      Armando Cobo | | #12

      Maybe my analogies are not scientifically correct, but my point still is that after 10-15 years of educating and training Builders, Consumer, the Design and Real Estate communities to ACH50, now some folks want to change the rules. Once the lexicon is embedded into the culture, not only makes it hard to change, but it confuses the issue for most people... other than the nerds.
      Did anyone thought about this 15 years a go? if so, why wasn't this changed then? Has Resnet and BPI agreed to this new forms of reporting?
      It reminds me when 20 years a go Building Scientists explained why we needed to stop saying that houses need to breathe, or thst hot air raises, or that we need to say Building Enclosure instead of Building Envelope... How's that working out 20 years later!
      Try to explain that to Joe Consumer when you are educating them on what's a high-performance house is. 9 out of 10 times we end up using those old terms because that's what they've known and read on the internet.

      1. Malcolm Taylor | | #16

        Armando,
        I think there is a lot in what you say. You can see from the comments when people on this board post their ACH50 results that they are immediately understood by everyone, and I'm not convinced the code requirements that may be brought in would be stringent enough to penalize any decent builder, even if certain building shapes did provide slightly anomalous results.
        The debate does seem a bit to me like the one on cooking circles over using measuring cups rather than scales for ingredients. Does the increased accuracy move the whole exercise out of the everyday world of builders into the rarified one of specialists?

  7. Gio Robson | | #10

    We may also want to include "Normalized Leakage Area at 10 pascals" (NLA@10pa) in this discussion. NLA@10pa provides the ratio of the equivalent leakage area to the building surface area (all 6 sides).

    The (once a leader, now barely hanging on by a thread) R2000 program in Canada has always included a minimum airtightness requirement of 1.5 ACH@50 pascals OR the option of using NLA@10pa, with a maximum leakage area target of 0.7 cm2/m2 (1.0 in2/100ft2) at 10 pascals.

    To arrive at a NLA@10pa, in addition to more time consuming and complicated (and I suppose also more prone to error) surface area calculations, the airtightness tester will also have to complete a multi-point, rather than a single point test, and software entry (more time, but also perhaps more reliable as one can verify error, flow exponent, and correlation coefficient parameters).

    One advantage of NLA@10 is that this provides a metric builders can easier envision (how big is a 0.1 CFM hole!!!??) AND it provides a result closer to pressures the building more normally experiences throughout the year (ie 10 pascals vs 50 pascals).

  8. User avater GBA Editor
    Martin Holladay | | #13

    Response to John Semmelhack (Comment #3):
    John,
    Thanks for alerting us to the fact that PHIUS has switched metrics for its airtightness target -- that's an interesting development.

  9. User avater GBA Editor
    Martin Holladay | | #14

    Response to David Goodyear (Comment #4),
    Thanks for letting me know about the photo credit confusion. I apologize to you and Eric Whetzel. I've fixed the problem.

    When I was putting this article together, I knew that this debate had a high nerdiness quotient, and was worried that few GBA readers would be interested in the minutiae of this debate. I was evidently mistaken, if the number of comments posted on the article's first day of publication are any guide. Thanks for the feedback.

  10. User avater GBA Editor
    Martin Holladay | | #15

    Response to Armando Cobo (Comments #7 and #12),
    Armando,
    Thanks for representing the viewpoint of residential designers and builders. I'm sure you're correct -- most builders who have paid any attention to airtightness are familiar with ach50, but haven't the foggiest idea of what a cfm50 per sq. ft. of shell number might indicate. The likely solution will be for blower door reports and code targets to reference both metrics, at least for a transitional period.

    Education never ends -- for all of us.

  11. User avater GBA Editor
    Martin Holladay | | #17

    One of Andy Shapiro's points is very important and merits further discussion. Shapiro wrote, "Seems to me ACH50 is an energy value, if you accept some conversion/correlation to natural ACH. And if this is in support of an energy code, that is what you want — after all, it’s NACH that drives energy usage."

    I'll elaborate. Let's say that I want to buy a new house, and I am considering two options. Both houses have the same "rating" -- namely 0.16 cfm50/sq. ft. of shell. So an uneducated buyer might assume that they are equally "efficient."

    But House A has all kinds of bay windows, dormers, and ells, resulting in twice as much shell area as House B, which is admirably compact. House A may have the same "rating," but the house has double the rate of air leakage as House B, and therefore has significantly higher energy bills than House B.

    But you can't tell which house is better if all you are looking at is cfm50 per sq. ft. of shell.

    1. Trevor Lambert | | #18

      Martin,

      It just dawned on me that both volumetric and surface area measurement are problematic reference points, as they don't linearly track the most relevant unit for comparison: conditioned living space. What about a value for leakage per area of conditioned space? That way the efficiency of the two houses in your example would be accurately reflected. And it should also fairly represent the relative efficiencies of houses of varying size.

    2. Andy Kosick | | #19

      I understand what you’re saying but I think your energy nerd is showing. I’m all for educating people, but the idea that the average home buyer will (or even should for that matter) be looking at a very specific performance metric like shell air leakage is probably ridiculous, or if they’re even asking they likely have a reasonable understanding of what it means. A large and small home with the same ACH50 could just as easily be misconstrued to have similar energy loss from air leakage without a clear understanding of what it means. I’d say homebuyers are more likely to look at estimated utility bills, perhaps a HERS rating, or even an estimate of carbon footprint; with any luck in time some standard of IAQ. Buildings are complex, as a consumer in the modern world we are forced to trust the details to professionals.

      What I’m trying to get at is that no individual prescriptive element of the code paints the whole picture and shouldn’t be confused with doing so. The question is, which performance metric does the best job of ensuring consistent quality in that element of the structure. Having inspected and tested many Habitat for Humanity homes (some of the highest performing homes I get to work with) and some 10,000 sf mansions, I’ve concluded that ACH50 does a poor job of consistently assessing the quality of the air barrier.

      In terms of calculating surface area I'll say this, it's a different and much bigger discussion. I haven't seen a hand drawn plan in years. The flow of information from architectural software to energy models, HVAC design, and onsite data collect should be effortless by now. Let's draw it in a computer once and give the data to everyone involved. The failures of residential construction computing compared to other industries is a topic unto itself.

      Thanks for putting this whole discussion out there.

  12. User avater GBA Editor
    Martin Holladay | | #20

    Andy,
    Thanks for your comments. For further discussion of the topic you raised, see "Do Homeowners Need to Understand Home Performance?"

  13. Scott Shell | | #21

    I have a large kitchen exhaust at home that I measured at 190 cfm of leakage at 50 pascals (which is about 1 ACH on my home). I got up on the roof and looked in and it has sheet metal flaps that gravity holds closed until the fan pressure gets strong enough. In Passive House world people are adding make-up air supply inter-connected to operate when the kitchen exhaust turns on. Seems to me that this is not an issue at natural air pressures a home would experience, and isn't necessary. Should our blower door testing protocol acknowledge this? Other recommendations?

  14. User avater GBA Editor
    Martin Holladay | | #22

    User 6947031,
    First of all, can you tell us your name? (I'm Martin.)

    Whether or not you seal the exterior vent termination of your range hood fan before conducting a blower-door test is up to you. As I noted in my article, "Blower Door Basics":

    "Ventilation system intake or exhaust vents (and passive air inlets) are usually (but not always) sealed, depending on the aims of the blower-door test. If the test is being performed to comply with section N1102.4.2 of the 2009 International Residential Code, the section requires that 'Exterior openings for continuous ventilation systems and heat recovery ventilators shall be closed and sealed.' Moreover, in many cases a builder will seal passive air intake vents during a blower-door test to determine the theoretical leakiness of the building’s envelope without any passive inlets."

    Building codes require the installation of a makeup air system in a home with a range hood exhaust fan rated at 400 cfm or more. For more information on code requirements for makeup air systems, see "Makeup Air for Range Hoods."

  15. Todd Eidson | | #23

    I have to wholeheartedly agree with Armando's first paragraph in Comment #12, and with Andy's statement "homebuyers are more likely to look at estimated utility bills, perhaps a HERS rating." It's been years of education to finally get some homebuyers to understand the very basics of home performance. There still even exists that perception that "green building" means every house has bamboo floors.

    The typical American consumer wants everything instantaneous and for the lowest cost. You can order a 30cuft refrigerator on Amazon from the palm of your hand and have it delivered to your door. There is such information overload, and bad information on the internet about building, that we should keep it simple and streamlined. At least on the single-family residential consumer side, we shouldn't try to fix what isn't broken.

  16. Derek Roff | | #24

    I disagree with a number of points raised by Armando (#12) and Todd (#23). I think the nerdly discussions here are very interesting, and more generally, we can thank the nerds for the tremendous advances in building performance and codes over the last forty years. Our codes can't, and won't stay static. Discussing how they are best improved is a very prudent topic.

    Building codes are not consumer documents. I doubt if 1% of home buyers understood them in detail back in 1968 or 1948, and the percentage is similar now. That's fine. Codes exist to guide building professionals in producing better, safer structures. So it is quite relevant to know the viewpoints of architects, builders, raters, and others involved in realizing the code requirements. Education of the consumer is also important, but it will never extend to a complete understanding of the code and all its numbers and calculations, except for a vanishingly small group of consumers.

  17. Todd Eidson | | #25

    I agree, Derek, that the nerds are to be revered for getting us to where we are now.

    When the MLS finally started including "green" features and HERS information, and the 2015 IECC required us builders to post home performance data visible inside the home, then codes therefore became much more consumer documents than ever before. That's a great advancement. And we have this very forum on GBA to thank.

    My earlier point in #23 was specifically to address that 'vanishingly small group of consumers.' That's a sad, but true, statement itself. Therefore, if we've finally educated just a small portion on some generally accepted standard, why any rush to change the rules and further reduce 'a complete understanding of the code?' Let's increase an understanding of the generally accepted current code by taking one idea and running with it full steam.

    I'd be interested in hearing Sam Rashkin weigh in on this article and comments.

  18. PioneerBuilders | | #26

    One comment regarding being required to use a certified tester. In Washington State, the code official or AHJ (I don't remember which) is allowed discretion with this.

    I had a very disheartening experience. We are a new construction home building co. When I was having our blower door test, the "professional" asked me what I wanted the number to be. He was willing to doctor the numbers to whatever I wanted, very easy to do. I'm sorry if this sounds prideful or arrogant, but my conscience wouldn't allow that. And we failed our test at first. Since then we have become much better at sealing up our homes.

    This experience motivated me to buy our own blower door and manometer. Now we test our homes at drywall and at final. We have honed a repeatable process for air sealing. In summary, we are a better builder now, and this will benefit the homeowners and others for a long time.

    I'd sure like this same provision to be allowed; allowing for someone like me to continue testing our houses. Fact is, we have an impeccable reputation with our inspectors because of our decades of experience and not being shysters. Feel free to check us on Instagram. My brother's account is @awesomeframers and mine is @pioneerbuildersinc

    I'm not looking for followers but for getting more eyes on our work to make us better.

  19. User avater GBA Editor
    Martin Holladay | | #27

    Pioneer,
    I'm sorry to hear the story about the crooked blower-door tester. I guess crooks exist in every profession -- which is depressing.

  20. User avater
    Nathaniel May | | #28

    I'm surprised there is any debate about this. It's not a question of a simple vs complex metric. It's a question of accuracy and there is no question that the CFM50 metric for airtightness is a more accurate metric than ACH50. One of the primary reasons we focus on airtightness is that we need to have an accurate understanding of the potential for air movement to transport moisture in to/through an assembly. An assembly that encloses a large volume will yield a much lower ACH50 result than the identical assembly that encloses a small volume. Since CFM50 is testing the assembly irrespective of volume, CFM50 will yield a consistent test result for an identical assembly enclosing various sized volumes. To put it simply, CFM50 is a better tool than ACH50.

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