Blower Door Metrics Explained: ACH50 vs CFM50/SF

Blower Door Metrics Explained: ACH50 vs CFM50/SF

The following was written in 2015 and is a critique of a single document that Building Science Corporation and PHIUS collaborated on.
Very good enclosure airtightness is a cornerstone characteristic of low-energy, resilient, high-performance (Passive House) buildings. Air control is so important that the Building Science Corporation ranks it second only to keeping the rain out - one notch above vapour control and two notches above thermal insulation - in making a functional enclosure.[1] The Passive House Institute (PHI) has singled out airtightness as the one physical attribute to be tested on-site as part the certification protocol and forms one of only three primary performance metrics for the Passive House Standard.[2] As we demonstrate in our free eBook, High Performance Historic Masonry Retrofits, airtightness is at the core of responsibly making historic buildings low-energy.

Because airtightness is so fundamental to performance, it is disconcerting to read the Building Science Corporation’s recently released report – in collaboration with PHIUS - Climate-Specific Passive Building Standards. The report gives a particularly confusing and perfunctory treatment of airtightness - one that purports to propose a change to the PHI developed airtightness metrics in the service of a new PHIUS Standard.

Since the passages are short, here are the three separate instances where airtightness is addressed in the report:

On page 4:

“The air-tightness requirement comes from consideration of building durability and mold risk. The air-leakage study is beyond the scope of this report, which is focused on the space conditioning.”

On page 32:

“The top priorities for future work at this point are: …5. Studies on relaxing the air-tightness criteria by climate. Again, the air-tightness requirement is driven by moisture risk (energy savings being a side benefit). It stands to reason that the danger threshold would be climate dependent. Also, it may be appropriate to revisit the field testing protocol: perhaps the test should be done two different ways – One for energy modeling purposes being realistic about leakage in normal operation, and another protocol for durability, focusing on leakage through the assemblies, with more of the nonthreatening things like door thresholds and vent dampers taped off.”

On page 33:

“6.1 Summary - … 1. The air-tightness requirement was reconsidered on the basis of avoiding moisture and mold risk, using dynamic hygrothermal simulations to be published elsewhere. The proposed change is from a limit of 0.6 ACH50 to 0.05 CFM50 per square foot of gross envelope area. This allows the airtightness requirement to scale appropriately based on building size. Before, a larger building that met the 0.6 ACH50 requirement could be in actuality up to seven times more leaky than a small single family home that tested the same.”

Martin Holladay, in writing a review of the published report in GreenBuildingAdvisor, has taken issue with the idea that airtightness is, as BSC/PHIUS writes, “driven by moisture risk (energy savings being a side benefit).” Martin says “Instead of abandoning Feist’s basis for establishing an airtightness target, the PHIUS committee appears to have embraced it — while simultaneously noting that the target will need to be changed if the argument for its basis is retained.”

We believe both the BSC/PHIUS report and Martin's review are misconstruing the emphasis the Passive House Institute (PHI) is placing on moisture risk, and the resulting airtightness limits. The BSC/PHIUS report has an agenda to make a new PHIUS Standard out of PHI's Passive House Standard. And as we'll see, by focusing ostensibly on moisture risk BSC/PHIUS justifies switching the airtightness metric from the volumetric ACH50, to CFM50/SF surface area leakage, not coincidentally giving apparent relief to those trying to certify small houses, as the surface area metric is less stringent for tiny houses than the volumetric .6ACH50 standard. (More on this below.)

It is clear from PHI writings, and other reports by BSC, that the level of airtightness achieved has great effects on efficiency, comfort and indoor air quality - but that yes, moisture protection is foremost. So while the marginalization of efficiency seems to suit the BSC/PHIUS report's agenda, it provides a false context, and is therefore unhelpful.

First Do No Harm

PHI has written that:

  • “Regarding energy saving construction, airtightness isn’t a pastime, it is vital for the prevention of moisture penetration in building components.” (here) [Emphasis by PHI.]
  • “A superiorly airtight building will ensure favourable ventilation and temperatures while preventing moisture damage.” (here)

A better way to think about the focus on moisture damage prevention is to consider the medical doctor's oath: First do no harm. In making more efficient buildings we are increasing insulation levels, making the enclosure assemblies colder, reducing their drying potential and thereby increasing moisture damage risk. Airtightness is "vital" in ameliorating this condition - allowing us to safely make very efficient building enclosures. First, do no harm.

Germans expect their buildings to last 100 years or more, making it practically a priori that airtightness would first address preventing moisture damage. However, like a doctor is expected to deliver positive results far beyond doing no harm, so too airtightness is expected, as a core function, to provide additional benefits that define a high-performance building - such as indoor air quality, comfort, resilience and efficiency. These are not "side benefits".

It's worth a quick look at these other benefits and the moisture protection benefit too.

Indoor Air Quality

The only simple and predictable way to control the quality of air is for it to exist in an airtight environment. Therefore an airtight enclosure is a precondition for reliable high-quality indoor air. Coupled with balanced high-efficient heat recovery ventilation every room can have desired low pollutant levels. It should always be possible to utilize natural ventilation but this should be a discretionary option - at the occupants wish, not necessity, for the wind may not be blowing when you need it.


Drafts and noise are great nuisances. And big heating and cooling systems are wastefully deployed to overcome the drafts, often exacerbating the noise problem. With airtightness, drafts are eliminated to an extent that the mechanical systems are minimized while providing a level of thermal comfort typically not previously experienced by occupants - as external noises practically vanish. It is this "thermal and sound quietness" that can provide a visceral "aha moment" to many.


Because we expect instances of extreme weather to increase as climate change worsens, resilience is an ever more important objective. New York City's Building Resiliency Task Force described this important quality as Maintain Habitable Temperatures Without Power (#26) after wide-spread power outages caused by Superstorm Sandy forced many residence to flee their homes in cold winter weather. We've experienced the power of airtightness to deliver such resilience in a very cold December several years earlier as we reached an airtightness of 1ACH prior to the installation of thermal insulation - the house interior maintained temperatures in the 50s - heated by contractor bodies, light bulbs and daylight. No need to flee.


Passive House is an energy efficiency standard after all. So just how important is airtightness in regards to efficiency? To find out, let’s take the example of the first Passive House, Darmstadt Kranichstein[4]. Let’s compare what happens to the building efficiency when we adjust the airtightness level versus the insulation levels. The building is super-insulated and is very airtight, at a tested value of 0.22ACH50, with a heat demand of 14kWh/m2a and a heat load of 10W/m2 as indicated in the Passive House Planning Package or PHPP.

  • First let’s leave the airtightness limit the same but cut the insulation levels in half at the floor, walls and roof. The result is a heat demand of 29kWh/m2a and heat load of 17W/m2.
  • Now if we instead retain the original super-insulation levels but simply decrease the airtightness to 3ACH50 we get a heat demand of 27kWh/m2a and load of 25W/m2 – virtually matching the effect of removing half the insulation!

From these results, we'd be right to conclude that airtightness has an impact on efficiency disproportionate to its relative cost and effort. In fact experienced teams will design in very good airtightness numbers, well below the 0.6 limit, achieving what we might consider the first case of actual "value engineering".[4]

German physicist, Helmut Wagner has written that a 1989 study by the Stuttgart Institute of Building Physics (DBZ 12/89, page 1639ff) showed that relatively small cracks (3mm wide) in airtightness control layers can produce up to a 10 fold reduction in insulation value (as well as harmful moisture loading over 1000 times greater than what was possible with diffusion loading alone).[5] Similarly we understand a crack of just 1mm in the Stuttgart study produced a thermal value reduction of 4.8 times (a more conservative, yet still striking value we tend to refer to). 4.8 times reduction is a different building enclosure – it's effectively a different building.

BSC has embarked on its own Thermal Metric Research Project that attempts to quantify in laboratory conditions actual effects of leakage, among other factors, on insulated assemblies. Martin Holladay described the complexity of the issue and preliminary results in GreenBuildingAdvisor, writing, "The percentage effect is much larger on high R-value walls because the heat flows were lower to begin with,” said Schumacher. “So with higher R-value walls, it’s more important to take care of air flow." And, "According to a summary released by BSC, “All wall assemblies experience a loss in thermal performance due to air movement through the assembly. This is true for all of the assemblies tested regardless of the type of insulation material used (e.g. cellulose, fiberglass, ocSPF, ccSPF, XPS). The energy impact of airflow depends on the flow path, the interaction between the air and the solid materials in the assembly, and the installed R-value of the assembly."

And Passive House Consultant, Graham Irwin of Essential Habitat presented a convincing argument at the North American Passive House Network Conference 2014: that extreme airtightness can provide significant benefits in a place like San Diego California where moisture/mold is typically not a primary issue, and natural ventilation is readily available. Graham shows that because airtightness effectively chops-off the peak heating load spikes that still occur, dramatic downsizing of the heating system is still possible, with minimal insulation. Graham concluded that across California airtightness is the most important "lever" effecting heating demand and load.

Moisture Risk

It is well established from studies by BSC and many others, including the Stuttgart study mentioned above, that moisture loading of the assembly via air leakage poses a substantial risk to building structures - let's call it an existential risk for emphasis.[6] And as noted above, as the insulation levels rise, so too the need for greater airtightness. To first do no harm, we need very strong and durable airtightness.

ACH50 vs. CFM50

Let’s first recognize that ACH50 is the needed metric for the energy efficiency calculations, as it represents the rate at which the volume of conditioned air will need to be conditioned anew. And to state the obvious, Passive House is an efficiency standard. Intuitively ACH50 fits the process of making a more efficient building. To get a more efficient building, a lower ACH50 number is helpful and to do that a more compact building shape is more helpful – they reinforce each other.

But when we switch to CFM50, something funny happens: to reach the certification limit of .05 BSC/PHIUS is proposing we may actually want to make the volume less compact and more inefficient. Here's a simple example of this phenomenon with a home of approximately 20,000 CF volume and approximately 1,600SF floor area:

Scenario A: a volumetric cube (27.15' x 27.15' x 27.15') is the most efficient shape and results in .6ACH50 of 200 CFM, and at .05CFM50/SF 221 CFM.

Scenario B: we elongate the volume and area to 20' x 50' footprint, and we get 200 CFM at .6ACH50 (it's the same volume) but now the surface area has significantly increased so the .05CFM50/SF number increases to 240 CFM.

Scenario C: we have an even more inefficient shape (yet still relatively compact compared to many houses) elongating the plan to 20' x 100' footprint, resulting in, you guessed it a 200CFM number from .6ACH50 but now a .05CFM/SF number of 320.

Scenario B and C clearly provide progressively worse energy performance without any discernible effect on moisture protection (arguably worse as well, but more on that in a moment). The only benefit is that there is a pathway to make certification allegedly easier for the designer and builder: make an inefficient shape and switch metrics and you get to increase your leakage allowance by 60%! (60=320-200/200). Really. It seems an odd choice to move away from the volume metric that intuitively and practically corresponds with the task at hand.

Big Buildings

But you ask, what about big buildings? While the BSC/PHIUS report is focused solely on single family homes they do note that their proposed PHIUS Standard surface area metric of .05CFM50/SF is more stringent than PHI's Passive House Standard, because as the buildings get big, it, as the report notes, "..allows the airtightness requirement to scale..." consequently providing better moisture risk protection. But BSC/PHIUS neglect to mention that PHI recommends that buildings of approximately 140,000 CF or greater meet a target value of 0.033CFM50/SF of surface area. And while the recommendation is for good building practice (First do no harm) the Passive House Standard is an efficiency standard and so the 0.6ACH50 remains the certification limit. This seems sensible.

Passive House Airtightness Limits - What's the right Question?

In Martin's review he expresses the frustration of many observers and practitioners when he writes: "No convincing argument has ever been presented to show that the 0.6 ach50 target is necessary to prevent condensation, mold, or rot. On the contrary, there is plenty of evidence that buildings with air leakage rates of 0.6 to 2.0 ach50 are performing very well."

We'd propose that Martin and others are essentially asking the wrong question. There is no distinct target number to prevent condensation, mold or rot and it isn't logical that such a number would exist. Moisture risk is always present - even in tight enclosures there may be a nasty leak hiding - the best we can do is track down every conceivable leak. As we get tighter more leaks become apparent, more leaks can be fixed, and better protection against risk provided. Even when the certification limits are met PHI recommends continuing to search and close all holes that might be found.

The right question to ask is simply: How tight can we reasonably build? PHI has answered 0.6ACH50 for new construction up to approximately 140,000 CF and 0.033CFM50/SF above that. For retrofits PHI says 1.0ACH50 is reasonable. The basic position is that, given proper planning hitting .6ACH is no harder than hitting 1.5ACH - practically providing better risk insurance and energy efficiency for free. Our experience corroborates this perspective. What we see are teams struggling initially, with half-baked approaches and a lack of commitment, to get below 1.0ACH - but that after some experience is gained and planning is improved Passive House limits are confidently achieved. This is the right direction.

On Testing Protocols

The report proposes two testing protocols. This is just as bad an idea as the metric change, as it is hard enough to get a single protocol understood and executed properly. It is all too common to see US practitioners taping off doors and various non-HRV mechanical equipment and other miscellaneous openings. Let's make the protocol as simple as possible to ensure we are getting correct values. The PHIUS Standard should not make testing more complicated.

Keep it Tight - The New Normal

Let's build as tightly as possible. Like Darmstadt-Kranichstein at .22ACH50, we see experienced practitioners hitting much lower numbers. Experienced teams can readily hit .6, .5, .4, .3 and lower - and so they should. Let's make that the new normal.


[1] BSC describes the control layers and order of importance in BSI-001: The Perfect Wall

[2] The other metrics are for heat demand/load and source energy. See the Passive House Institute's certification criteria.

[3] There seems to be a parlor game afoot about who said "passive house" first. But it isn't such a useful conversation. For usage today, Darmstadt-Kranichstein is the first Passive House as we currently understand the term - see here. And read about the history of passive houses on PHI's website here. Learn what is meant by the term Passive House, see an explanation here.

[4] "Value engineering" is a term of art used in the construction industry by contractors and owner's representatives to reduce construction costs, ostensibly without sacrificing the design intent but too often making a substantially inferior product in the process - quite the opposite of the positive effects that a change to greater airtightness provides.

[5] See translated excerpt and German original here.

[6] See: PHI's Passipedia here. Article Air Leaks: How They Waste Energy and Rot Houses by John Straube here. BSC RR-0004: Air Barriers vs. vapour Barriers by Joseph Lstiburek here.