January 5, 2015

BSC & PHIUS Passive Building Report Not Airtight

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 vapor 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.

Comfort

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.

Resilience

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.

Efficiency

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. Vapor Barriers by Joseph Lstiburek here.

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11 Responses to BSC & PHIUS Passive Building Report Not Airtight

  1. Elrond Burrell January 7, 2015 at 6:24 pm #

    Fantastic blog! And that is the best description / illustration of why ACH is the right measurement rather than CFM, thank you! I had that on my radar to write about also and may still at some point.

    • Ken January 7, 2015 at 6:49 pm #

      Elrond,
      Thank you so much. And please do write on it when you get the chance. It’s a tricky thing and deserves more attention.

  2. norm farwell January 13, 2015 at 3:12 pm #

    Thanks, this clarifies things. Clear and honest efficiency standards will help us build better buildings. I am disappointed that PHIUS and BCS seem lost in the weeds.

  3. Don Pulfer January 13, 2015 at 3:51 pm #

    Thanks for this article. Clear, well-reasoned, and much easier to read than the draft PHIUS/BSC report. It appears to me that the draft is not so off-base as we might fear, but that the standards of communication are lower than we’d hope. It is a draft, though, and I hope you have sent this criticism to the PHIUS/BSC working group.

  4. Vere Nekoninda January 13, 2015 at 5:08 pm #

    Thank you for an interesting and informative blog posting. You’ve introduced me to some new ideas and clarified a few things for me. I have a question about your three scenarios for the comparison of building shape and its effect on ACH50 vs CFM50. You say that the three variations compare “a home of approximately 20,000 CF volume and approximately 1,600SF floor area”. However, I calculate the square footage of the three examples as 737 sq ft, 1,000 sq ft, and 2,000 sq ft. Those floor areas are dramatically different, hence we are comparing very different, rather than similar structures. Can you comment on the varying floor areas?

    • John January 14, 2015 at 9:51 am #

      Vere, thanks for the comment. The footprint is not synonymous with floor area. You can fit the target floor area within these footprints and volumes – you could have a double height space, for instance.

  5. CA Snyder January 13, 2015 at 11:12 pm #

    Thank you for pointing out the differences between a volume metric and a surface area metric. And I totally agree that if job number 1 is preventing condensation within the building envelope so as to ensure building durability and healthy conditions for occupants, than an air-tightness metric alone is insufficient to the task. Condensation is determined by where in the envelope assembly the dew point occurs for a given temperature differential between inside and outside and the relative humidity of the warmer air sample, since the heat is flowing from warmest to coldest. Consequently the location of the air barrier relative to the insulation is critical to prevent condensation upon impermeable surfaces within the wall, in general most of the insulation should be on the warmer side of the air barrier within the wall, but the exact ratio is climate dependent, base upon the size of the temperature differential between indoors and outdoors. Its not difficult to figure out, all it takes is the psychrometric chart to find the dewpoint for the warmer air sample, then making a ratio of the amount of insulation on the inside of the air barrier, (figured as sum of R-values, not inches of thickness) to the amount of insulation on the outside of the barrier, then divy up the temperature differential proportionate to that to see what the temperature of the air barrier is and whether it is colder than the dew point of the warmer air sample.
    Yet somehow building professionals fail to check for these problems, even those good folks from the Building Science Corporation (BSC).

    Today I got an e-mail from the principal architect on a project who persists in trying to bring me in at the end of design development when he’s already set most everything in stone. He was arguing with me that his proposed wall assembly was fine because the person who answered the phone at BSC told him that a wall having a half an inch more exterior insulation than the code minimum of 1.5″ over 2×6 framing would be a “great wall assembly” for us here in Michigan, no problems with condensation. I was able to show him the math to prove that even at the bottom of the human comfort zone for humidity (40% RH) the wall that the BSC employee approved of would not only have a wet air barrier every night in January in Michigan, but also that most of the cellulose insulation and the wood framing in the wall would also get repeated wettings from condensation most winter nights. The BSC approved wall assembly had only about 2/5ths of the insulation on the cold side of the air barrier, compared with the rule of thumb for MI of 2/3rds to 3/4 of the insulation should be on the cold side of the air barrier. You are better off leaving out the air barrier than locating it improperly and trapping moisture in the wall on the cold side of the dew point where it can nurture rot and mold. Even the proposed upgrade the principal architect gave of increasing the exterior insulation to 4″ so that slightly more insulation was outside than inside was not enough to prevent condensation in Michigan’s winters, which still would have occurred on the air barrier at least 1/3rd of the January nights.

    Too many building professionals defer to authority, and mistakenly assume that the building codes must be sufficient to prevent sick building syndrome, when building code officials really haven’t figured out how to address condensation issues in all climates. “Code compliant” is not a synonym for quality, all it says is if it were any worse, it would be illegal. There is a difference between “building science” based on physics (Dr Feist, founder of PHI is a building physicist), and the more common “builder’s science” which says if its allowed by code, go ahead and build it, and if no one calls you back within your warranty period (sometimes only a year), then it must have been alright. Unfortunately, sick building syndrome usually takes longer than a year to make the occupants sick, and often people never figure out what is making them so ill unless the mold permeates all the way to the surface of the walls, roof, etc, or the do renovations to the building and open up the structure and find a bunch of black goo coating devouring everything organic in the building envelope. Unfortunately, I’ve seen too much “builder’s science” coming out of highly reknowned organizations like BSC. Buyers beware!

  6. Michael Hindle January 14, 2015 at 10:42 am #

    Hello Ken and all,

    This is a very thorough post! Where do you find the time?!

    Unfortunately I do not have the time today, but I thought I would point out a couple of things.

    1) All are encouraged to provide technical feedback to the tech committee on the evolving PHIUS standard adaptation process. All technically relevant and constructive comments will be reviewed.

    https://docs.google.com/forms/d/1pwBAxZY8HWxWWpUNuQ47hTR8w6mbc8G69GL2BXpSlxo/viewform?c=0&w=1

    While it is safe to say that all standards rely on assumptions and setting limits by certain metrics and none will ever be perfect in all situations, I have great confidence in the PROCESS that PHIUS has set up of intelligent and committed practitioners providing feedback that is relevant to the goal we all share. With a process like this, the standard will evolve to reflect the information brought to the table by the best and brightest, not according to partisan orthodoxy.

    I have closely reviewed the report and submitted questions of my own, and I can assure you, Graham and other Tech Committee members have been very responsive, and are fully committed to making the standard the best it can be for all climates in North America!

    2) Ken, you lay out a compelling argument that for the hypothetical case you articulate (and one can always find such hypotheticals whenever you have a fixed standard of any kind) that the change to CFM/sf could in this case lead to a more lax air tightness standard. In the rest of the post (and I agree with you) you are asserting that it is not ONLY mold and degradation risk, it is also energy efficiency. We should all remember that the energy efficiency metrics are still a governing criteria, so the net efficiency would still be assured. So there is still a penalty for complicated geometry or poor surface to volume ratio, and of course we must rely to some degree on the designers’ enlightened discretion, on which any such standard will still rest to some degree.

    3) The hypothetical I am more concerned about is the one I am faced with on several multifamily projects, and even a regretably large home for two. The larger volume buildings would allow for substantially more air leakage using the ACH50 metric. The proportionally larger air flow could (and most likely would) be focused in localized leakage in predictable places, and the deleterious effects would be proportionally far more serious in my multifamily projects than the concerns raised in the hypothetical homes you describe! So in my view, finding and appropriate threshold and metric that does not let larger buildings off the hook in terms of building durability and safety is paramount.

    4) So perhaps we should be choosing the metric by building type, size and geometry. You have acknowledged that the PHI already agrees in principal with this by including a recommendation for So perhaps we should be choosing the metric by building type, size and geometry in a way that is simple and coherent.

    You pointed out that the PHI itself acknowledges the issue and sanctions this approach with the referenced 0.033CFM50/SF for buildings over 140,000 CF . So perhaps there is really not much space between the two standards at all. It may just be a question of setting the threshold cut-off between different building types and sizes.

    I think the ACH 50 target works very well in buildings that most of us have been working on, but as the market is growing quickly for larger buildings, the scalability of the PHIUS /BSC proposed standard adaptation draft becomes very important. It is also more consonant with other commercial metrics for envelope evaluation.

    This is a great conversation, and as I said before, please post comments to the comments link I pasted in above! the deadline is very soon, so read it carefully and make your most technically astute arguments. The process is there to ensure that no one small group of people set any part of this standard in a vacuum. The collective wisdom is important to us all, and I for one feel quite confident that no one standard or organization could ever be “RIGHT”. The right answer will come through a process of evolution and increased sharing of information.

    Best,

    Michael

    • Ken January 14, 2015 at 2:03 pm #

      Hi Michael,
      Thanks so much for the comments. We are insane at the moment with the office move, but I do want to briefly note that the example is not meant to be a “gotcha” in any way – it is just our attempt to make clear a complicated issue. The phenomenon illustrated is not unique but a geometric given.
      We vary much appreciate a diversity of opinion and a good discussion of these important topics. We look forward to discussing further from our new office!
      All best,
      Ken

  7. Matthew Cutler-Welsh February 24, 2015 at 2:49 am #

    Ken, this is great. Just found it thanks to Derek Ward of Clioma.

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