The recent proposal by PHIUS and BSC to create what is being described as ‘a passive building standard adaptation for the U.S.’ is being widely promoted and distributed to various journals, publications and cities for adoption. Here is a critique – one of a growing number of articles expressing concern over this proposal.
But first let’s clarify some important distinctions between the definition of ‘Passive House’ and the Passive House Standard. These are confusing constructs, so it’s important to start with clear definitions in order to have any good discussion regarding this topic. Many advocates and detractors misunderstand the subtleties between the theoretical, or functional definition of a ‘Passive House,’ the Passive House Standard, and the practical implementation of the standard. Therefore, a brief overview of how Passive House is defined is necessary.
The Functional Definition of Passive House, as described here in Passipedia, is pegged to the goal of thermal comfort. This is a subtle, but very important distinction. A ‘functional definition’ is a term more typically used by physicists and not commonly used in the building industry. It commonly describes a theory or thought experiment/Gedankenexperiment and often uses a ‘first principles’ approach ie. a core issue or function that a product/item/design needs to solve. PHI chose a very humanist perspective to define the ‘first principle’ of all buildings as optimal thermal comfort for occupants. (Added emphasis – read that again, since it’s so simple as to be revolutionary! Thermal comfort first, not cheapest or lowest energy building. The cheapest, lowest energy structure would be a tent with solar PV. Job done.) Dr’s Bo Adamson and Wolfgang Feist – and presumably some of their precursors – were able to crunch the numbers to find a theoretical point at which we could adjust the indoor temperature “solely by post-heating or post-cooling of the fresh air mass, which is required to achieve sufficient indoor air quality conditions – without the need for additional recirculation of air.” (Yes, this is confusing!) The functional definition was defined as the target goal, based on their first principles approach, where it become theoretically viable via physics calculations to determine that heating load was low enough for it to be possible to deliver sufficient heat for the building to achieve that human comfort, via an air stream. (Important note: THIS DOES NOT MEAN HEATING OR COOLING MUST BE OR SHOULD BE DELIVERED THIS WAY. NOR IS IT NECESSARILY THE MOST EFFECTIVE OR CHEAPEST HEATING OR COOLING DELIVERY DESIGN FOR A PASSIVE HOUSE.) Due to the complex nature of building use, shape, orientation, proportion and interior partitioning, delivering heat via an air stream is often too complex and not cost effective. Most buildings designed to the Passive House Standard separate their supply of heating and fresh air delivery systems. Pragmatics aside, the theoretically viable target, based on the calculated physics, was used by the Passive House Institute to determine the metrics upon which they based the Passive House Standard. The Passive House Standard was thus defined by the Passive House Institute (PHI), Darmstadt, as a set of widely achievable performance benchmarks. These benchmark targets reliably enable designers to create buildings that attain the level of human comfort described by the functional definition. Most notably, PHI developed a design tool, the Passive House Planning Package (PHPP), that has enabled thousands of buildings to predictably perform as ‘Passive Houses.’While there is still much argument about how the Standard metrics were calculated, and how the Heat Load baseline of 10kWh/m was translated to the Specific Space Heat Demand number of 15kWh/m2yr, both numbers have been consistently shown to be achievable and viable for all but the most extreme climates. Since certification is allowable via either Heat Load, or Specific Space Heat Demand, the discussion is somewhat moot. As PHIUS has ironically discovered while defining their own standard, some ‘heuristics’ are necessary to pick numbers when defining a standard.
The Name, “Passive House,” (upper case letters) makes reference to a building that has used a globally recognized performance methodology during its design and construction. A ‘Passive House’ (functional definition) and the ‘Passive House Standard,’ while intimately linked, are two separate terms. One can theoretically fulfill the functional definition of a ‘Passive House’ without meeting the exact criteria of the ‘Passive House Standard.’ (It’s assumed there is no need to define the generic word ‘passive,’ which describes a broad range of actions and items not restricted to buildings, other than to denote it by using lower case letters.) Note: a Passive House is not always a ‘house.’ It may be a school, office, clinic, apartment building, retail outlet, etc.
The Passive House ‘Promise’ of Cost Effectiveness (aka ‘Tunneling through the cost barrier’): an oft-cited graph showing the theoretical point at which the Functional Definition occurs on a hypothetical Cost vs Heat Demand axis. The theory goes that the mechanical system – and cost – can be reduced at the point where heating may be delivered via the air stream. It should be noted that for most places this graph currently represents an aspirational goal. Emphasis on currently because in most of North America we either don’t have many cost-effective/competitive components just yet, eg. high performance windows, high % recovery HRV’s or optimized mechanical systems that combine water, ventilation and heat delivery in some simplified and integrated manner. Additionally, our designers are relatively new to Passive House and therefore still somewhat inexperienced. (We may not have discovered a few key design elements that could deliver on this promise.) In countries with more advanced Passive House ‘ecosystems’ (products, designers, construction methods) are routinely able to deliver on the promise of cost-effectiveness. These advanced practitioners are building projects to the Passive House Standard at cost parity or near cost parity to code-compliant buildings. [It is difficult to speculate whether the North American Passive House promise of cost effectiveness is one that has been either delayed or deferred, or simply not well celebrated when it has already been achieved by the likes of practitioners such as Tim McDonald on his Onion Flats project or Adam Cohen on his Emory & Henry College dormitory building.]
Surprisingly, these three fairly simple items have caused a cacophonous amount of cage rattling. This noise has grown louder and more boisterous since the Passive House Institute canceled its contracts with PHIUS as an authorized provider of Passive House related services in 2011. (Among reasons cited were failing to abide by the terms of their agreements.) Noises generated have ranged from a marathon trademark tussle over use of the term ‘Certified Passive House,’ to general derision of the functional definition. They have worked up to a frenzied crescendo of accusations of “dishonesty” over the last item – the Promise of Cost Effectiveness, accompanied by flag-waiving claims to the origination of the concept (more on that later.) Curiously, all of these discussions have managed to entirely miss the original definition and focus of Passive House, which is thermal comfort.
This background informs the following critique of the BSC/PHIUS proposal for ‘Climate-Specific Passive Building Standards,’ which may be viewed here.
No measured performance data: Cost and performance information for real projects located in any U.S. location are entirely missing from this study. While making sweeping claims about the failure of the established Passive House Standard to perform in U.S. climates, the authors offer no evidence of this claim in either this study or elsewhere in PHIUS’s protracted list of complaints against the established Passive House Standard. In stark contrast to this claim, many other DOE studies are emerging from across the country, documenting actual performance (and often costs) of projects designed to the Passive House Standard. (Links provided at end of this article.) These are all performing incredibly close to the predictions modeled in the PHPP. Incremental cost increases compared to code compliant buildings range between five and twelve percent – hardly an outrageous amount for the significant energy and comfort benefits associated with these buildings. Ironically, many of these monitored studies include projects certified by PHIUS to the same metrics defined by PHI both before and after PHIUS’ decertification.
No proposed new standard measured data: Acknowledged in the report is that “it would be an exaggeration to claim that this new system would deliver cost-optimality/competitiveness for any particular real project.” With no measured data from existing projects (nor economic analysis) from either older projects that perhaps aren’t optimized and aren’t working, or new projects built to these new recommendations, this report makes a great show of calculations and graphics, but confirms neither the ineffectiveness of the Passive House Standard, nor the effectiveness of this new regime.
No comfort check: I was surprised to see that PHIUS/BSC have pegged the criteria for their standard to cost of electricity and cost of PV and all but omitted any review of any comfort criteria, except obliquely via ‘window u-value constraints.’ Omitting comfort as a basis for this proposed standard is a total re-calibration. It shifts this standard outside the functional definition of ‘Passive House.’ Instead, PHIUS/BSC have set their performance benchmark against the local cost of electricity and photovoltaic panels. This is inherently unstable. It makes their standard just as utility- and local-labor-rate-specific as it does about climate. PHIUS/BSC obliquely acknowledges this and has conceded that this will require that they adjust their standard ‘every three-five years.’ Administering this will be challenging. From a designer’s perspective, it creates added complexity that defies their oft-stated goal of allowing this standard to become more widely adopted.
Tools: PHIUS chose to use BEOpt (with EnergyPlus) and WUFI Passive as their preferred calculation software. This makes sense on one level as the PHPP currently requires the user to create their own ‘dashboard’ for cost optimization, while BEOpt offers a standardized framework to perform cost optimization and was specifically designed for this function. What gives me pause is this segment of the report:
“The experimental data turned out to be not-so-apples to-apples. Also, despite some weeks of effort, the 3-zone dynamic model in WUFI Passive doesn’t reproduce the annual heating demand of the single-zone BEopt model (70% higher), even though the geometry, assemblies, windows, and shading schedule all match and the internal gains, natural ventilation, attic climate, and ground temperatures are all driven by external hourly data files from EnergyPlus. Suspicion is now focused on the zoning and related differences in the mechanical ventilation setup, but the issue is not resolved, and it didn’t seem prudent to proceed with comfort evaluation until the energy results matched more closely. The BEopt bug mentioned in 5.1.2 above accounts for some of the discrepancy (probably at least half of it).”
It appears that sufficient discrepancies exist between these software packages to warrant further research prior to a full market launch.
PHIUS is asking the North American Passive House community to adopt its derivative interpretation of ‘Passive House.’ It doesn’t appear to be based on any actual evidence that the International Passive House Institute’s Passive House Standard is not working. What I see when I read this report is a minimally ‘re-jiggered’ calculus, buried under a very thick layer of analysis – something it has tried to do for at least the past six years (PHIUS tried, unsuccessfully and without the knowledge of PHI, to trademark the term “Certified Passive House” back in 2009.) While I accept that the Passive House Standard is not and should not be static (indeed, it’s about to shift radically to include calculations for renewable energy generation and storage,) this particular report has not convinced me that it should be minutely tuned to each location in the U.S. It gives no data. As for the debate regarding economics and cost effectiveness, our local product marketplace is simply too immature to accurately include economic evaluations just yet.
This proposal asks that we invest all our stock in a handful of analysts at PHIUS/BSC in the hopes that their tools and predictions will provide a cheaper pathway to achieve ‘Passive House,’ based only on predictive simulations. Or instead, we can stick with the tools and methodologies that many of us have measured to be working remarkably well, are continuously being refined, and are based on a twenty-five year track record of collected performance data that has allowed for adjustments and calibrations.
There are already multiple Passive House certification entities operating across North America. Those numbers are about to expand and will continue to do so as the market grows. Instead of new metrics, what we desperately need is more education and many, many more locally produced high performance products. We need to promote radical innovation and market growth, which we’ve seen lead to cost competitiveness in every other industry – why should Passive House and the building industry be any different? Many more projects need to be built and monitored before we have a decent sample set to accurately begin to consider re-jiggering the numbers, if that is warranted at all. It is premature for PHIUS/BSC to attempt these changes. Because this proposal is incomplete, one should use extreme caution when contemplating its adoption. Let’s continue to build a much stronger project- and knowledge-base first.
– Bronwyn Barry
Bronwyn is a PHI CPHD and PHIUS CPHC, based in San Francisco. She is the Co-President of both Passive House California and the North American Passive House Network. All views expressed here are her own.
Monitored Data Reference Links:
Hood River Passive House DOE Study: http://www.nrel.gov/docs/fy14osti/60999.pdf
1st Passivhaus Retrofit in the US: http://apps1.eere.energy.gov/buildings/publications/pdfs/building_america/sonoma_house.pdf
For other reviews of the PHIUS-BSC proposal see: