By August of 2011, eight years had passed since we completed the Smith House, the first home in the United States to be built to the European Passivhaus standard. Those eight years were heady and full: We founded the national non-profit Passive House Institute U.S. (PHIUS). We created a Certified Passive House Consultant (CPHC) training curriculum and delivered trainings to hundreds of professionals from coast to coast.
Those pioneering professionals began building their own projects from coast to coast, and from north to south, in all U.S. climate zones save for Florida. Because PHIUS had a good deal of practical experience building its own projects, because it provided training and certification (at the time under the auspices of the German Passivhaus Institut), PHIUS was quite naturally closely involved with nearly all of these projects.
And that’s when we — PHIUS and CPHCs and builders across the United States — began collectively to learn the limitations of the European Passivhaus metric in varied climate zones outside of Central Europe.
Some buildings were overinsulated and overglazed
To be sure, the concept of a single, relatively easily understood, internationally applicable energy metric for heating and cooling was (and is) enormously attractive. And in Central Europe the metrics have been well verified and tested.
But between consulting on some projects and certifying and reviewing many others, we learned that the concept of a single Holy Grail standard for North America’s varied climates is just too good to be true.
In practice, designers have arguably been forced into non-optimal decisions and designs in pursuit of the European 15 kWh/m²•year metric. For example, in the colder climates they tended to seriously overinsulate — with diminishing returns in the outer layers — and tended to overglaze (with expensive high-performance windows, no less). The projects relied heavily on solar gain to make the energy balance work.
With some exceptions (e.g., the Pacific Northwest), the North American continent has design temperatures that are much more challenging than central Europe. It gets significantly colder during the winter, even while the number of heating degree days (HDDs) on an annual basis can look very similar to those in Europe. Madison, Wisconsin, is a perfect example: It has a colder design temperature than Oslo, Norway, while its HDDs are almost 2,000 lower than Oslo’s.
An overreliance on solar gain
Although design temperatures are colder, there is generally very good solar potential in North America. Therefore designers in the U.S. and Canada tried to compensate by becoming essentially “solar Passivhauses” to get closer to the target, which in return caused overheating and comfort issues. (The passive solar movement learned those lessons in the 1970s. Ironically, those lessons were the ones that led to the development of the original passive house concept that deemphasized solar and reemphasized insulation.)
Let’s face it: the annual heating demand of 15 kWh/m²•year was a result of meeting 10 W/m2 peak load in a specific climate, the European climate, with less extreme design temperatures — which as a bonus also allowed “supply air heating only” — the flagship core definition of a Passivhaus as established by the Passivhaus Institut (PHI).
The specific relationship of annual demand and peak load in the European climate has led to the characteristic definition of the standard. Yet, the relationship of annual heating demand and peak load is not a strong one, and is very different on the North American continent. This is likely the reason why the pioneers in the 1970s and 1980s had identified generally similar peak loads as energy targets but paid little attention to limit their peaks to “supply air only,” because they could not get there, and comfort was still assured with slightly higher peak loads and greater annual demands.
Solutions that weren’t cost-effective
Overinsulation and overglazing both resulted in overspending beyond cost effectiveness, seriously challenging the claim that 15 kWh/m²•year is somewhat magically the cost optimum/sweet spot between demand and supply everywhere in the world. (See many earlier articles by Martin Holladay questioning the 16-inch-thick subslab insulation of early Passivhaus projects and the discussions that followed.)
Conversely, in warmer and milder climates (a prime example being California), the target of 15 kWh/m²•year is actually too high, allowing projects to leave significant cost-effective energy savings on the table. In extreme hot and humid climates like Florida, we learned that energy targets for cooling were simply unattainable.
It appeared that the European standard had simply mirrored the heating demand of 15 kWh/m²•year for cooling without verifying it in hot climates. In practice, insulation does not yield the dramatic return in energy savings in cooling-dominated climates as it does in heating climates; in fact too much insulation can increase the cooling load.
PHPP problems in hot, humid climates
These issues also manifested in the Passive House Planning Package (PHPP). Because PHPP is a massive Excel spreadsheet, users can “look under the hood,” which makes it a nice teaching and learning tool. But while well validated for heating-dominated climates, the tool proved inaccurate when we consulted on the LeBois House in Lafayette, Louisiana.
The project was intended to be a proof-of-concept project in the Lafayette climate, and to demonstrate that designers could confidently use PHPP in hot and humid climates. The project plan included monitoring for two years after it was inhabited. During that period it became clear that in PHPP, cooling demand and sensible peak algorithms were off by a large margin. Moreover, we learned that latent loads really need to be accounted for in the standard (they were not at the time).
The project was performing significantly better on the sensible cooling demand side than PHPP had predicted, by about 30%*, but worse on the peak — a situation that makes system sizing difficult. On the other hand, RESNET’s energy modeling tool REMRate predicted the actual performance almost spot on.
Overall, the project was a huge success. We proved that hot climate passive principles do apply, resulting in superior comfort and significant energy savings.
In California, PV beats passive house
But this project was another example of an overarching conclusion: the original German standard and tool were inadequate when applied in climates other than the cool, moderate, heating-dominated baseline climate. Results did not support the one-size-fits-all standard concept.
In cold climates, unreasonably high investment costs led people to abandon the concept, and uptake in northern cold climates remains to this day insignificant. In warmer climates like coastal California, a European passive house is easily beaten by a house with a photovoltaic system, because the standard does not go far enough and does not harvest enough through conservation to make it a financial slam dunk.
Standards are tools that help us to quantify, measure against and meet certain goals we have agreed upon. It’s only logical that they need to be updated and refined as economics, materials and other conditions change and as we learn more. It is an evolutionary process.
Standards should evolve, informed by feedback loops, or they become a hindrance, not a help. We can’t blindly trust: we need to verify and validate to assure that our models remain applicable.
Developing new passive house standards for North America
In 2011, the PHIUS Technical Committee, a volunteer body based on modified consensus and comprised of international building science experts and North American passive house practitioners, embarked on the plan to identify a methodology to generate new passive standards for all climate zones. The tech committee has identified four foundational principles that the standard should follow:
- 1. Being biased towards conservation by constraining the envelope design through definition of annual heating and cooling demands and peak loads per climate that must be met using passive measures first. The climate-specific annual demand thresholds should pay back the investment and peak load thresholds should assure comfort.
- 2. Meeting a total primary energy maximum per person for all energy uses in a building. This is essentially the equivalent to a carbon limit, responding very directly to the amount of carbon savings that need to be achieved in the building sector to stabilize the climate.
- 3. An airtightness requirement assuring building envelope durability, verified by climate and measured in air leakage per square foot of envelope area.
- 4. Cost-effectiveness using national average costs for materials and energy.
The sweet spot or characteristic energy use intensity (EUI) is then defined as the optimum design between demand and supply, or more specifically, between conservation and generation.
Lower PV prices have changed the conversation
In a sustainable world we must look at zero energy as our goal. We are no longer only trying to justify the cost-effectiveness of a certain level of stand-alone conservation, we are trying to justify the optimal combination of both, conservation and generation, to reach zero energy.
The energy supply would be expected to come from renewable sources; for buildings this would most likely come from photovoltaic (PV) systems. The cost for these systems has come down dramatically over the past few years. This changes the conversation significantly. Figuring that zero is our goal, the cost of PV has a significant impact on where the optimum lies. Now zero has indeed realistically become our new target; positive energy is next. That alone is reason to redo the calculations and refine the standards.
In 2013 we pitched the idea of refining the standard depending on climate and cost to Building Science Corporation in Westford, Massachusetts. They liked the idea and submitted a research proposal with PHIUS as an industry partner under their DOE Building America contract to define passive standards by climate zone according to U.S. cost data. The calculations are being done using the energy modeling tool WUFI Passive (developed by the Fraunhofer Institute for Building Physics, Owens Corning, and PHIUS) and the energy and cost optimizer BEopt (developed by the National Renewable Energy Laboratory).
A one-hundred-year payback period is unrealistic
The effort is running calculations for all climates for a typical single-family home, with carefully chosen and defined design constraints and energy baseline features, first in BEopt. All baseline decisions were carefully conceived and evaluated by the PHIUS tech committee. In the process, it became clear that the European case for cost effectiveness of the 15 kWh/m²•year standard is based on a 100-year lifecycle period for a single-family end townhouse.
The tech committee found this to be an unrealistic value for a North American economic feasibility assessment of conservation measures. One hundred years might be accurate in an ultimately sustainable energy economy, but we are not there yet. The measures need to be cost-effective in the old economy as we are transitioning to the new. Consequently, the tech committee opted to use 30 years instead of 100.
The committee also settled on using a detached, average size single-family home — the predominant housing type in North America. The detached home is also arguably a worst-case scenario to use as a benchmark; any other building type, larger or attached, will perform better.
European internal load assumptions don’t work for North America
In reviewing base assumptions for the model, the tech committee also decided that the internal loads currently assumed in the European model are far from realistic. While the committee agreed that the defaults for internal loads should be stringent compared to the current national average use of miscellaneous electrical loads, they also acknowledged that the current European defaults are only one-seventh of the actual current internal load average in the United States. This leads to a significant mismatch of what is assumed and what happens in reality.
Corrected higher initial internal loads in turn impact heating and cooling demand criteria on an annual basis, and have an impact on where those demand criteria need to be defined when setting standards.
As of this writing, the standard adaptation test plan is almost complete and the parameters and the methodology for the study have been decided. As the project progresses, the dynamic modeling side of WUFI Passive will be used to verify hygrothermal wall assembly performance by climate and to assure that the comfort criteria by zone are maintained when annual heating or cooling demands are slightly increased or reduced.
Preliminary results are looking very promising. PHIUS is already accepting projects under a pilot certification program.
As the work has moved forward, questions have arisen as to how granular these new standards should be. The final format is still an open question. Originally, a zone-based standard model was envisioned, but it is also possible that the study will result in the development of an equation that accurately calculates the respective heating, cooling demand and peak loads by location.
The new climate specific standards findings are scheduled to be presented for the first time during the Ninth Annual North American Passive House Conference in San Francisco, September 12-13, 2014.
* Because of the limitations in PHPP discovered in the field, PHIUS partnered in 2011 with Fraunhofer Institute for Building Physics and Owens Corning to collaborate on a new passive design tool that would appropriately predict energy performance for passive buildings in all climates. We now use WUFI Passive, capable of static (similar to PHPP) as well as a more detailed dynamic simulation to assess whole building energy performance, comfort conditions, hygrothermal performances of envelope assemblies, and hygric interaction of the enclosure and the living space.
Katrin Klingenberg is the co-founder and executive director of the Passive House Institute U.S. (PHIUS). She has spoken and published on passive building topics nationally and internationally, holds a masters degree in architecture, and is a licensed architect in Germany.
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