Image Credit: All images: Passive House Institute U.S. The cumulative square footage of Passive House projects in North America. The cumulative number of Passive House residential units in North America. Most of the bars in this graph (all of the bars except the blue bar) refer to a Passive House building in New York City. The last four bars of the graph compare monitored energy use to modeled energy use (as predicted by three different modeling runs). The blue bar shows the modeled energy use (using WUFI Passive software) of an imaginary building of the same size as the New York City building — one that meets the minimum requirements set by ASHRAE 90.1. Most of the bars in this graph (all of the bars except the blue bar) refer to a Passive House building in Hillsboro, Oregon. The last three bars of the graph compare monitored energy use to modeled energy use (as predicted by two different modeling programs). The blue bar shows the modeled energy use (using WUFI Passive software) of an imaginary building of the same size as the Hillsboro building — one that meets the minimum requirements set by ASHRAE 90.1. Three of the lines in this graph (all of the lines except the dotted blue line) refer to the Passive House building in New York City. The dotted blue line refers to the modeled energy use (using WUFI Passive software) of an imaginary building of the same size as the New York City building — one that meets the minimum requirements set by ASHRAE 90.1. The numbers in parentheses refer to the year (2015 or 2016). Curiously, the creator of the graph did not place the months in chronological order. Three of the lines in this graph (all of the lines except the dotted blue line) refer to the Passive House building in Hillsboro, Oregon. The dotted blue line refers to the modeled energy use (using WUFI Passive software) of an imaginary building of the same size as the building in Hillsboro — one that meets the minimum requirements set by ASHRAE 90.1.
After a period of growth in the ’70s and ’80s, and a brief hiatus in the ’90s, passive building principles and metrics are making an impressive comeback in North America. Passive principles were developed 40-plus years ago by pioneers including William Shurcliff, Rob Dumont, and Joe Lstiburek — to mention just a few. Today, these principles are broadly seen as critical for a renewable energy future.
Market transformation is in progress and policy makers have taken notice across the country. The last few years, according to data obtained by the Pembina Institute, show an exponential growth rate in passive building certifications in North America.
At the core of this development has been the Passive House Institute US (PHIUS). In 2002, I built the first house in the U.S. to follow the German Passivhaus standard as a design guideline. The house was built in Urbana, Illinois. We built more homes as affordable housing, designed training curricula, and created the CPHC (Certified Passive House Consultant) and builder training programs. We’ve also facilitated sharing experiences through our annual conferences. These efforts helped to build a robust community of professionals that is now the driving force behind the growth of Passive House building.
The German Passivhaus Institut (PHI) briefly partnered with PHIUS to offer their certifications in the U.S., but the organizations parted ways in 2011 over disagreements about the appropriate passive standards approach. Starting in 2012, PHIUS significantly changed its certification protocols in order to assure climate concerns were addressed; to incorporate accepted industry practices recommended by the U.S. Department of Energy; and to introduce a third-party verification of the quality of implementation (working in concert with RESNET).
In North America today, two very different approaches to passive standards are used in the market, with the vast majority of units certifying through PHIUS. Beginning in 2014, PHIUS launched a research effort to develop the PHIUS+ 2015 Standard. Based on data from projects across North American, the PHIUS+ 2015 standard was developed specifically for North America’s varied climate zones. Since that launch, PHIUS+ certifications have experienced exponential growth, indicating that the program was successful in removing barriers that had limited adoption.
A smaller number of projects and units are being certified to the European Passivhaus standard promoted by PHI. This certification method also is experiencing growth but at a much slower rate (see the graph at the top of the column, and Images #2 and #3 below).
A performance-based standard
Passive building is a performance-based standard. With the first measured data coming in from multifamily buildings, we were anxious to know how well we (the community, the certifiers, and the tools) were doing. This was with an eye to becoming even better at assuring not only the quality of passive building construction, but also to becoming better at predicting and ensuring actual performance — and then making sure our buildings maintain those great levels of performance over time.
With a market that is starting to go gangbusters, reliable measured data is highly valuable from many different perspectives. For one, if we could show that passive standards can guarantee performance not to deviate more than ±10% (I am probably lax here, I’d like to see more like ±5%) from what was modeled, it might actually convince our critics and modeling naysayers that there is something to the methodology and that the modeling is worth it.
And what a potential this could have for the financing sector of high performance, energ- efficient homes and buildings! Fannie Mae and Freddie Mac will currently only underwrite 75% of predicted (modeled) energy savings, which seems to indicate that they are expecting that models are not particularly accurate (and in fairness, a lot of building energy models haven’t been very accurate). And lastly, measured results might finally answer the question between passive house folk: Which one of the two very different certification approaches to the standard is more accurate in predicting actual performance? Wouldn’t that be something useful to know?
The data is coming in
A handful of our first certified passive multifamily projects are now in and have been occupied for one year or more. Measured data is available. It is by no means a representative proof-of-concept study (although such a study is underway — a project funded by the PHIUS Industry Advisory Council will monitor 30 projects of all typologies in all relevant North American climate zones). But the data sets do offer good first insights as to where things are headed: How well do our tools work to assure performance in the field? Where are they not working? And where is there room for improvement?
The main questions guiding analysis are:
(1) How do passive building standards compare to an ASHRAE 90.1-2010 code baseline? How much better than code are the modeled predictions?
(2) Does the predicted modeled performance align in terms of total site energy use intensity (for normalization purposes) and how do the two passive building standards, models, and protocols used in the market today (PHIUS+2015 and WUFI Passive, PHI and PHPP) compare and/or differ?
(3) Does the measured performance align with the predicted performance in terms of total site EUI and how do the two passive building standards, models and protocols used in the market today (PHIUS+2015 and WUFI Passive, PHI and PHPP) compare and/or differ?
(4) How significant are the performance gaps, if any?
(5) If there are performance gaps, how do we explain them and what can we do to improve the issues to close that gap in the future?
For this blog we’ll limit the discussion to two examples. The projects are located in New York City and Hillsboro, Oregon (Climate Zones 4 and 4 Marine). We graphed what the two passive models had predicted for the projects to consume. We graphed what has been monitored and, for comparison, we also graphed the predicted (modeled) results if the project had been modeled and designed to meet ASHRAE 90.1-2010. (This can be potentially confusing and should not be compared to the monitored data, only to the other modeled data. It can be assumed that a building designed to meet ASHRAE 90.1, once it has been built and monitored, would also show a performance gap; it’s just that we don’t know what that would be for this particular project. Note that the modeling software used to model the imaginary building — that is, the building complying with the minimum requirements of ASHRAE 90.1 — was WUFI Passive.)
Results are varied
The two passive models predicted surprisingly different results for the total expected site energy savings over the imaginary building complying with the minimum requirements of ASHRAE 90.1. WUFI Passive, using PHIUS+2015 modeling protocols, predicted a ±30% improvement over the ASHRAE 90.1-2010 model. The Passive House Planning Package (PHPP) using PHI modeling protocols predicted an improvement over ASHRAE 90.1-2010 of 50-60%. This is a difference in predicted improvement of almost a factor of two between the two models.
At first glance one might think that the PHI standard is just that much more stringent, but keep in mind that the building specifications for the modeled Passive House buildings are exactly the same. The difference can only be due to either the difference of algorithms calculating the energy savings or due to modeling assumptions that each calculation protocol prescribes. (An independent NYSERDA study recently compared PHIUS and PHI standards to ASHRAE 90.1-2010. The study is about to be published. A preview was presented at this year’s NYPH conference in NYC. It confirmed results very similar to those that we were seeing in our study).
The first example project (the one in New York City) consumed significantly more energy than either of the passive models had predicted — one-third more than PHIUS had predicted and more than twice of what PHI had predicted — but performed overall 17% better than the imaginary building meeting the minimum requirements of ASHRAE 90.1. (See Image #4 below).
This project did not go through full certification and only had the design pre-certified. This points to the fact that onsite quality assurance during construction, and thorough commissioning and testing of all systems at completion are indispensable to actual performance. In this particular case, the team was already experienced in building very efficient projects but not to this level of performance. The team might have underestimated the importance of additional passive building specific quality assurance check points onsite and omitted them. This project also did not have a monitoring system and all that was available to assess performance were utility bills.
Monitoring and feedback systems for passive level buildings are emerging as a very useful and maybe needed tool for building operators and managers to run their buildings appropriately and to identify where something might have gone wrong to be able to correct for it once the building is under operation.
To explain the performance gap on this project, further investigation is needed. It could be that manufacturers overpredict performance, which is then used in a model. Some newer technologies are still fairly uncommon, and could have problems with installation. Other quality assurance related issues, such as lack of onsite verification, missing or inaccurate blower door tests, higher ventilation rates, and a lack of final commissioning could play a role. Such outstanding concerns seem to be at the root of the performance gap.
Second project results
The second project (the one in Hillsboro, Oregon) was the best performer, beating the PHIUS+2015 predictions by 2% while it still consumed about 30% more than PHI predicted. (See Image #5 below).
The PHIUS+ model is just a bit conservative and overpredicts actual measured results slightly. This is the result we like to see! But here again, circumstances matter. Success was likely. This team was the best-prepared team of these three pioneer teams, with a very experienced CPHC, an experienced builder, and a committed developer, and a project model that integrated the process with all team players involved.
Passive principles were not just added on to business as usual. The PHIUS+ onsite verification program had evolved by the time it was under construction. The project followed all check points as required by the certification protocol onsite. The team also specified a rather sophisticated monitoring system with occupant feedback. This system allows for very detailed data and performance analysis as well as continued tuning of the building operations. (See Image #4 below.)
What it means
(1) We at PHIUS may not be able to guarantee passive performance to be within 10% of the modeled results just yet, but we are working on it.
(2) Interestingly, occupants are better than their reputation; they were not major contributors to the performance gaps.
(3) Both passive models predict significantly better performance for the Passive House building than an imaginary building meeting the minimum requirements of ASHRAE 90.1, but the predictions differ by almost a factor of two between the two certification protocols. PHI claims almost twice the amount of savings as PHIUS for an identical building. The PHIUS+2015 modeling protocol is significantly closer to measured reality than the PHI modeling protocol (by about 20-30%).
(4) PHIUS+2015 algorithms and assumptions appear accurate in predicting the envelope and passive-measures performance. In the two cases where the projects underperformed, it was possible to trace the performance gap back to mechanical issues, and/or quality assurance issues during the construction process. To identify potential reasons for the gaps we adjusted the initial PHIUS+ model for potential problems: for example, the model was adjusted for a reported higher ventilation rate the building has been running at and suspected lower efficiencies of systems. Those plausible explanations and changes to the model made it a closer match to the measured results (see the adjusted PHIUS+ model, shown as the red bar in Image #4 below).
(5) Quality assurance and verification are critical to success. The experience of the team members (CPHC, builder, and verifier) and the accuracy and trustworthiness of the tools they have available to them to back-check their assumptions against reality are indispensable to achieving passive levels of performance.
I thank all of the multifamily teams for sharing their data with us. This defines another important step in the passive building evolution: Measured data feedback loops help to define this phase of our committed effort to improving and honing our tools so that we can confidently close the remaining gaps!
Coming up soon: In September at the 12th Annual North American Passive House Conference in Seattle, James Ortega, certification staff at PHIUS, and Marc Rosenbaum will team up in a key session to further investigate and explain the impact of systems design and manufacturer’s claims of efficiencies on the accuracy of our modeled predictions.
Katrin Klingenberg is the executive director of PHIUS.