Countering a policymaker’s concern regarding passive house

We recently heard from a PHIUS constituent who had these comments about an experience with a policy maker who was skeptical about passive building:

In recent discussions about building performance compliance options for our city’s Stretch/Green Code, a committee member raised a concern about a provision for passive house design in commercial projects. He thought that current modeling software isn’t reliable enough for reasonable accuracy and cited it as a “fatal flaw.”

Monitored energy use is tracking closely to WUFI Passive models.

Click on the image for a pdf report: Monitored energy use is tracking closely to WUFI Passive models!

Knowing that there are large and small commercial passive house projects being successfully built, I hope you can speak to his concern. The more specifically this can be addressed – the good and the bad – the better.  We are strong believers in passive house design but need to confidently understand how reliable the available technology is before adding it to our building codes.

Essentially, then, the question many policy makers have is this:

Is Passive House Certification too Risky for Code?

Short answer No.

PHIUS+ Certification is based on the same science, data, and energy programs that building codes are built on. However…

Our community is indeed successfully designing and certifying residential and commercial buildings to the PHIUS+ Passive House Standards.

We have two terrific resource sites for both applications and with great examples. Here are the links to them:


WUFI Passive, the design/certification and energy modeling software, has proven to be accurate in predicting energy use. We have the largest pool of certified projects in North America and actual measured data is available for many of those projects.

Where we have measured data,we have found that on average we achieve modeled vs. measured results of +-7%, which is the best I have heard of in the energy modeling industry.

That said, there is much confusion out there regarding two passive house certifications. PHIUS Senior Scientist detailed the substantive differences between PHIUS+ and the European approach. In short, ours is a climate-specific passive building standard developed under a DOE grant for North America that has proven to produce very accurate predictions (here is a link to the NREL publication:

The German Passivhaus Institute uses a different standard, not derived from North American climates but instead from only one central European climate (one set of standards for all climates in the world). They also use a different modeling tool, a spreadsheet called PHPP. They have to my knowledge very few projects certified to date in the commercial and large MF residential sector in North America and have not published any modeled vs. measured data.

We initially used their tool for our projects and found a significant difference in modeled vs. measured performance. Overheating in summer was also a problem. In our experience, the performance was off by 25-30% pretty consistently from what was predicted by PHPP. That’s why we switched to a different, more accurate methodology and modeling tool.

So, to the policy maker who raised the concern, I would agree that the PHPP and German standard do have a problem in North America. I do not expect that policy makers take my word for it, either. We encourage code and other officials to vet the standards and design tools carefully before including them. And to all of the PHIUS community who are fighting the good passive building fight, we will be happy to provide you data that proves the performance of the PHIUS+ standard and the WUFI-Passive modeling tool.



One thought on “Countering a policymaker’s concern regarding passive house

  1. The issues with PHPP are not so much with its calculation methods per se – WUFI Passive and PHPP have a lot in common there. Rather, the problems are in some of the input assumptions, particularly about residential lighting and miscellaneous loads, and with the performance targets/criteria for certification. Our calculation protocol for certification is based on RESNET formulas for lighting and plug loads, that we consider to be more realistic. Importantly, we also incorporate those assumptions into the studies that we use to set the heating/cooling performance targets for certification. This matters because those assumptions affect the internal heat gains.

    An additional point on software validity: As I understand it, both PHPP and WUFI Passive have passed the tests of ASHRAE 140, “Standard Method of Test for the Evaluation of Building Energy Analysis Computer Programs”. (Report on WUFI Passive compliance to be published soon.) But that standard basically does not test the software’s input assumptions – it imposes standard parameters about the climate data and internal gains, so as to get at the validity of the calculation methods per se. Thus, a software can be 140-compliant and still be off if it the energy model isn’t set up right, and that is more likely when it comes out-of-the-box with wrong assumptions.

    Another important point about PHIUS+ is that compliance is not based entirely on the energy model. There are numerous quality assurance requirements derived from the Energy Star, ZERH, and Indoor airPLUS programs. These include some prescriptive minimums on things like assembly R-values and window performance. Our certification protocol also entails quite a bit of scrutiny around moisture risk. These features should keep anything from going too wrong with the energy design.

    Energy Star and PHIUS+ were both treated similarly when written into the Massachusetts energy code – basically following the “above code program” mechanism in the IECC. That calls out specific provisions of its Chapter 4 (Energy Effiiciency) as “Mandatory”. In the MA code and in the proposal I made for the WA code, those provisions are imposed on PHIUS+, as additional “guardrails” so to speak. It’s a bit style-cramping and not entirely satisfactory from a passive house designer’s point of view, but it should satisfy concerns about “risk”.

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