Passive house certification in the USA

WUFI Passive as standard tool

© Photo Passive House Institute US

Climate-dependent limit values for passive house design are available for over 100 locations.

© Photo Passive House Institute US

The new headquarters of the Rocky Mountain Institute – certified using WUFI Passive.

Passive houses can work even when the climate is warm and humid. However, they require adapted tools and project planning. In the United States to date, passive houses have been certified to PHIUS+. This standard is largely equivalent to the German passive house standard, where cost-effectiveness is based on the calculation that higher expenditure for measures such as insulating the building is more than offset by not needing an expensive waterborne heating system and through reduced energy needs. In the United States, however, these savings are less pronounced: for one thing, homes there generally have much cheaper air heating and cooling systems; and for another, the costs of fossil fuels are significantly lower than in Europe.

Nevertheless, the use of passive building principles offers many more advantages besides lower energy consumption: it can reduce the effect of daytime climate fluctuations, which in parts of the U.S. are much stronger than in Central Europe – meaning the room climate stays pleasant all year round. Even under extreme weather conditions, which can sometimes lead to power blackouts that last several days, the building does not incur any damage.

If we look at projects that have been certified in the U.S. to date, we see the following: if the passive house standard is to be translated cost effectively, then the stipulations contained therein have to be adapted to the various climates. Fraunhofer IBP’s Hygrothermal Building Analysis working group assisted the North American passive house institute PHIUS in two ways here: the researchers helped develop a climate-adapted passive house standard and also implemented it in the established building simulation software, WUFI® Passive.

One piece of software for many questions

First launched in 2012, the original version of WUFI® Passive was already the product of collaboration between Fraunhofer IBP and PHIUS. The goal of the collaboration was to develop a planning package that was adapted to the requirements and needs of the American passive house community. In contrast to the moderate climate in Central Europe, the warm and humid climate that prevails in some regions of the United States makes it necessary to cool and dehumidify the room air. This throws up various moisture engineering questions that the usual monthly-balance-based energy assessments are unable to adequately answer. In the first version of the software, the researchers had already taken this factor into consideration: the software allows users not only to design and certify the energy layout of the building in the planning phase, but also to view the individual components and the building as a whole in terms of hygrothermal dynamics. This makes it possible to ensure over the long term that the proper hygiene conditions are in place, that the building is free from damage, and that the residents will be able to live in comfort.

Development of a climate-adapted passive house standard

As the next step, the working group helped PHIUS develop a climate-adapted passive house standard that also takes into account the different building traditions in the United States: the PHIUS+ 2015 standard. The background to this move was as follows: as a result of the stipulations in the original passive house standard, it was very difficult to employ energy-saving building styles cost effectively in many regions, which led to buildings with uncomfortable living conditions in some cases. As a result, the standard struggled to gain acceptance. In response, the researchers sought to define standards that achieved ambitious reductions in energy consumption and CO2 emissions, but were also cost effective. To do this, they calculated climate-dependent limit values for more than a hundred locations – covering aspects such as the annual heating and cooling requirements or the peak heating and cooling loads. Then they integrated the technique into WUFI® Passive.

As a result, Fraunhofer IBP’s software is currently the standard tool for the design and certification of passive house projects in the United States.

Alongside the full version, there is now also a free version called WUFI® Passive Free. It has proved very popular, attracting over 500 new users in under six months. The free version allows users to design and certify all aspects of a project. However, the dynamic hygrothermal simulation is available only in the full version.

Now the working group wants to bring its American experience to bear on the development of passive buildings in China, helping its partners there develop guidelines and standards that are adapted to the local climate and usage behavior.







Florian Antretter

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Florian Antretter

Fraunhofer Institute for Building Physics IBP
Fraunhoferstr. 10
83626  Valley, Germany

Phone +49 8024 643-242

Fax +49 8024 643-366