From Vienna, With Airsealing: A Passive House Office Tower

Last year we blogged about Vienna’s RHW.2, the world’s first Passive House office tower. This year we were able to bring René Toth, one of RHW.2’s engineers, stateside for our annual conference, Façade Face-Off. Toth also spoke the following morning to give more insight into the projects’s technical details—and judging from the nonstop questions, it was a good thing he did. Though Toth was happy to answer, he honestly seemed a bit surprised, remarking that “in Austria this is more or less normal.”

A quick snapshot: RHW.2 is a 450,000-square-foot mixed-use building (mostly offices) with six underground and 21 aboveground floors. It uses 80% less energy than the average NYC commercial high-rise. This is mainly because of a great envelope—air sealed, insulated, and with the right thermal mass.

Toth revealed that a high-performance envelope was a necessary component of the project, but not the whole story. The design team followed the energy reduction process through to its logical conclusion, using ultra-energy-efficient equipment to eliminate unwanted heat within the building. Since many big buildings actually need wintertime cooling because of the waste heat from building equipment, that’s a very logical place to start for deep savings.

The team also focused on relentlessly demolishing standby losses. Toth said the building has 6,000 light fixtures, so just a single watt of standby power consumed by each LED or fluorescent fixture—even when they are off—adds up to tens of thousands of kilowatt-hours annually. You could power four or five average New York City homes just with the standby losses from the lighting on one commercial building! RHW.2 fixes this by putting hard switches on the drivers for these lights—turning them all the way off and eliminating standby loss when they are not in use. It’s ideas like this that make me confident that very deep energy use reductions are within reach.

The building was carefully tested for airtightness during construction. By testing a small area early on, the team was able to catch holes left by a contractor before these mistakes were repeated throughout the rest of the building. This helped them to pass their second test when construction was complete. While U.S. code sets air sealing standards for commercial buildings, it doesn’t insist on verifying it through testing. As the RHW.2 experience shows, blower door tests reveal problems that can be fixed before it’s too late (and too expensive), and is worth looking at for all types of construction.

The end result is a building focused on user comfort. It boasts natural ventilation, no annoying fan sounds or drafts, and daylighting everywhere—everything but free marzipan. The building gives users more control over their space than many U.S. office buildings—so in theory, the building’s energy balance could be affected if someone left a window open between the inner and outer skins during the winter, causing an unwanted stack effect. In practice, the owners have found that since the building is very well regulated for comfortable temperatures, this has not been a problem.

RHW.2 has a few unusual features. For instance, it benefits from waste heat from a neighboring data center (whose energy use isn’t included in the Passive House calculations) and uses the nearby Danube Canal for free cooling water. Does this mean its success would be hard to replicate? Toth doesn’t think so. “Every site should make use of the benefits it has. We had the Danube and the data center. If the building had been somewhere else, it would have had some other option we could have taken advantage of.”

About the author

Cecil Scheib
Cecil Scheib is Chief Program Officer for Urban Green Council.