A new study from FXFOWLE finds that Passive House is definitely achievable for the large, tall multifamily buildings that dominate the New York City market.
As the Passive House standard gains traction in the public and private sector, decision makers of all stripes are eager for information about applying the standard to a wide range of building types. Local examples of Passive House projects, though growing quickly in number, are mostly residential and smaller in scale. What about very large or very tall buildings? What about buildings that include multiple uses? Will Passive House make the project more expensive? Will it affect marketability?
Now a major contribution has been added to this conversation in the form of an extremely detailed study by FXFOWLE looking at the feasibility of applying Passive House to tall multifamily buildings. The study uses the current design for a real project as a baseline and provides an incredibly useful resource for everyone from policy makers, to building owners, to architects and engineers.
First developed in Germany, the Passive House standard has now been successfully applied in myriad countries and many different climates. The standard hinges on developing a highly efficient envelope, with continuous insulation and limited thermal bridges. Passive House envelopes are also significantly more airtight than is typical and, coupled with constant volume energy recovery ventilation and high-performance windows, provide an extremely comfortable interior environment using a very limited amount of energy. (To learn more about the basics of Passive House, check out our briefing, Passive NYC.)
The comparative study by FXFOWLE applies Passive House requirements to an existing project which their office had already designed and is currently under construction. Nearly 300 feet tall, the 26-story, mixed-use, residential project selected for study is located in Queens, New York. Roughly 600,000 SF, the building includes 100,000 SF of retail and 450 apartments, 20% of which are affordable. The project was originally designed to meet LEED Silver criteria, including 20% energy cost savings relative to ASHRAE 90.1-2007.
FXFOWLE set out to bring their baseline project up to Passive House specs with as few changes as possible, while relying only on widely available materials or systems and without demanding obscure construction techniques. As a result, the Passive House version of their project could be built tomorrow. Their study demonstrates that this could be achieved at a cost increase of only 2.4% and without any perceivable changes to the aesthetics of the project. The latter is a critical point. The proposed project retains the 36% window-to-wall ratio of the baseline building, a fairly common figure familiar to projects with punched openings, and the study is another example of achieving the goals of Passive House in an elegant package. But Passive House projects require windows that are higher performing, and significantly more expensive, than the energy code would dictate- typically they need to be triple-glazed to meet the stringent comfort requirements. A 36% ratio can provide a bright, comfortable interior, but projects that wish to rely on significantly more glazing will find it much, much more difficult to meet the Passive House standards. The luxury residential market for instance, is dominated by facades with minimal depth or texture and floor-to-ceiling, highly-transparent glass. Many of these projects would need to either reduce the area of glazing, add external shading systems, or both.
So the Passive House version costs slightly more than the baseline project. What do you get for that money?
First and foremost, a big upgrade in comfort. Regular, code-compliant buildings are full of compromises that impact the comfort of the occupants but aren’t really accounted for in the typical design process. Take, for instance, window performance. Although our energy codes require windows to meet certain performance criteria these are largely in place to reduce direct heat loss (or gain) and to avoid critical failures like condensation build up. Passive House takes window criteria a step further by taking into account how the temperature of the surface of the window impacts comfort. Even if the inside air is comfortable, say 72 degrees in winter, if the surface of your new code-compliant window is 55 degrees the radiant impacts will make you quite uncomfortable. And even though the cold window surface is the cause, you’re only available response is to walk over and turn up the thermostat. Passive House tackles this by limiting the temperature differential between the temperature of the air and the temperature of the exterior walls and windows to about 7 degrees (the threshold when the delta causes discomfort.) Other issues that impact comfort but are largely ignored by standard codes include drafts (caused by by leaky exteriors, but also by that temperature differential I just described) and thermal bridging (think of a floor slab that extends out to become a balcony, and how cold it will make the floor in winter.) On top of these thermal comfort issues, the constant volume ventilation provides cleaner air, and more of it, and does it quietly- all of which ensures Passive House interiors are among the healthiest and most comfortable available.
Next on the list of benefits is the low cost of heating and cooling the building. One of the major benefits of Passive House is the minimal amount of energy required to heat and cool the buildings. In this scenario, the heating demand is reduced by about 85% and the cooling demand is reduced by 40%. Needless to say, these are significant when one considers that the base building project was slated to reduce energy cost 20% relative to ASHRAE 90.1-2007. One of the key factors in this reduction is the improved energy performance that results from a significantly more airtight building. The study notes that the airtightness of the Passive House project is nearly 7 times as stringent as the code, and since airtightness is not policed one assumes actual performance of code compliant buildings is quite a bit worse.
Using what look to be very conservative numbers, FXFOWLE estimates annual energy cost savings to be $225,000. This figure produces a relatively long 24 year payback period, but there are reasons to be confident the actual payback period would be shorter. In their simple payback calculations FXFOWLE assumes flat energy costs across the payback period, which is standard procedure for this type of calculation. But the reality is that energy costs will climb, and the more they do so the shorter the actual payback period would be. Secondly, the expensive glazing system required by Passive House is a major component of the additional project’s costs, but as demand grows more manufacturers will enter the market. In Europe, triple-glazed systems are roughly at cost parity with standard systems and here in the US the costs of these systems are coming down every month.
My last point on cost is that, in addition to the other very significant benefits, Passive House is a strong risk management strategy. A building that we anticipate using significantly less energy in a static universe insulates the owners and tenants from increased energy consumption due to operation as well as spikes in utility costs. Smart financiers and investment bankers hedge their positions in the market, Passive House allows building owners to do likewise.
The FXFOWLE study is comprehensive and considers a broad array of additional subjects. There are extensive diagrams of the energy recovery ventilation systems proposed, at the unit and whole building level. And the report delves into a host of other issues that must be addressed to pursue Passive House, including structural impacts, material and product selection, zoning, security, and others. Perhaps most impressive are the many, many details provided that compare base building construction details and how they would be modified to suit the thermal bridging and airtightness requirements of Passive House.
Anyone with even a passing interest in how buildings use energy, or how that consumption can be dramatically reduced to play a pivotal role in meeting our local climate action goals, should read the FXFOWLE study cover to cover.
The complete FXFOWLE study is available for download, here.
In October the Building Energy Exchange organized a presentation of the preliminary findings of the study. Although a few of the numbers have been refined since that presentation, I highly recommend you watch the video of the event, available here.
– Yetsuh Frank, BE-Ex