How to Build a Passive House Apartment Building

There are over 10,000 certified Passive Houses worldwide, so the movement is big enough that debate is growing about whether the Euro-centric Passive House standard should be adapted for different climate zones and other factors on our side of the pond. Much of that debate focuses on single-family homes, but a recent talk by Lois Arena (Steven Winter Associates) on the challenges of making high rise buildings Passive still had designers and builders filling the Center for Architecture’s balconies.

At northern latitudes, most building energy use and equipment sizing calculations are driven by heating needs in winter1. But in New York, Arena says, Passive House high rises may actually have their energy and equipment dominated by cooling needs because of the city’s higher density of people and equipment. Buildings may also contain energy-intensive areas like fitness centers, which generate even more heat. Arena found that even a couple of treadmills can make a big difference to Passive House calculations for the whole building. Because multifamily buildings generally don’t have sun shades, windows that severely curtail heat gain from the sun may be needed2. The net result? Less insulation may be needed to meet Passive House requirements in NYC. Losing heat to the environment in summer, perhaps through the foundation into the cooler earth below, may even be part of the plan.

Arena says typical curtain wall construction may need some tweaks to get to Passive House. The steel framing may create thermal bridges that leak heat through the required continuous insulation. Since they’re made in a factory, curtain walls assemblies can be of very high quality with tight tolerances—at least when they leave the factory floor. All that can go out the window3 once they get to a job site, however, since every curtain wall attachment to the building and other parts of the wall offers a chance for thermal bridges and air leaks.

At the top of the building, roofs need special attention to ensure the integrity of the air barrier, which prevents air infiltration at details like parapets and drains (features not typical in home construction). At the bottom, you can’t just shove polystyrene foam under the footings of a high rise, so insulating the foundation can be a challenge. 

A cooling-dominated Passive House high rise makes highly efficient equipment a must. Lighting and refrigerators are obvious examples for upgrades. Whether the HVAC system is central (assuming there’s room for the ducting and piping) or individual to each unit (and you can find small enough units and deal with the individual thermal bridges and air leakage), it will need to be ultra-efficient. Individual unit heating/cooling systems mean you need to find a place to put all the separate external components (see photo). They also make it very easy to submeter occupants for both heating and cooling, which might be very attractive for developers.

Passive House ventilation requires heat recovery, and it is standard practice to allow occupants the ability to boost ventilation rates in kitchens and bathrooms. But, Arena says, there’s a code wrinkle in NYC: optimum air flow for Passive Houses is below that required by NYC codes, meaning units would be required to “over ventilate.” To make things trickier, some European products for venting apartments are not legal in New York (for example, units with concentric vents).

But Arena says Passive House high rises are possible in NYC, and there are several projects that show it can be done. Designers and builders who are ready to rise to several specific challenges––and work with an overseas certifying body––will find themselves on the forefront of deep green construction. 

1 In addition to the famous air tightness standard (0.6 air changes per hour at 50 pascals pressure—actually easier to meet in large buildings, since they have less surface area compared to their volume than small ones), Passive House has three “energy budgets” buildings must meet. These are: total energy use by the building of 38 kBTU/ft2/yr (meaning source energy, so it includes efficiency of the offsite power source), and heating and cooling energy of 4.75 kBTU/ft2/yr. Passive House also has several “soft” requirements projects must meet: opening windows, rare overheating, and “user-adjustable ventilation rates.” Arena notes that in office environments, people already fight over space temperatures, so giving them control over their ventilation rate is a real can of worms! While technically, exceptions may be made to these requirements in Passive House projects that otherwise maintain comfortable environments, Arena’s experience is that these exceptions are hard to come by.

To get around the lack of sun-shading overhangs, vertical shading elements can be used. Arena says even setting the window back in the thick wall (towards the interior, known as an “innie”) can help, as can shading from balconies above can help. In any case, windows are key in balancing the heating, cooling, and total energy use requirements, so these projects should expect to do a lot of modeling.

Speaking of windows, they get very confusing. The European standard (ISO) and the North American standard (NFRC) for rating windows have no direct conversion between them. This is a challenge for manufacturers who need to have their products tested and labeled under both standards. Arena believes domestic products tested and labeled to Passive House requirements using the NFRC standard will be hitting the market, so hopefully this will become simpler soon.