Urban Multifamily Passive House in the Northeast Corridor

Condos and Rental Buildings in Washington, DC, Philadelphia, New York City, and Boston

As you may know, the Passive House standard applies to all building types, not just single-family houses. In fact, the first Passive House was a block of four townhouses built in 1991. The Northeast Corridor is a great location for multifamily buildings that meet the Passive House standard due to the high cost of energy, relatively mild climate, and good solar access. The Northeast Corridor also generally has great transit access, which reduces transportation costs and increases home values.

Passive House Design Basics

Form and orientation: Multifamily buildings in this region don’t have to be simple boxes facing south. Site and zoning constraints will likely determine the shape and size of the building more than energy performance concerns. That’s OK because larger buildings have relatively less surface volume, so they lose less heat. As an example, one multifamily Passive House retrofit project Duncan Architect consulted on in Brooklyn had the majority of its solar gain from north-facing windows, but we were still able to reduce the annual heat demand by 90 percent.

Structural System: Wood has less embodied energy and a smaller carbon footprint than steel or concrete. However, fire codes prevent the use of wood structures in many urban areas. Fortunately, the Passive House standard allows any structural material as long as thermal bridges (interruptions in the insulation) are avoided.

Windows: High performance triple-glazed windows are comfortably warm to the touch even on the coldest winter days. South facing windows can actually provide a net heat gain in the winter. Properly designed exterior shading will keep the apartments from overheating in the summer. Passive solar gains can offset some of the heating costs of the building, but because of their expense, windows should not be oversized. Residents can open their windows as they wish, but they don’t have to in order to get fresh air because a special ventilation and air filtration system is part of the Passive House package. Triple-glazed, airtight windows also reduce street noise, which is an important selling point in urban areas. The Passive House Institute certifies windows for “cool, temperate” and colder climates. On this map “cool, temperate” is shown in cyan and includes most of the US. weltkarte_und_klimazonenhttp://www.passiv.de/_images/03/00_zertifizierungskriterien/01_transparente_bauteile/weltkarte_und_klimazonen.jpg

Insulation: Passive House designers optimize the thickness of insulation using an energy modeling tool called PHPP. In brownstones, rowhouses, and midrise apartment buildings, this typically results in walls that are as thick as the traditional buildings in the area. In New York City developers can take advantage of floor area deductions for high-performance walls more than 8” [200 mm] thick. A zoning analysis by Duncan Architect for a proposed hotel tower in Brooklyn showed its developers that they could add an entire floor by improving the performance of the walls.

Thermal mass can help regulate temperature. However, it is more useful in regions other than the Northeast, where there are cool summer nights.

Airtightness is a key requirement of the Passive House standard. An airtight building enclosure with proper ventilation and walls that can dry out if they get wet prevents “sick building syndrome”. An airtight building is also more comfortable because there are no drafts. Last but not least, it costs less to operate because there is virtually no heat loss (or heat gain in the summer) due to uncontrolled air infiltration.


Special ventilation units provide constant fresh filtered outdoor air to the apartments while exhausting kitchens and bathrooms without significant heat loss. They are called heat recovery ventilators (HRVs). Some also recover humidity and are called energy (or enthalpy) recovery ventilators (ERVs). Humidity recovery allows mechanical ventilation without drying the air too much in the winter or bringing in too much humidity in the summer.

Heating: Within a Passive House building enclosure any type of heating system is possible. Radiators under the windows are not required for comfort in a Passive House, so the heat source can be located anywhere. Mini-split air source heat pumps provide heat two to three times as efficiently as direct electric radiators. Electric warming mats under tile in the bathroom are nice luxury features that don’t cost a lot to install. Gas boilers can provide heat and domestic hot water. Natural gas is currently a fairly inexpensive fuel, but that may change in the future with carbon taxes on fossil fuels. Ground source heat pumps are typically overkill for a single-family Passive House because the heat loads are so small. However, a large multifamily may be able to take advantage of the economies of scale. Rental buildings should include heat in the rent to avoid the costs of individual metering and to match market expectations. Condo buyers, on the other hand, are more accustomed to paying for their heat and owning their heating equipment. In this case it makes sense to provide decentralized heating units and to market the extremely low heating bills.

Air conditioning is needed in DC, Philadelphia, and NYC. In Boston many apartment dwellers survive without A/C, but the summer humidity does exceed recommended levels. Central air is expected for condos while being an opportunity for market distinction for rental buildings.


Affordable housing should take into account heating and cooling costs as well as transportation costs. Urban multifamily Passive House addresses both. For luxury apartments, the Passive House standard assures unparalleled comfort and healthy indoor air. (And, yes, affordable apartments get those benefits, too.)

Contact us to design your next urban multifamily Passive House. Email greg@duncanarchitectpllc.com for more information.


Big Changes for New York Buildings to Address Renewable Energy and Resiliency


New York is preparing for the potential closure of the Indian Point nuclear power plant by investing in demand reduction, on-site energy production, and energy storage. The Combined Heat and Power (CHP) Acceleration Program from NYSERDA is part of this effort.


CHP provides resiliency in case of power outage. Because the equipment runs 24/7, it is likely to continue operating during an emergency, as long as natural gas supply is not disrupted. Backup generators, on the other hand, risk not starting when they are needed and generally do not have the fuel to operate for days at a time. For commercial buildings over 50,000 square feet (5000 square meters), multifamily buildings with more than 200 units, and buildings that have a high hot water demand, CHP is economical, especially with state incentives. In smaller buildings with Passive House levels of efficiency, however, it may make more sense to disconnect from the natural gas connection to avoid fossil fuel use. Providing hot water using direct electricity is more efficient within a renewable structure.  [2014 PHI Conference Proceedings P. 651] Domestic hot water tanks heated by air source heat pumps can act as energy storage for a renewable smart grid infrastructure.

More information about installing CHP microturbines in NYC buildings.

Currently CHP relies on natural gas supplied by utility companies. However, there is a potential synergy with a new technology called Power-to-Gas (P2G). P2G is useful for seasonal storage of surplus renewable energy. With P2G the amount of methane gas required for a typical house would be about 3.5% of the amount of natural gas equivalent (fossil energy) currently consumed. [2014 PHI Conference Proceedings p.641] Why not use biogas? Biogas is problematic because it requires a lot of agricultural and forest land that could otherwise be used for food, raw material, and transportation fuel.

Renewable energy is likely to continue its fast growth. In fact, the investment bank UBS sees residential PV as a huge growth market in the near future, even though utility-scale PV is less expensive per installed peak watt. As onsite PV energy production becomes more common, there will be a challenge with overproduction on sunny days and the need to use energy from storage at night, especially during cold winters. A change to New York State’s utility structure may be needed to address the cost of renewable energy storage and transmission. In addition to energy storage, one solution is to use onsite PV energy as much as possible before exporting it to the grid. This has been called—a little inelegantly—self consumption. German architect Kay Künzel presented his self-consumption strategies at a New York Passive House presentation on February 27, 2014. His home automation system timed certain appliances to run during the day to take advantage of direct PV energy. Another self-consumption strategy is to use DC power as much as possible to avoid the DC-to-AC inverter losses.

Solar panels on vegetated roof of Etrium Passive House office building near Cologne. Photo by Greg Duncan

Solar panels on vegetated roof of Etrium Passive House office building near Cologne. Photo by Greg Duncan

New York City recently passed the Zone Green amendments which make installation of solar panels easier, although in order to qualify for New York property tax rebates for installation of solar panels, you will need to have a registered architect or engineer file with the Department of Buildings. To calculate the photovoltaic potential in your location and to get information about incentives, use the PV Watts calculator.

Duncan Architect can help you create buildings that are energy-efficient and resilient with onsite renewable energy where feasible. Email greg@duncanarchitectpllc.com for more information.


Passive House Institute to Include Renewable Energy in New Ratings

The Passive House Institute has announced [PDF] that it will start rating buildings that produce renewable energy while meeting the clearly defined Passive House Standard. In order to do this, PHI had to reexamine the calculations for primary energy. PHI uses the term primary energy to refer to what the US EPA calls source energy which you may be familiar with for Energy Star ratings. I’ll use PHI’s terminology in this post. See the Energy Star Portfolio Manager Technical Reference [PDF] for national source energy ratios for the US and Canada. These ratios depend on the mix of electricity generation. EPA chose to use national averages instead of local or electrical grid ratios. The choice of a “correct” primary energy ratio is complicated and not apolitical. For example, some people may disagree that burning coal on site is equivalent to onsite PV (both have primary energy ratios of 1.0 per EPA).  And the ratios are constantly changing. When designing buildings that will last decades, should we use past primary energy ratios or future forecasts? These choices only get more complicated as renewable energy becomes a larger fraction of the electric grid because wind and sunlight are intermittent, requiring energy storage.

PV panels on vegetated roof of Etrium Passive House office building near Cologne. Photo by Greg Duncan

Solar panels on vegetated roof of Etrium Passive House office building near Cologne. Photo: Greg Duncan

Current definitions of Net Zero Energy Buildings—there are at least four—ignore the energy storage and transmission requirements due to renewable energy produced on site. Different uses of energy have different requirements for storage and transmission. Because of these problems, PHI is replacing the primary energy ratio with a new concept, Primary Energy Renewable (PER), that takes into account storage losses as well as production and distribution losses. For example, onsite PV can power air conditioners during summer days when there is plenty of sun, so there is no need for storage and the PER is lower. Conversely, PV systems are not particularly ideal for heating. The PER for heating with electricity—ideally with an air source heat pump instead of direct resistance—is therefore higher. In the future, it will be much more environmentally sound and cost-effective to use renewable energy to cover energy demand for any cooling that may be needed than for heating. These new application-specific PER ratios are also climate specific, dependent on seasonal solar and wind production potential. [Proceedings of the 18thInternational Passive House Conference, pp 648-9. Aachen, Germany 2014. ISBN 978-3-00-045216-1.]

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Utility-scale solar at Nellis Air Force Base USAF – 070731-F-8831R-001

As you can see just from the examples of heating and cooling, calculation of a Net Zero Energy building is complicated. All of them rely on the building itself as the boundary condition for renewable energy production, although sometimes including parking lots and public rights of way. The Net Zero concept discriminates against tall buildings because it is not feasible to install enough PV or wind turbines to meet the energy demands of a 4+ story building. WNYC has an article on the challenges of going solar in an urban environment and why Passive House should be the basis for the design, in order to reduce energy consumption. The article uses the example of a Net Zero Energy school on Staten Island with the luxury of a more suburban location than most of New York City that will open in 2015. This building uses the “trick” of including PV panels over a parking lot.

P.S. 62, being built on Staten Island's South Shore, will have 2,000 solar panels to generate as much energy as the building uses. (Chris Mossa/WNYC)

P.S. 62, being built on Staten Island’s South Shore, will have 2,000 solar panels to generate as much energy as the building uses. (Chris Mossa/WNYC)


To address the discrimination against tall buildings that Net Zero definitions perpetuate, PHI is proposing new classifications based on renewable energy produced on site per lot area rather than floor area:

  • Passive House Classic: unchanged except for the new definition of Primary Energy Renewable. No onsite renewable energy required.
  • Passive House Plus: for a single family house, about as much energy is produced as is consumed.
  • Passive House Premium: for a single family house, produces an energy surplus.

Without changing the Passive House definition, PHI will also start recognizing “Energy Conservation Buildings” that meet a certain threshold for energy efficiency but fall short of meeting the Passive House standard.