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.

HVAC

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.

Conclusion

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.