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5 Considerations for Passive House Design for Multifamily Housing

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Multifamily developers will often come to architects and ask, “What’s next in sustainable design?” They do this because they want to stay ahead of the curve with ever improving code requirements and to offer unique living environments for their tenants to differentiate themselves in the marketplace. Over the past few decades various building certification programs have entered the industry and are part of the common parlance when discussing sustainability, such as LEED, WELL, and the ENERGY STAR programs. Each certification program has driven the construction industry to develop innovative ways to improve the health, wellness and safety of both occupants and the natural environment, so what's next?

Net Zero revolves around a simple concept; a building will maintain a perfect balance between the energy it consumes with the energy it generates. There are two components to consider to achieve this harmony, 1) a super-efficient building, and 2) a renewable energy source for that building. The renewable energy source has, again, become so commonplace almost everyone can identify them: PV solar arrays and wind turbines. However, not too many people know or understand the specifications of a super-efficient building. And that is where the Passive House design standard enters the picture.

Like many other innovations in the construction industry, the current iteration of Passive House entered the US market by way of European influence. Beginning in the early 1970’s in North America, Passive House was a reaction to the US oil embargo where designers became keenly aware there had to be a shift away from the heavy reliance on fossil fuels towards a more independent, passive use of energy. The 1980’s saw German scientists and engineers take the reins on perfecting these concepts, with physicist Wolfgang Feist founding the PassivHaus Institute (PHI) in 1996. By 2007, the PHI expanded to the United States and re-named itself the Passive House Institute US (PHIUS). In 2015, the PHIUS+2015 Passive Building Standard was introduced as a tool for designers to apply the highly tuned specifications to the multifamily housing typology. According to PHUIS, “Certified and Pre-Certified projects now total more than 1 million square feet across 1,200 units”, covering every corner of the nation.

Passive House is guided by five values which all projects must follow. Each of the values informs the next the one and if they are not adhered to with conviction the house of cards can fall apart. The key thing to keep in mind, however, is regardless of how intensive these values may seem, by adhering to them you will end up with a super-efficient multifamily building.

Super Insulation

This is not too unlike the current best practice of wrapping a building envelope with continuous insulation (CI), however, this is insulation on steroids. Providing CI at the exterior sides of the roof, walls, and under the floor slab, effectively wraps the building in a giant blanket. But this is not just a slight increase in insulation thickness; take to point most under-slab insulation is currently specified around R-10, or nominally 2” of rigid insulation. Depending on the climate region, Passive House specifies that same building will have an under-slab requirement of R-22, or nominally 6” of rigid insulation, a 200% increase. This logic applies to the wall and roof surfaces as well.

Air Tight Envelope

The biggest threat to compromising the super-insulation envelope can be the smallest things, the holes and seams that are common to construction. Every time a fastener or pipe penetrates the envelope, we are punching a hole right through our blanket and providing an avenue for to escape. A few small holes here and there don’t seem like much, but when you add them all up around the entire building, those holes can equal a rather large opening. Air holds a surprising amount of moisture, known as relative humidity, and when it transmits through cracks there is potential for that moisture to condensate within the wall cavity. The most effective way to achieve a robust seal is by utilizing a fluid applied or fully adhered air barrier, traditional house wrap just won’t cut it.

Optimized windows

We demand windows to perform a relatively complex set of functions to meet our needs: they must stop air and moisture from entering the building, prevent tempered indoor air from escaping, be able to open and close to provide fresh air, and perhaps most importantly, be transparent. Essentially, we expect windows to perform like a wall, but to remain invisible to our view. 

Due to this conflicting relationship between the window and the wall, now is not the time to skimp on the performance of the fenestration. Passive House requires the implementation of triple-glazed windows with insulating gas between panes, regardless of climate region, to meet the stringent insulating requirements. Traditional double-glazed windows have a U-value range between 0.2-0.4, while triple-glazed can get as low as 0.12-0.18.

In addition, Passive House expects 50% of indoor heat to be provided directly from the Sun, also known as free heat. This requires special attention to be paid for specifying the solar heat gain coefficient (SHGC) of each window depending on their placement orientation. North facing windows will require a different SHGC than those facing South, and the same goes for windows sitting on the East-West facades. By studying each façade independently, an architect can create a fenestration design that truly matches the demands of the building.

Dedicated fresh air

One byproduct of a super-efficient, leak resistant building is that it does a great job of holding onto its indoor air. In a short amount of time that air will become stale and in extreme cases, dangerous. To overcome this unfortunate circumstance, a Dedicated Outdoor Air System (DOAS) with Energy Recovery Ventilation (ERV) can simultaneously supply fresh air while managing humidity levels and reducing heating/cooling demands. Put simply, the system works by combining outdoor air with used indoor air to provide continual pretempered fresh air. In the case of a cold climate, chilly outdoor air combines with the warm indoor air, resulting in a pretempered air supply that requires less additional energy to bring up to a comfortable temperature. This logic also applies in warmer southern climates, except in the opposite relationship.

Minimal mechanical demand

The building is now to the point where the exterior envelope is wrapped in a layer of super-insulation, penetrations and cracks are robustly sealed, and the windows are optimized for performance. Combining these attributes with the decoupled ventilation, there is far less demand for a large, whole building mechanical system, offering greater flexibility for the source of tempered air. In-slab radiant heating, geo-thermal, thermal mass walls, trombe walls, heat sinks, and as previously mentioned, passive solar heat gain are all strategies that can be explored to supplement the heating load. This is where cost savings from a reduced mechanical system can be carried over to pay for the improved envelope systems.

By following these 5 design principles, a building will be on the path towards the new gold standard in sustainable design, Passive House Certification. By tying the building to a renewable energy source, Net Zero Energy is easily achievable. This is applicable to wide variety of multifamily project types, including residence halls, dormitories, senior living communities, hotels, and apartments. For those looking for the next big thing, Passive House is the answer.

Sam Bell,
Project Architect

Sam Bell is a Project Architect with EUA working in the Milwaukee office. Sam is a member of the Living Environments Studio.