Pushing the Envelope on Envelope
The PLACE Team was hired to provide technical assistance to award-winning, affordable housing developer, Aeon. Aeon wanted The Rose to be the best performing building of its kind in the country, but on an affordable housing budget.
PLACE's title was Sustainability Champion.
Minneapolis, Minnesota has the harshest climate of any big city in America, so the challenge for PLACE was to create a building envelope capable of keeping the residents cool in a scorching summer with high humidity and a frigid winter with temperatures plunging to minus thirty degrees Fahrenheit. Building codes requiring minimum standards of performance just aren’t good enough for a high performance building. In some of the coldest places in the world, like Northern Scandinavia, the PassivHaus movement successfully delivers buildings that require no conventional heating system. Yet buildings in the U.S. are built each day to standards that cause them to consume 70% of America’s primary energy, with 52% of that energy going to heating and cooling.
The building envelope is a dynamic interface between the interior of the building and the outdoor environment. The envelope acts as a thermal barrier that determines how much heating and cooling energy will be required to create space comfortable for its inhabitants, and how much artificial lighting will be needed.
A building’s envelope is a system that includes its foundation, walls, windows and roof as well as its cladding, insulation and air sealants. The envelope can cost effectively reduce the need for heating, cooling and lighting energy. Additionally, the building’s orientation can significantly affect it’s performance.
Many building teams seek to improve performance through the use of renewables, or by changing light bulbs. No effective high-performance building goal can be achieved without a high-performance envelope. The Rose used the building envelope as a primary energy performance strategy.
Orientation and Massing
Before any envelope strategies were developed, the team researched the impact of building orientation and massing on performance. Five rejected options for building massing and orientation were generated during design research. The approaches were bounded by zoning requirements, the site characteristics and the neighborhood context. The proposed strategies offered different ways to activate the streetscapes, optimize solar PV potential, improve daylighting, and minimize corridor space.
Each option was modeled by DOE2 software to determine its energy use intensity (EUI), expressed in kBtus per square foot annually. Though each of the options represented the same amount of space for the program (i.e. dwelling units, common space, office, etc.), we found a wide variation in the amount of energy consumed annually. The data showed a fifteen percent range in EUI among the six strategies.
Solar gain on West-facing façades, which tended to increase air-conditioning costs, turned out to be a major factor. An orientation and massing that placed two building on an east-west axis was modeled to perform better than any other strategy. The “double bar” strategy fit the neighborhood context and allowed for solar penetration into the central courtyard.In addition, the space was designed to minimize interior corridor space, which must be heated and cooled, but is occupied less than five percent of the time. Efficient floor plans also removed thousands of unnecessary square feet from the project.
Considering the fact that orienting buildings is an essentially free decision, our work on orientation and massing turned out to be the single most cost effective energy performance strategy for The Rose.
A Super Envelope
After maximizing energy performance through optimal building orientations, the team began its research on the best thermal envelope to meet our goals. Minneapolis endures the harshest climate of any major city in the US, experiencing over 130 degrees of temperature range and relative humidity levels running from desert to swamp. The team’s assumption was that a “super envelope” was the most cost-effective way to lower energy load demands for the project in our climatic extremes.
7 Initial Super Envelope Strategies:
- Optimal orientation & massing
- Super roof with R value of at least 80
- Super walls with R value of at least 40
- Super windows with U value of at most .18 and carefully-designed assemblies
- Slab insulation with R value of at least 24
- Foundation insulation of at least R-18
- Minimize envelope penetrations
Choosing a building envelope is a long-term decision. Framing and insulation is not easily or cost-effectively changed. Our light bulbs can be swapped. We live with our envelope for a long time. And a building’s envelop is a dynamic interaction of framing, insulation, windows, window assemblies, roofing, cladding and more. The cost of each of these affects the cost of the others. The question: How to deliver the best envelope at the best price?
Let PLACE help you. Contact us for technical assistance.