This home is situated near Lanesboro, Minnesota in the middle of a broad valley, close to the Root River, woods, trails and a community of people who share Nancy and John’s love for the outdoors. From the start, a primary goal was to generate most of the home’s energy from the sun. From this perspective the site is close to ideal except for a stand of trees directly west of the house. However, the afternoon shade provided by those trees is arguably beneficial for reducing summer overheating.
Foundation & Ground Floor
The biggest design challenge was identifying a foundation and floor system addressing accessibility, durability and a super-insulated, air-tight wall system all at once. My colleague Lou Host-Jablonski and I began our discussion about the building envelope with a healthy debate about the foundation. We’d seen many super-insulated, cold-climate buildings with shallow, frost-protected foundations—the kind that look like a raft on concrete sitting in a sea of foam insulation. One advantage of this system is the ease with which a continuous thermal envelope can be achieved. However, there are also disadvantages to a shallow foundation. First, the required frost skirt eliminates substantial planting next to the house, which didn’t jibe with Nancy’s permaculture goals for the site. Second, it takes a lot of concrete and the owners had already procured several thousand board feet of salvaged elm for flooring. Third, exterior EPS foam breaks down quickly, even underground, and can be damaged by things like lawn mowers, compromising the thermal envelope. Finally, we prefer to design buildings for 100+ year use and we still aren’t sure about the raft system’s longevity in our neck of the woods. Additionally, farm fields in this area have frozen up to eight feet deep, and the jury is still out on “the raft”, so we recommended a six-foot foundation wall.
Walls & Roof
For exterior walls, we first intended to hang site-made Larsen trusses from a 2” x 6” structural wall bearing directly on the foundation wall. This method fills exterior and interior walls with dense pack cellulose insulation separated by a plywood or OSB air barrier. The technique has been used in Passive Houses, but we aren’t keen on the space it creates at ground level for leaves to collect and rodents to hang out. Additionally, the exterior walls and ground floor must connect to create a continuous thermal envelope and air barrier.
Eventually it became obvious to us that this double-wall system needed to be reversed so the Larsen trusses “hang” on the inside instead of the outside. We were aware of a rule-of-thumb often used for super-insulated houses, in which an air-tight layer separates two layers of insulation with approximately two thirds of the insulation on the exterior. Because we reversed this ratio, we ran a WUFI model to test for risk of moisture build-up and mold growth. The model showed we were in the clear. We also ran a THERM model on our wall system, which showed good performance and no thermal bridging. At the roof we ran a continuous OSB air barrier onto the underside of wood roof trusses and called for 2” x 4” strapping to create a cavity for additional insulation and a chase for electrical and ventilation runs.
Windows & Doors
Super-insulated houses require ultra-performing windows and doors. It would be pointless to work on the home’s envelope, only to cut a bunch of holes in it and fill them with low-quality, leaky windows and doors. So we looked to Canadian and European manufacturers who offer a wide variety of ultra-performing windows and doors. Typically these are triple-paned with a high capacity to let in the sun’s warming rays. They also have thermally-broken frames and an ultra-airtight seal. Availability of windows and doors of this quality has been one of the biggest challenges for the ultra-performance home movement in the U.S. The situation has improved, but there is still a call for a high-quality U.S. manufactured product.
The homeowners chose Zola Thermoclad windows and doors for this project. The devil is in the details when it comes to using European windows! Unlike U.S. windows—where frame and sill are integral to the window, water drainage detailing is figured out, and nailing flange installation is easy—European windows require head scratching to think through these things.
Because loads on ultra-performing buildings are so low, mechanical systems can be minimized. Most important is the ventilation system. A high-efficiency energy-recovery ventilation (ERV) system ensures good air quality and constant, fresh air in airtight buildings. We chose the Zehnder 350 system for quiet function and easy installation. Air is exhausted from bathrooms and kitchen and supplied to other living spaces.
The passive design of the house minimizes the need for traditional heating/cooling. To supplement this in extreme situations we chose two on-demand Mitsubishi mini-split heat pump units; one for the first floor, one for the second. Mitsubishi now offers the h2i model, which operates effectively down to -13˚ F, which is important in cold climates.
Water is heated using a domestic solar hot water system including a roof-mounted collector and insulated lines, pump/controller station, and 50-gallon water storage tank with electric element for back-up heating. Electricity is generated through a 4.9 kW roof-mounted photovoltaic array, which is expected to produce 6,500 kWh of electricity yearly.
Passive House & Net Zero
Nancy and John learned about Passive House in 2008, but did not judge it to be affordable. Later they connected with Lou and me through the Passive House Institute U.S. where I had earned the Certified Passive House Professional (CPHC) designation. Naturally, we had discussions about Passive House certification from the beginning.
Throughout the design process we utilized Passive House Planning Package (PHPP) software to guide decision-making about insulation levels, glazing, and orientation. The shape of the house (a two-story with one-story “lean-to”) grew out of design program requirements.
Throughout the design process we consistently met Passive House (PH) criteria for primary energy and cooling demands. We were very close to meeting heat demand criteria, but couldn’t get there without removing already minimal north-facing windows, adding even more insulation, converting the one-story “lean-to” to a two-story space (adding unnecessary square footage), and installing a sub-soil heat exchanger. All of these extra resources reduced heat demand by an extremely small amount, but just enough to meet PHIUS’ current PH criteria.
In the end we decided not to pursue PHIUS certification. In addition to the realities above it was clear that removing the only north-facing windows compromised natural ventilation and daylighting needs. We also realized this house would be net-zero or close to it even without formal PH certification because of the photovoltaic system on the roof. Now, that’s something to write home about!
It’s cold in Minnesota, with approximately 7750 Heating Degree Days, as compared to 5600 in Germany (the climate on which PH criteria are currently based). That means Germany’s climate is more like central Illinois, not the northern Midwest. There has been a lot of discussion about how PH criteria are translated in the U.S., and we in northern climates will be watching closely to see how this plays out and affects us. [Ed. note: Revised criteria are expected to be released by PHIUS late in 2014.]
Give the design process time! More than you might expect goes into planning an ultra-performance home. Energy modeling, thermal and moisture analysis, window and door research, coordinating all the pieces and more take a lot of time. Rushing through the process leads to cut corners and disappointing compromises. The best-quality, best-performing house grows from an integrated, thoughtful design process.
* During this project, Christi worked with Design Coalition Architects.