On July 16, I served as co-presenter at an event for the Twin Ports Climate Conversations. Hosted by the Minnesota DNR, the Climate Conversation series brings together concerned citizens to learn about climate-related topics across many disciplines, including natural resource management, public health, infrastructure, and more.
This event’s focus was buildings, and the event was held at a house currently under construction I designed as one of my last projects at Wagner Zaun Architecture. During the tour of this ultra-low-energy home, the homeowner (a mechanical engineer who specializes in renewable energy systems) and I talked with two dozen interested climate activists, scientists, educators, and laypersons about the importance of changing the way we build and power our buildings.
To address the ongoing impact buildings have on the climate, we need to do more than reduce energy use. We need to rapidly and dramatically reduce the CO2 emissions associated with creating and operating our buildings. The 3,000 square foot Hoxie house was designed to the U.S. Passive House standard, and the owners plan to have a solar PV system installed to make the home zero net energy. While this particular home won’t go through the Passive House certification process, it will be registered in the Architecture 2030 database.
No one thing makes this house climate-appropriate. Rather, it is the integrated design of the building form, the materials and assemblies, the equipment and the systems design working together (along with homeowner behavior) that result in a home with very little negative environmental impact.
Here’s some of what makes this home low-energy, low carbon, and climate-appropriate:
Explaining the air barrier assembly
Hoxie residence under construction, winter 2018
ENCLOSURE
The house sits on a south-facing slope, with breathtaking views of Lake Superior and great orientation for passive solar operation. Triple pane windows and solar-optimized overhangs help manage heat gain from the sun, allowing solar gain in winter but reducing unwanted summer heat
The house is a simple form: one-story with a walk-out lower level. A simple form allows for more efficient use of materials; it also simplifies the detailing needed to maintain the thermal and air barriers of the enclosure
The walls above the ground on the lower walk-out level are wood framed, unlike the insulated concrete foundation walls situated below grade. Concrete is a more carbon-intensive material than wood, so using less of it reduces the home’s CO2 footprint
The wood-framed walls are 2 x 6 studs, insulated in the stud cavities, sheathed and sealed, then wrapped on the outside with 6” of rigid mineral wool insulation (which isn’t made from petroleum, like most foam insulation products). The finished framed walls have an R-value of ~R-45, more than twice what current code requires
The attic is filled with more than two feet of cellulose insulation, with an R-value of more than 100 (again, more than twice what current code requires).
The home is connected to the garage with a screened porch, providing shelter, outdoor living space, and connection to the landscape. This light house-to-garage connection helps maintain the integrity of the thermal enclosure of the home, and keeps the air barrier of the super-insulated home completely separate from the garage
SYSTEMS
The sun’s energy will help heat the house in winter. When the sun’s energy isn’t adequate, the whole house can be heated with two small super-efficient cold-climate air source heat pumps
Hot water is provided with a heat pump water heater, more than twice as efficient as a conventional electric water heater and four times more efficient than a typical gas water heater
The home is all-electric, and will have a 10 kW PV array likely to offset all the annual electricity use of the family for net-zero energy.
The tested air tightness of the home is .28 ACH50, more than 10 times as air tight as current residential energy codes require. A heat-recovery mechanical ventilation system (HRV) ensures a steady exchange of fresh air to keep occupants healthy
Southern exposure on a summer day