Goals & Energy Modeling

Building Sustainably 

The adjacent lot from our old house, and the site of our net zero house.

The site of our net zero house.

It was our dream to build a super energy-efficient house on the adjoining city lot beside our old home. In the midst of the alarming global climate crisis, we felt compelled to build a house that’s environmentally sustainable in its design, composition, and energy use.

Physical comfort was also a factor. With Minnesota’s seasonal temperatures spanning from -34°F to +108°F, we also wanted the new house to feel consistently warm in winter and reasonably cool in summer without feeling guilty about the energy expenditure. We started researching options.

Aiming for Net Zero

A late 1970s course at Shelter Institute in Bath, Maine introduced us to the fundamentals of designing and building a passive solar home. But, nearly 35 years later, with the dramatic improvements in the building science of energy efficiency, indoor air quality, and structural longevity, we needed to expand our research.

Beginning in 2013, we read articles and attended local lectures to learn the refined super energy-efficient homebuilding practices developed by innovators like the German Passivhaus Institut. We also toured the sprinkling of houses within 150 miles that were using advanced energy-saving methods.

The more we learned, the more we felt both excited and compelled to build a house that would produce the energy it consumed. We decided to aim for a building that performs at net zero.

Home Energy Rating System (HERS)

Home Energy Rating System (HERS)

Technical Support  

We knew the types of living spaces we’d best use and the aesthetics we enjoy, so we set up shop (old school style) on the dining room table with graph paper, architect scale and mechanical pencil to begin drawing floor plans and elevations. We also had Tim's carpentry and general contracting expertise in our toolbox, and we could call on his skilled subcontractors in the various construction trades. But, we needed help navigating the science and technology involved in meeting our sustainable energy goals.

Our architect friend Lucas Alm of ALM Design Studio was our perfect match—a natural teacher and super collaborator with a passion for energy efficient design. Lucas enlisted his team of engineering, energy, and solar experts for the project, designed the home’s super-insulated envelope, drew the wall cross-sections, made 3D projections of the sunlight’s reach through our window configuration, and consulted with us on our floor plan and elevation drawings. He also led onsite construction tours for people interested in understanding the home’s unique building components and the unfolding construction process.  

Tim (left) and Lucas (right).

Energy Modeling

Ours is a 2,079 square foot, one and one-half story house. To determine the heat gains and losses from our window and door glazing, as well as the insulation combinations and quantities needed to achieve our net-zero goal, our energy modeler, Philipp Gross, used the Passive House Planning Package (PHPP)—an energy modeling software program that calculates a building’s energy loads based on 1) the R-values (the rate of a material’s resistance to heat flow) of all the exterior surface areas of the envelope: the floor foundation, walls, windows, doors, and roof, and 2) the occupants’ projected energy use.

Once the envelope design was finalized, the PHPP program calculated the home's peak heat load to be 13.2 kBTU/hr, or almost 60% less than a similarly sized home meeting the MN 2015 energy code. Annual energy use intensity (EUI) was calculated at 11.6 kBTU/ft2/yr. This is a savings of 77% compared to a similarly sized house meeting the MN 2015 residential energy code modeled at 51.1 kBTU/ft2/yr. 

Efficiency Enhancements

The PHPP program uses 20° Centigrade (68° Fahrenheit) as a baseline for heating load; but, because of health issues, we keep the indoor temperature at 74° in our cold winters. In order to compensate for this, we filled the 1.5-inch electrical chase with mineral wool insulation, increasing the wall system’s R-value by 6, bringing it up to R45.

The PHPP energy modeling called for 5.7 kilowatts (kW) of PV solar panels to reach net zero. As we anticipated driving an electric hybrid car, we ended up installing 7.695 kW, which was the maximum capacity of our south-facing garage roof, generating more than enough renewable energy to offset the home’s energy expenditure.

Projection of Energy Usage in kWh

Heat Load: 3.9 kW [13.2 kBtu/h]

Total Site Energy: 8031 kWh