Near-Net-Zero in Northern Minnesota


The Isabella Ecohome is LEED Platinum-rated and Passive House certified. The project team approached the net-zero-energy goal primarily through a highly insulated, airtight building envelope.


If a home in northernmost Minnesota, where the average January temperature remains around 4°F (minus 16°C), can achieve net-zero-energy use, imagine the possibilities for buildings in less-challenging climates.

An immensely complicated goal? Maybe. But we deemed it a risk worth taking to build a 2,134-square-foot lakeside ecohome in tiny Isabella that doubles as an “experiment station.” With this effort, we wanted to “pay it forward” to prove that achieving
net-zero could become standard building practice.

The total cost to build our home was approximately $342 per square foot (based on
5,384 total square feet, including unconditioned space). Much of the cost was associated with our 8.4-kilowatt photovoltaic (PV), 92-tube solar-heating and experimental heat- storage systems and associated energy monitoring equipment; the relatively remote location; rainwater collection and a vegetative roof; labor to repurpose salvaged materials; and a home energy monitoring system. We justified the additional costs for several reasons:

Isabella EcohomeFirst, because we could afford it; second, because we felt it was an environmentally worthy cause; and third, because we were curious about the challenge of achieving net-zero in an extremely cold climate. Our utility company and the state granted us rebates for energy-producing and conserving systems totaling $20,800.

In addition to meeting net-zero-energy goals, we wanted our home to replicate the ecological balance found in nature, being both sustainable and aesthetically pleasing. Although this goal further complicated the project, we decided that the most advanced passive houses should virtually live and breathe.

Isabella House SpecsAfter discussing the project with our project team, we felt that we had the technology and the building science to achieve these lofty imperatives. We agreed on the following criteria for our home:

  • Generate more renewable energy than it used
  • Sustainably manage the use of water
  • Waste nothing
  • Adapt to new conditions
  • Work symbiotically with all other living things
  • Eliminate toxins and pollutants
  • Add beauty to our world

Our Isabella Ecohome, as we call it, was completed in September 2010. We began planning and design in August 2006 and construction in March 2007. The Isabella project incorporated an extreme waste- and material-management system. One workman commented he had never built a house without a waste material dumpster! Repurposed materials, fast-growing bio-fiber products like Dakota Burl, a sunflower-seed-shell composite board, and reclaimed tile and wood helped us achieve zero waste and low lifecycle-assessment values.

The house is certified to the highest levels possible by both Passive House Institute US and the U.S. Green Building Council. We believe many features, too lengthy to describe here, make this one of the most advanced energy-efficient buildings of our times.

Creating a Tight Building Envelope

Without data to prove our claims, the experimental aspect of the Isabella Eco House would be meaningless. A Siemens TALON Techmar 420 building control system verifies that our home’s systems meet our original design criteria. Baseline and historical data allow us to adapt and make refinements. Alarm messages are emailed whenever power, pumps, temperatures or water levels are not performing as intended.

Reclaimed Wood

The project team incorporated an extreme waste- and material-management system in the Isabella project. Repurposed materials, fast-growing bio-fiber products and reclaimed tile and wood helped the project achieve zero waste and low lifecycle-assessment values.

Our home was designed to have an annual heating energy consumption of 4,500 Btu per square foot of conditioned space, or about 9.6 million total Btu per year. We approached this goal primarily through a highly insulated, air-tight building envelope. The walls and roof have average R-values of 55 and 90, respectively. We avoided unnecessary thermal bridges and paid meticulous attention to preventing air leaks. The house eventually reached a final air tightness of 0.5 air changes per hour at 50 kilopascal pressure differential.

This extreme air tightness was achieved by first using Bayseal’s PP 1.1 closed-cell spray polyurethane foam for the difficult-to-access areas, as an air barrier and insulating layer. While this type of insulation has greatly reduced ozone-depleting propellant problems of the past, it is still a petroleum-based product, so we tried to minimize its use. We filled most cavities in the double-wall system and attic spaces with Modern Insulation’s Weather Blanket, a dense-packed cellulous material made from 85 percent post-consumer recycled newspapers and borate fire retardants.

High CeilingsWe chose wood-framed high-performance windows made by Mueller in Lautenbach, Germany, and marketed under the Optiwin label in the United States. The glazing, selected to optimize the solar gain for each orientation, was an inner/outer PPG 3/16-inch Starphire low-iron glass/double-heat mirror, suspended coated film. It became apparent during energy modeling that we needed to maximize the south-wall glazing and/or reduce the west-, east- and north-wall glazing to achieve Passive House certification.

Unfortunately, the lake view was to the north, and putting no windows on that side of the house was not a real option. The thermal performance ratings of the south-facing windows are 0.088 U and 0.52 SHGC; and for the north-, west- and east-facing windows, 0.07 U and 0.37 SHGC.

A heat recovery ventilation system (HRV) helps ensures air quality in the airtight house. A ground-loop heat-recovery system connected to the HRV preheats the incoming sub-zero-temperature fresh air. The monitoring system reveals that we gain a 10°F to 15°F preheating benefit through this system.

A rainwater-collection system and vegetative roof assure that we continue to replenish the aquifers and support plants and animals for ecological balance. The living roof provides insulation, as well.

Testing Solar Heat Storage

There are about 9,700 heating degree-days in Isabella and only 189 cooling degree-days. Thus, in addition to orientating the house and windows to maximize passive solar gain in the winter, our high priority was maximizing solar heat collection, and also storage for winter use.

When designing and sizing the active solar heat collection system, we estimated that there would be about 120 days of sun in the spring, summer and fall for solar heat collection that could be stored. We installed 92 SUNDA Seido evacuated solar heat collection tubes. From July 2010 through June 2011, according to our TALON monitoring system, we collected 42 million Btu. As expected, most of the solar heat collection occurs in the summer.

solar Heat Storage

The experimental solar heat-storage system stores heat in an insulation-lined crib beneath the house. Tubes of water transfer the heat through flooring under the living space.

To better match the solar energy available in Northern Minnesota with the seasonal heating demands, we designed an experimental seasonal solar heat-storage system. In the space that would have been the basement of the main living area, we created a crib for sensible heat storage, lined with 16 inches of expanded polystyrene (EPS) insulation. We would transfer heat stored in the crib through tubes of pumped water. The tubes would continue from the crib through slabs beneath the living space. Thereby solar heat collected months earlier could be used in mid-winter.

We created two compartments in the crib separated by 2 inches of EPS insulation, one containing a sand/gravel mixture and the other containing taconite concentrate. Taconite is unique to Northern Minnesota, and the heat these pellets hold per unit volume is almost twofold that of sand or gravel.

During the first year, we found that when the outside temperatures fall to the minus 40°F (minus 40°C) range and the water delivered to the in-slab heat pipes falls below 85°F (29°C), the rate of heat delivery from the in-slab heating is inadequate to keep the house at 70°F (21°C). We hoped we could get the crib temperatures close to 140°F by the beginning of winter, but we’ve found that when the crib temperatures reached about 120°F (49°C), the heat begins to be lost across the EPS insulation as fast as it is added to the crib.

We believe we can get closer to net-zero by installing a small heat pump to transfer heat from the taconite/sand into the water circulating in the home’s in-slab heating tubes when the crib temperature falls below the 85°F, in order to ensure adequate space-heating throughout the winter. The heat pump would only need to raise the temperature of the water 10°F to 20°F above the crib temperature, and it would use far less electrical energy than the backup resistive heating by the boiler.

Energy From GridThe system also monitors two additional solar heat-storage areas — a 500-gallon water tank and an 80-gallon domestic hot water tank. We use data gathered from them to determine how many cloudy days it takes to deplete the stored energy supply. When starting at 150°F (66°C) — possible with a few days of sun — this tank can heat the house for several days even when outside temperatures fall well below 0°F.

Once the stored solar energy supply is depleted in both the sand/taconite crib and 500-gallon tank, a 10-kW electric boiler comes on for backup space and domestic water heating. In summer, our PV system supplies the modest domestic hot water heating need, but in mid-winter we need to buy power from the grid. From July 2010 through June 2011, our total net electricity use was slightly under 1,000 kilowatt-hours (kWh). We supplement with a wood stove for space heating. Last year, we burned about a cord of wood.

Backing Up on-Grid PV with Batteries

Our PV system comprises 42 Sanyo HIT 200-watt modules mounted on the south-facing garage roof, tilted 60 degrees to slightly optimize winter over summer electricity generation and, more importantly, to facilitate snow shedding. With an 8.4-kW peak generating capacity, this system is projected to generate 11,000 kWh per year. The total cost of the grid-connected system and a lead-acid battery backup system was approximately $100,000 installed. We received a $16,800 rebate from the state and an additional $4,000 rebate from our local utility. Through net metering, we earn a credit on our utility bill from late spring to early fall.

In the basement mechanical room, we have two SMA Sunny Boy 6000U inverters along with two SMA Sunny Island 4248U units that interface with 12 sealed lead-acid batteries. The addition of batteries to a grid-tied system allows our solar heat collection equipment to continue to operate most of the building’s mechanical, lighting and appliances in the event of a power failure.

Control Room

The Isabella experiment station uses a Siemens TALON building control system to verify that the home’s systems meet the original design criteria.

The figure on left shows the stored solar heat flowing to our in-floor heating loops, as well as the grid electricity used for our backup boiler each month (for backup heating) and our total grid-electricity use and contributions throughout the year. Most grid electricity in wintertime goes to supplying the backup electric boiler, which not only heats the living spaces, but also heats the domestic hot water when there is insufficient solar heat collection.

The TALON monitoring system indicates that the total energy supplied to the floor space- heating loop for the 2010/2011 heating season was 7.1 million Btu (2,080 kWh). When we add the energy supplied by burning wood, our space-heating total comes to 22 million Btu, or about 10,000 Btu per square foot of conditioned space annually. Though we can’t independently monitor the boiler’s electricity use for space heating vs. water heating, the monitoring system data suggests that perhaps half of the its power consumption goes to heating domestic hot water.

Achieving Energy, Ecological Goals

Our home has a Home Energy Rating Systems (HERS) score of 3, within a breath of a net-zero-energy home’s score of 0. Nevertheless, based on data collected since the project completion, we are contemplating a few changes to achieve net-zero.

We plan to better insulate the garage basement, reducing heating demands for our rain- water collection system’s storage tanks located there. We’ll install motion or daylighting controls on all lights and investigate automatic shutoff for all devices powering transformers. We also plan to modify the PV system with a DC circuit, to avoid conversion losses for some of the appliances clustered above the basement utility room.

Social justice and beauty are parts of ecology that acknowledge the value of engaging people through art while supporting the notion of equal access and opportunities for all people. Because social justice was a project imperative, 85 percent of the labor was provided by people living in an economically underutilized business zone. The Isabella project embraced adding beauty through the creation of a place that is welcoming, educational, inspiring, healthful and fun.

See full details of the Isabella Ecohouse project:


John Eckfeldt & Nancy SchultzJohn Eckfeldt, M.D., Ph.D., has served as director of clinical chemistry and director of clinical laboratories for the University of Minnesota Medical Center for many years. He now holds the endowed Ellis Benson Professorship and is vice chair for Clinical Affairs in the Department of Laboratory Medicine and Pathology in the University of Minnesota’s Medical School.

Nancy Schultz ( is the principal manager and owner of Compass Rose. She has more than 24 years of experience in the architecture, engineering and sustainability industries serving in positions such as director of architecture, director of marketing and development, board member and numerous other leadership and project management positions. Schultz is a licensed architect and a LEED-accredited professional certified with the National Council of Architectural Registration Boards.

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One Response to Near-Net-Zero in Northern Minnesota

  1. David Durrant Reply

    June 10, 2014 at 8:18 am

    I also live in far northern Minnesota – in Tower.

    I am considering solar heat and note that you state that taconite pellets hold heat better than sand or gravel – why is that please?

    Best Regards,
    Dave Durrant

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