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First Test of Taconite for Thermal Storage in Minnesota PassivHaus

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When the principal architect at an 80 year old Minnesota architectural, city planning, scientific and engineering consultancy firm built a vacation home in Isabella, Minnesota, a region that routinely gets to 60 below zero, she decided to test an experimental thermal insulation technology.

It has paid off. This year, the house was awarded both PassivHaus certification and LEED certification with an expected energy savings of 97%. (Put another way, this is like a comparison between a car that gets 200 miles per gallon, to a car that only gets 25).

This means the house needs virtually no fossil fuels to heat and light, despite being in a region that needs 9,700 heating degree days a year. This kind of weather is almost uninhabitable for humans.

LEED AP Nancy Schultz is the principal architect with SEH.  Working for a firm with such a technical focus, it is not surprising that her interest would be in very serious energy savings, and her blog of the progress of the Isabella Project is full of great detail. (Shultz also established the US Passive House Institute.)

The house, featured at USA Today, has the airtight envelope of a PassivHaus. The windows have an R-value much higher than most walls. (Some building codes only require R-1) The walls were R-55 and the roof R-90.

An 8 KW solar system provides the electricity. A solar hot water heater using vacuum tubes averages 172,000 btus a day in heat. For daily household use an normal 80 gallon tank suffices, but an additional huge 500 gallon water tank ensures that even a month of cloudy days will be toasty inside.

But the real innovation is in a test of a solar heat storage containment area under the building. It is filled with waste taconite , a low grade iron ore from Minnesota’s Mesabi Iron Range mines and sand, that has been used in steel making since the high grade iron mines were depleted in the 50’s after World War II.

There have been several patents on heat storage using taconite.  But this LEED and PassivHaus will be the first house to provide a test of its efficacy as thermal storage.