Two years ago, an engineer in the Interior Alaska city known for its unforgiving climate built a house that is kept toasty without fossil fuels and uses a mere fraction of the energy typically needed, even at 30 below.

Bruno and Judith Grunau's house sits on one of Fairbanks' rolling hills that span out from the vast Tanana Valley. On a clear September afternoon, autumn had painted the hillsides a vibrant yellow as Bruno Grunau toured his home.

Grunau is a research and testing engineer with the Cold Climate Housing Research Center, an organization which focuses on developing sustainable energy technology in sub-Arctic and Arctic climates. Judith Grunau is an architect.

"Our vision was to have a super energy-efficient home. That was an unspoken understanding between us," Grunau wrote later.

The home is considered "net-zero ready." That means if the couple were to add solar panels to offset the monthly electric costs, they would be completely off the energy grid.

The house is also, in a way, a testing ground. Since it's "one of only a few in this area, we can learn a lot from the system," Grunau said. And like many experiments, Grunau later said he'd do some things differently if he had the chance.

Grunau makes one concession: Elevated from the valley's temperature inversion, the house has never been hit with Fairbanks' infamous minus-40-degree cold snaps. "To be fair, I'm cheating a little bit," Grunau said. The coldest temperature the house has faced is 30 below.

READ MORE: Fairbanks engineer focused on energy efficiency brings his work home