Standing proudly on the outskirts of the Danish city of Aarhus is an experimental eco-home where one brave family is testing out life with the latest cutting-edge sustainable design. For the past year, Sverre and Sophie Simonsen, along with their three children, Axel, nine, Anna, seven, and baby Marie, have been living in the world’s first Active House—a building so technically advanced that in 40 years it will have created enough energy not only to support the family inside along the way but also to pay back the energy used for its materials and construction: a house, in short, with no carbon debt.

It’s one of eight prototype homes being constructed across Europe by the Danish window company Velfac. A test family will occupy each of the houses for a year while the building’s performance and the family’s experience is monitored. Having seen an advertisement on the internet, the Simonsens jumped at the opportunity. “Like most Danes we were interested in the environment beforehand,” says Sophie, a secretary, “but here was a chance to actually do something about it.” Sverre, a chemical engineer, was also keen to road-test this slick bit of architecture. “Our old house is a typical 1970s suburban home with lots of small rooms, little windows, and dark corners,” he says. “I’d always wondered what it would be like to live in a really modern open-plan building.” Having seen off the competition from 40 other families, they made the move—less than a quarter of a mile up the road but a gargantuan leap in terms of their carbon footprint.

A different architect designed each Active House. The Simonsens’, an angular building perched on a sloping plot, is by Aart Architects, an up-and-coming local practice. “The starting point was the roof,” says architect Anders Tyrrestrup. “We were restricted to one-and-a-half stories by local planning regulations and the south-facing side needed to be angled at 35 degrees to optimize solar gain.” By creating a steep northern pitch and stretching the south-facing side, the firm cleverly maximized the roof area that gets the most sun.

The materials—a timber frame, and slate and Douglas fir cladding—were selected for their low embodied energy and easy maintenance. The monolithic slate skin merges brilliantly with the solar panels positioned on the roof. “We imagined it as a cross between a home and a stealth bomber,” says Tyrrestrup.

Most impressive is the subtlety with which the building’s forward-looking environmental agenda has been woven into the thoroughly modern design. “Hopefully, the project demonstrates how by integrating new technologies at the design stage, rather than adding them as an afterthought,” Tyrrestrup says, “we can produce a new architectural language with a deeper meaning.”

Unsurprisingly, considering the sponsor, glass features heavily in the house with the equivalent of 40 percent of the 2,045-square-foot floor area (roughly twice as much light coverage as in the average home) bathed in light coming through the triple-glazed, argon-filled windows and their super-insulated frames. In most European eco-homes, large windows on all but the southern facades are frowned upon due to the material’s poor insulating performance, but here every room receives light from at least two different directions.

According to Sophie this influx of natural light has proved the greatest change. “It’s really made a difference to our mood,” she says. “But it got very hot in the summer, and it was so bright you almost needed sunglasses in the kitchen.” Privacy also proved tricky. Sophie reports that “the perforated solar screens are virtually transparent, so passersby could see what we were having for dinner.”

Although fiddling with the house’s temperature and comfort settings and installing a few manually operated blinds set things right, the glazing is critical to the house’s green performance. In the winter, solar radiation provides half of the home’s heating, while in the summer, light and heat transmission is restricted by an automated system that controls interior and exterior sunscreens and opens windows to naturally ventilate the space.

To keep the house cozy during the dreary Danish winters, 72 square feet of solar collectors sit on the southern face of the roof. These provide the energy to heat 50 to 60 percent of the annual hot water and, combined with a solar heat pump, power the underfloor heating. Also jostling for space on the roof are 540 square feet of solar cells that generate electricity for the lighting, household appliances, and the home’s automated control system. They should produce nearly double the amount required by the family—so for eight months the excess energy is fed back to the grid. In the depths of winter, when they’ll need it most, that surplus is returned from the grid free of charge.

The family keeps tabs on how much energy and hot water the house is producing and consuming, and they independently operate the windows and blinds through a complicated-looking control panel in the hallway. Sophie compares it to the family’s desktop computer, noting that “you don’t have to understand all the technology behind it to be able to use it.” The biggest problem has been the user interface. “At the beginning it was frustrating because the blinds and windows are numbered haphazardly, so there was lots of trial and error,” adds Sverre.

The vexations of such a high-tech home have been tempered by moments of hilarity, especially with the lights, which automatically turn off when sensors detect no activity. “If you’ve been sitting on the toilet a little too long, you’re suddenly plunged into darkness,” admits Sverre. Long stints on the computer require rocking back and forth, and the couple has installed bedside lamps as the lights kept going off during the children’s bedtime stories.

Despite the occasional hiccup, the Simonsens are enjoying life in the Active House, especially the children, who after a recent school visit have become the envy of their classmates. “The amazing thing is that we don’t feel like we’re making any sacrifices,” says Sophie. The house has a pair of flat-screen TVs, a kitchen equipped with top-of-the-line Siemens and Gaggenau appliances, a double shower encased in recycled glass tiles, and a washing machine. “We use less energy without even thinking about it,” she says.

Although all of the products within the Active House are, or soon will be, on the market, the cost of creating a bespoke system that ties all of the technology together pushed the final bill to about $750,000. This, combined with the unknown cost of future technical hitches, means the Simonsens have ultimately decided not to buy when the house goes on the market.

Instead, they’re hatching plans to improve the green credentials of their old home by adding insulation and installing roof lights. Sophie has her eye on some nifty kitchen appliances: “This oven is so efficient I can prepare meals quicker, which is good for me and good for the environment,” she reports. And they haven’t ruled out renewables. “I definitely feel more confident in the technology now that we’ve lived with it,” says Sverre.

So will they be glad to return to a lower-tech brand of normality? “We won’t miss the constant visits from technicians and the people peering through our windows, but we’re proud to have taken part,” says Sverre. “The really interesting question is what impact the house will have made in 20 years’ time.” For now, we’ll just have to wait and see.

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