There are LEED-certified buildings and there are zero-energy buildings. And then there’s Villa Åkarp outside of Malmo, Sweden—an energy-plus residential home. In other words, a home that produces more energy than it uses.
Since construction was completed on the three-bedroom, two-bathroom house in 2009, it has raised the bar for what is possible in home construction. Villa Åkarp takes its design cues from the passive house, or Passivhaus, movement, a design program for creating super-insulated houses that rely on window orientation, thermal recovery and other elements for heating and cooling.
But Villa Åkarp ups the ante.
Besides the elements taken from the passive house, “the plus energy house should also have low consumption of hot water and usage of electricity for appliances,” write Karin Adalberth, Ph.D., and Magnus Kamstedt of Rockwool International A/S, the company behind Villa Åkarp’s insulation. “Finally, a plus energy house also produces its own electricity and heat…by means of solar collectors and PVs [photovoltaic panels].”
Insulation is the first step. The energy-plus house is virtually airtight, with 15.7 inches of polystyrene insulating the foundation, 21.3 inches of insulation on the walls, and similarly thick insulation on the roof. Windows are triple-pane and only three face south, which prevents sunlight from warming the building. The windows also feature krypton gas filling, which reduces heat loss and a solar-protective layer for those facing south and west.
With this level of airtight construction, ventilation is key. Villa Åkarp features a heat exchanger that keeps air moving inside the home and provides a steady stream of fresh air, with two highly efficient fans and “several acoustic traps in order to secure that no noise will reach the indoor environment.” The drywall in one bedroom ceiling contains wax granulate that responds to temperature changes. When the temperature rises above 77 degrees Farenheit, the wax in the drywall begins to melt, drawing heat from the indoor air. When the temperature falls, “the wax becomes stiff and energy is released back into the room,” Adalberth and Kanstedt write, helping to stabilize the indoor air temperature.
Because the house is in Sweden, a key design concern was preventing heat loss and minimizing energy used to produce heat. Heat is captured from the home’s wastewater via a pipe-within-a-pipe, pre-warming incoming cold water. Home heat also comes from a pellet stove, which heats the home’s water tank, particularly during the winter or when the home’s solar thermal collectors can’t meet demand.
The solar panels cover the south-facing roof at a 45-degree angle and produce more electricity than the home, which is outfitted with energy-efficient appliances and LED lights, needs. Specifically, the PVs produce about 4,200 kWh of electricity per year, or a surplus of 600 kWh per year.
All the energy the home produces and uses is tracked closely, says Adalberth, the owner and technical buildings specialist of Rockwool. “The energy is measured in several points,” she says, “heat for hot water, radiation heat, pellet stove heat, ventilation heat, sewage water exchanged heat, solar heat, electricity for household purposes, electricity for pumps and fans, electricity from PV, etc.”
She says that three more energy-plus houses have completed construction in Sweden since Villa Åkarp and she expects to see more. Up-front cost will be a factor—Villa Åkarp cost roughly $100,000 more than a similarly sized traditional home when it was built in 2009—but with residential solar costs falling significantly these homes are becoming increasingly within reach.
Top image: A rendering of the west façade of the plus energy house. Courtesy