by :  Is it a home, or is it a power plant? Way back in 2009 when construction was completed on Villa Åkarp, the Swedish “energy positive” (a.k.a. grid positive) house, the folks behind the design claimed that the structure would produce more energy than it consumed, becoming the first house of its kind. Now, the chips are in, the data crunched, and the results announced: Villa Åkarp produces an excess of 600 kilowatt hours (kWh) of electricity on an annual basis —  enough to power another super energy-efficient home like itself for 2 months.

GE’s ecomagination blog reports that this lean, green energy profile was achieved by relying largely on Passive House building strategies. These include a virtually airtight building envelope stuffed with a whole lot of insulation – 15.7 inches of polystyrene insulating the foundation, and 21.3 inches of insulation on the walls and roof. A fascinating feature here is the use of drywall in one bedroom ceiling that contains a granulated wax product that responds to temperature changes. When the temperature exceeds 77 degrees Farenheit, the wax in the drywall begins to melt. This phase transition works to draw heat from the indoor air, in much the same way evaporation does. When the temperature falls, the wax becomes stiff and energy is released back into the room, which helps to stabilize the indoor air temperature.

Villa Akartp

Heat here is handled first by a thermal solar hot water system coupled with radiant, in-floor heat, then — when that system maxes out — by a pellet stove. The home also makes use of  both a traditional heat-recovery system (tied to the home’s high-performance ventilation — a key health feature in an airtight house) and an in-pipe heat-recovery system that captures heat normally lost from the home’s wastewater via a pipe-within-a-pipe. This in-pipe system uses the heat of hot shower water running down the drain, for example, to pre-warm incoming cold water, lessening the demand on the home’s more energy-intensive systems.

LED lighting and energy-efficient appliances also do their part to reduce the amount of energy required to power the household.

With energy demands kept to a minimum, the home’s solar panels have less work to do. This solar array, which covers the south-facing roof of the home at a 45-degree angle, produce more electricity than the home requires, and therefore sends energy back to the grid. The photovoltaic solar power system used by the home produces about 4,200 kWh of electricity per year, or a surplus of 600 kWh per year.

Based on the success of Villa Åkarp, three more grid positive houses have completed construction in Sweden since 2009, and Karin Adalberth, Ph.D., and Magnus Kamstedt of Rockwool International A/S — the company behind Villa Åkarp – say they expect to see more. Initial costs will remain a factor, as Villa Åkarp cost around $100,000 more to construct that a conventionally built home of similar size  back in 2009, and those numbers are still current today. But with residential solar costs falling significantly in Sweden, homes like Villa Åkarp are becoming increasingly within reach for both builders and buyers.