The students used a prototype sliding mechanical window system to transform the porch into a sun space in the winter months.

The students used a prototype sliding mechanical window system to transform the porch into a sun space in the winter months.

Credit: Ian Allen

Some factors that make Virginia’s Unit 6 Unplugged affordable can’t be realized in isolation. Students were originally designing the house to be part of a six-unit multifamily dwelling, meaning that certain costs would have been shared between the units.

“Our overall estimated construction cost was somewhat higher than our target,” says John Whitelaw, the team’s project manager. “That’s primarily because we used a prototype sliding mechanical window system to transform our porch into a sun space in the winter months. It’s a cool feature that added a lot to the house, but it could have been done less elegantly with much-lower-cost systems.”

Whitelaw says the two motorized windows that the team employed cost about $40,000. A BioPCM phase-change material in the sun space’s floor calibrates the release of heat from the thermal mass and helps keep the space at room temperature—an innovative use of an existing product to improve the performance of a passive system. And rather than introduce the cost and potential complexity of a major home-control system, engineers fashioned a simpler system that uses information from the security system’s monitoring sensors to turn off the HVAC when the building envelope is open for an extended period. “This has the potential to save significant waste energy over the life of the building,” Whitelaw says. “And it’s a simple hard-wired system not easily prone to failure.”

Whitelaw says that because Unit 6 is intended for Virginia’s more temperate climate, only a small 2.6-kilowatt solar array was needed to achieve net-zero energy consumption, making the model house relatively cost-efficient and boosting its marketability. The team will continue to develop the design of the six-unit project that inspired the decathlon entry. “It’s got developer interest, as well as support from the local housing authority,” Whitelaw says. “It is very likely that two or three of the net-zero, affordable six-unit buildings will be constructed in Norfolk in the next couple of years.”

Estimated cost: $325,613.44