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July 7, 2011
A homebuilder in the New England area has been building net zero energy single family homes since 2008 and is continuing with multiple small-scale subdivisions of 20 or more homes. This builder specializes in net zero affordable homes and sustainable net zero communities, while retaining houses with a familiar local vernacular appearance.
Some of the key features of these houses include solar orientation, superinsulated double-stud above grade walls (R-45+ nominal), triple glazed low emissivity krypton-filled windows, and exceptional airtightness.
The mechanical design takes advantage of the reduction in enclosure-based heating loads by using single-point (or one point per floor) heating, in the form of mini-split or ductless split air source heat pumps. This modification substantially reduces installed HVAC system costs relative to a conventional ducted system. The effects of this single-point distribution in superinsulated housing are a matter of further measurement in this study. In addition, the ability of an air source heat pump to meet heating loads in a cold climate is a matter of concern; the ability to maintain setpoint (and dependence on backup heat) is another research topic. The renewable energy component is a roof-mounted photovoltaic system, sized to meet the modeled loads; a PVT system (combined photovoltaic-solar thermal) was also used in some cases.
Controlled mechanical ventilation options have included heat recovery ventilators and simplified supply-only systems. When combining the factors of the installed cost of HRVs, the ventilation rates used, and the increased electrical (fan) energy of HRVs, the cost-benefit relationship of heat recovery appear to be not as advantageous as originally thought.
The builder also has been concentrating on cost control for these houses, given the shift into full-scale production, and has been actively involved in monitoring of the effectiveness of various measures. Energy modeling and analysis have also been used to examine the cost-effectiveness of various options. Avenues examined have included various solar domestic hot water systems, building-integrated photovoltaics, modifications to the wall construction, and modified foundations.
BSI-025: The Passive House (Passivhaus) Standard—A comparison to other cold climate low-energy houses