HERS Rating a Passive House
A NEW BUILDING IS TAKING shape at Full Revolution Farm, a “micro eco-farm” located just outside of Des Moines, Iowa. This project, which will include bed-and-breakfast accommodations, is being built to the standards of Passive House International, ENERGY STAR 3.1 and DOE Zero Energy Ready Home (ZERH), which includes the standards of the EPA’s WaterSense and Indoor AirPLUS programs. This ambitious 9,527-square-foot project represents a collaboration between motivated homeowner Celeste Yeager Karan, Bob Chomko, RESNET QAD for the Building Science Institute, and George Sullivan, principal at Eco Smart Building. Since the project broke ground, Sullivan’s company has since been reorganized into Net Zero Analysis & Design Corp. to better reflect the company’s revised focus and capabilities.
A Net-Positive Solar Home
The project at Full Revolution Farm is designed as a passive solar home, but because of its mechanical system, in actuality the entire structure operates as an active solar thermal collector.
The home has no annual heat load, but rather a continuous cooling load throughout the year. This was achieved through an envelope that has no thermal bridging, thorough air-sealing, a significant amount of insulation, strategically placed windows and an advanced mechanical system.
Properly sized and shaded south-facing windows allow for a large amount of solar radiation into the home. Heat is also captured from normal human activity, mechanical equipment and appliance operation. This energy is collected via the multi-zone Mitsubishi CITY MULTI Variable Refrigerant Flow (VRF) geothermal system, which uses two 125-gallon solar thermal domestic hot water tanks as primary heat storage; the mass of the lower-level slab floor functions as secondary heat storage. After both primary and secondary heat storage is met, the CITY MULTI system will use the exterior glycol-filled geothermal field as a heat sink. An earth tube system (open-loop ground-to-air geothermal system) coupled with an energy recovery ventilator (ERV) provides ventilation and primary cooling.
We installed a 12-inch layer of high-density insulation under the basement slab and between the inner and outer walls of the double foundation. Twelve-inch-thick custom designed and engineered panels (CDEPS), manufactured without internal framing members, were applied as a curtain wall to the exterior of the home’s wood frame. Spray foam was used for air-sealing after the CDEPS were installed.
This design allows for net-positive energy production and therefore, a negative HERS rating.