Net-Zero Performance

In the February 2011 article “Heading for Zero—Smart Strategies for Home Design” (HP141), I described the design and computer-modeling processes for our passive solar, net-zero energy home. The design followed the German Passivhaus philosophy. Now, after a year of performance data logging, we can see how close the house came to our design goals and modeling.

We have two adults and two children in the household. Our home, called Heliospiti (Greek for “sun house”), is an all-electric house of 3,180 square feet, with a slab-on-grade foundation, and is located at an elevation of 7,000 in Monument, Colorado. According to the Western Regional Climate Center (wrcc.dri.edu), this area averages 6,324 heating degree-days and 149 cooling degree-days per year—a heating-dominated climate. The shell consists of R‑49 double-stud walls, an R-67 roof, and an R-21 insulated concrete slab main floor for thermal mass.

The Accurate Dorwin windows are triple-pane, argon-filled. On the south side, we specified windows with a high solar heat gain and low U-factor; north windows have a very low U-factor; and there are no windows on the east or west sides. The house is oriented with its long axis east–west to maximize solar gain on the south face. To take the house from simply being a high-efficiency passive home to a net-zero energy home, we installed a 4.5-kilowatt grid-tied photovoltaic (PV) system and a solar hot water (SHW) system with three 40-square-foot collectors.

The passive solar design meets all of our space-heating needs, using a 4-inch-thick polished concrete floor and 11/4-inch-thick gypsum walls as the primary thermal mass. A passive solar wall based on “Build a Solar Heater…for $350” (HP109) heats the thermally isolated wood shop and garage. A single Mitsubishi Mr. Slim variable-compression minisplit air-source heat pump provides backup space heating. An UltimateAir RecoupAerator energy recovery ventilator (ERV) provides balanced, efficient ventilation. The incoming air for the ERV is passively preheated by a 100-foot-long, 10-foot-deep Rehau earth tube.

Overall Performance

Our home’s energy performance is monitored through a variety of devices. Mountain View Electric Association, our electric utility, provides a net meter that displays the home’s net energy consumption (or production). Internet-based software from Enphase Energy provides detailed production data for individual PV modules. I installed a four-channel Onset Hobo data logger to track the temperatures of outdoor and indoor air, the concrete slab, and the earth tube air as it enters the house. Internet-based software from Nissan tracks the daily and total recharging energy required for our Leaf electric vehicle (EV).

So how did the house perform overall? For the 314 days before the EV’s first recharge, the house produced an excess of 2,981 kilowatt-hours (kWh), averaging 9.5 kWh excess per day. The large surplus was intentional—our long-range goal was to produce enough additional electricity to charge an EV and still remain net-zero. While it is premature to say if we met that goal, the initial numbers look promising. During its first 61 days, the Leaf consumed an average of 6 kWh per day. When subtracted from our average excess production, this still resulted in a surplus of 3.5 kWh per day. So far, our goal of net-zero was exceeded, even including EV charging.

Air Tightness

The physics behind modern building science clearly shows the large impact of building tightness on energy efficiency. This has led to the extremely low air leakage allowance by the Passive House Institute. Through meticulous attention to sealing during construction, our house tested at 0.40 air changes per hour at 50 Pascals pressure difference between the inside and outside (ACH50)—33% tighter than the 0.60 ACH50 Passive House limit. And with an effective leakage area (ELA) of 15.4 square inches, it was 20% better than the ELA design goal of 19.3 square inches that we had initially set for construction.