A single perforated tube was buried under the entire length of the slab and stubbed into the garage space for any future radon mitigation needs. The building design also follows many of the FEMA construction recommendations for earthquake and wind loads.
Ventilation. Operable, adequately sized windows help passive chimney-effect airflow, but for this tight house, mechanical ventilation is necessary. For the main house, an energy recovery ventilator (ERV) with a 140-foot-long “earth tube,” buried at a 10-foot depth for pre-warming, provides fresh air to the home. For the needed air changes, it runs 15 minutes out of every hour. A separate “spot” ERV was installed in the garage ceiling for ventilation and moisture reduction. The incoming air for this ERV gains some passive pre-heating from the garage below via a rigid-insulation thermal tunnel that encapsulates the ductwork above the ceiling.
In the greenhouse area of the garage, auto vents are opened with beeswax-filled pistons that are triggered by temperature increases to boost summertime ventilation.
Heating. At this elevation, no cooling is needed other than simply opening a few windows. Heating is the priority, with 6,964 average heating degree days and an average temperature of about 28°F in the coldest winter months. Lots of thermal mass—concrete floors, cisterns, dual layers of south-facing drywall, the kitchen countertop materials, and even the iron weights in the weight room—absorb solar gain during the day and then release that stored energy when temperatures drop. The county building codes required a backup heating system, which we met with a 2,500 W electric baseboard unit.
Renewable electricity. The site had access to utility power so it made economic sense to install a grid-tied system—but installed with an eye toward disconnecting from the grid in the future. The PV system consists of twenty-eight 260 W modules in two strings mounted in two rows on the roof. While pole-mounts for PV arrays are common in high-elevation snowy climates—they can be set high and seasonally tilted to quickly shed snow—installing one in our county required an extra permit. Coupled with additional construction costs and the fact that the only pole-mount locations would have negatively impacted the views of the surrounding areas, we opted for a roof-mounted array. So far, we have found that the strong Colorado sun, even in the winter, coupled with the standoff gap under the modules, cleans both levels of arrays fairly quickly, without any intervention.
While a single inverter could have covered both strings, we selected two for redundancy. Each inverter has a built-in AC electrical outlet that can be used when the grid is down during sunny weather. The system was sized to meet our daily usage (after a rigorous review of our monthly anticipated loads), a future electric car, and the monthly cost to be connected to the grid. A 13 kW propane generator provides electricity backup for the entire house.
We originally estimated a yearly consumption of 10,056 kWh (house only) or a 12,182 kWh (the house with a future electric car). For 2015, the PV system produced 12,786 kWh, with 6,202 kWh actually being consumed. That means it produced roughly 5% more than the expected amount, and we are using 38% less than planned—enough to readily accommodate an electric car. This overproduction also allows us to more quickly recoup our monthly interconnection fee ($28), as well as the one-time $945 net-metering connection fee. The utiilty co-op is a bit convoluted in terms of compensating us for any surplus energy production, paying us at their “avoided cost”—the most recent average wholesale cost for energy.
Water harvesting. Even though rain harvesting was legalized in Colorado in 2009, few local plumbers were comfortable with implementing it. I even had to educate the state plumbing inspector on the topic at times, and this was five years after the legalization. At one point, the entire rainwater harvesting system was in jeopardy because of those misunderstandings—the only saving grace was the head supervisor who I ended up working with directly on the design to get it approved.
The result is that captured rainwater is stored in three 4,000-gallon concrete cisterns. This provides the water for all house needs and the fire sprinkler system, and is large enough to cover significant drought conditions. High-efficiency water fixtures and low-water-use appliances, such as 0.5 gallons-per-flush toilets, decrease water use.
The rainwater harvesting system was filled to capacity for the first time in March 2015, about four months after its commission. Since that time, it has (at the most) been drawn down by one-eighth of its capacity, and that was after a couple of months of very low rainfall. Given the size of the catchment surface, just one-tenth of an inch of captured rain equates to a 1 inch increase in water level across all three cisterns, or roughly a 130-gallon gain. In the spring, the cisterns were at full capacity after the melt of a 12-inch snowfall.
Sustainable food storage. Cool rooms were included in the home’s design for food storage and a more comfortable sleeping environment. These rooms are air-sealed and have insulated walls—even inside the house perimeter—to thermally isolate them from adjoining rooms. In both the “cool pantry” and master bedroom a Kera Technologies DSD-2RT differential thermostat tracks the external temperature versus the internal temperature, comparing both against the desired temperature. When the external temperature drops below the internal temperature setpoint, a Fantech FR100 fan turns on, bringing in the colder external air to decrease the internal room temperature.