In the early 2000s, I was deep into my computer programming career, traveling across the country. It was during that time that I decided to build my dream house—a log home in the mountains of North Carolina. But after poor luck with contractors and design problems, I knew there had to be a more sustainable way to build a home.
My wife Diana and I were both in the technology field, so we had the option of working remotely from anywhere. We chose Colorado and launched our new dream.
Over the next two years, I spent as much time as available researching, planning, and designing our new home. We came up with a set of lofty design goals (see sidebar), and I knew I needed to be the general contractor to ensure the necessary attention to detail by all construction contractors working on the project.
Planning & Design
There was still a lot I didn’t know, so I enlisted Jim Riggins, whose superinsulated solar home, Heliospiti, was featured in Home Power (see Web Extras). We discussed house shape; orientation; foundation/wall/attic design; insulation profiles; air sealing; advanced framing; green building materials; efficient windows and doors; and efficient water and electrical fixtures. We went over his experiences and what to expect, which led to more research and even more questions.
Once I felt I had a solid understanding, I designed the general house layout and started incorporating the various systems. Including some of the systems was straightforward, but others were more experimental—like the water cisterns also used for passive heating and cooling, an internal greenhouse, and a “cool pantry” room that would use only fans and a differential thermostat.
Once in a final version I modeled the house in 3-D using Home Designer Pro software, laying it out in complete detail down to the locations of every electrical outlet and light switch. I also created several supplemental construction documents for various trades to remove any interpretation or ambiguity. This included the wall design, ERV ducting layouts, PV module locations, subslab plumbing and electrical, and the rain harvesting system.
We broke ground in early April 2014, which in Colorado is like playing Russian roulette with the weather. I anticipated we would need about eight months to complete the project before the winter cold and storms would prevent contractors from getting to the site. Favorable weather prevailed, and we were able to meet that timeline.
The house is single-level, with two bedrooms and two baths, and a rectangular shape for optimal passive solar gain. A floating slab foundation was selected (with a thermal break from the stem wall). The wall design differed from the main living area and the garage (a choice I would regret later). The main living area is a double wall—an advanced framed 2-by-6 exterior wall separated by a 3-inch space from a 2-by-4 interior wall—while the garage walls are an advanced framed 2-by-6 wall with 1.5 inches of EPS foam on the exterior.
Meticulous attention was given to air-sealing and minimizing penetrations through the building envelope. Knowing that proper sealing is as important as good insulation, I spent many hours in the evenings foaming and caulking gaps. I performed two blower door tests—one before the drywall was installed to identify missed penetrations and the second when the house was complete. There were a few electrical penetrations I had missed, but the majority was improper sealing of the plastic vapor barrier in the main ceiling area by my framers. The final blower door test resulted in a reading of 161 CFM at 50 pascals, much better than the related Passivhaus standard of 280 CFM.
The slab is insulated underneath to R-20 (5 inches of high-compression-strength EPS) and the external side of the stem walls were covered with 1.5-inch EPS board for another thermal break. The main living area walls have a weighted value of R-45.4 provided by 3 inches of closed-cell spray foam and 9 inches of dense-pack cellulose fill. The garage area walls are R-25.3 with 5.5 inches of dense-pack cellulose and 1.5-inch EPS foam board on the external face. The attic is R-60 from 18 inches of blown-in cellulose.
We built the walls 24 inches above the ceiling level, which allows full ceiling insulation depth all the way to the edges of the walls. I also incorporated dropped ceilings in northern sections of the house which simplified wiring, HVAC, and plumbing runs while minimizing penetrations. We choose fiber cement board on the exterior for wildfire protection and ability to install it ourselves to cut costs.
All the doors are insulated, even the interior ones, for thermal and acoustic separation. For optimal temperature management, the two exterior doors both open to a buffer room that is separate from the main living area. Triple-locking exterior doors increase air sealing and security.
All windows are fiberglass-clad, triple-pane units, but air-filled, instead of gas-filled (given the elevation gain, gas fill would have been lost in transit). There are few windows on the east, west, and north sides; and most windows are non-operable (“fixed”) for the best insulation. The majority of operable windows are awning type, which offer the best air-sealing. Casements were used only where required for emergency egress.
Rolling exterior insulated metal shutters by Rollac were installed for all windows. The economic justification was their additional insulation of about R-1.2 to R-2 to the thermally weakest spaces in the walls, providing security when we are away, and providing wildfire protection. To further decrease the risk of fire damage, all external surfaces are non-flammable.
The standing-seam metal roof met our requirements for ease of maintenance, wildfire protection, and effective rain harvesting. Most roof pitches in Colorado are 6:12 or greater to shed snow, but that was too steep to maximize rainwater harvesting. A 3:12 pitch was selected and snow guards added to “hold” snowfall until it melted and could be captured. Continuous soffit and ridge ventilation help prevent ice damming. A 3-foot overhang provides seasonal shading for the windows.
During the winter months, the average night temperature inside the main living area is in the high 60s to low 70s. Over an average winter night (anywhere from the teens to single digits), that space will lose 3°F to 4°F by morning. In general, with the house temperature “charged” with a couple of days’ worth of winter sun, it can sustain three days of cold temperatures in the low teens and no sunshine before dropping into the very low 60s.
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.
The cool pantry has maintained an average temperature of 55°F—even during the summer months—using just the external fan and differential thermostat.
Greenhouse growing. The greenhouse area in the garage space was one of the project’s biggest design gambles. The main goal was to grow food year-round without having to actively heat and cool that space. The cisterns, with their large concrete and water thermal mass, were employed, with the sun providing the heat source. Both raised beds and an aquaponics system were built into this space to see which would work the best, both efficiently and productively, year-round.
In an attempt to decrease the garage-building expenses, I chose 2-by-6 wall construction with a layer of external EPS as a thermal break. However, the labor to install the EPS, add angle iron to support the heavy fiber cement board cladding, and the use of extended-length fasteners negated my planned savings. Besides less insulation in the walls, it also introduced a moisture issue given the exposed and thinner part of the stem wall (versus the double wall used in the remainder of the house). This lower-temperature stem-wall section exposed in a warmer, higher-humidity space causes water condensation, introducing mold concerns if left unchecked.
The greenhouse area has performed very well (validating the passive heating and cooling approach with the cisterns), but has also introduced some other issues. The aquaponics system showed a growth rate of about 30% greater than the raised beds at growing the same vegetables, and I was extremely excited about expanding that system. However, when the temperature dropped into the high 50s and low 60s in the greenhouse, the Rocky Mountain white tilapia went dormant or died off, which then also slowed the symbiotic growth of the plants. This can be remedied by using slower-growing, colder-temperature fish varieties like blue gill, catfish, and carp, but tilapia have easier maintenance characteristics that make the other species less desirable.
The aquaponics system has also considerably increased the humidity levels in the garage. In the summer, the humidity is exhausted outside through the automatic vents. However, in the winter when the windows need to stay closed to preserve heat, the humidity levels rose from 30% to roughly 60%, causing some mold and moisture issues across that entire garage space. I installed an ERV there to conserve energy and heat while exchanging the internal moisture-laden air with less-humid outside air.
The winter temperature in the garage-greenhouse dipped to 58°F at its very lowest; it was usually in the low 60s. This is an almost 2,000-square-foot space that is not actively heated, but uses passive solar gain and the thermal mass in the concrete slab, along with the cisterns and the water within them. I expect that performance to increase next year, since the cistern water is estimated to gain 7°F to 10°F over the summer, becoming a significant thermal battery for the greenhouse to draw from in the winter.
I must admit, we were holding our collective breath the first year as we tracked how all the systems operated. When the initial results started coming back, I became cautiously optimistic. As the year progressed, those results continued to be very positive and some of the systems demonstrated performances above their predicted capabilities. By the end of the year, we knew we had made good choices and now we can’t imagine living any other way. There are some things I would have done differently in hindsight, and some that I didn’t predict well, but overall, we are thrilled with the results.
Our friends and neighbors walk through our house and are amazed at how it operates so well with so few energy inputs. Many of them expect to find us huddled, wearing blankets for warmth, and trying to figure out which few appliances we can run from the PV modules. Instead, what they see is a “normal” house—but one that operates with few external inputs besides the sun and rain.