Ever thought of building your own energy efficient cabin ... for $11,000? Prior to moving to Colorado this past summer to work at RMI, one staffer did just that and shares his thoughts on passive solar design, thermally massive materials, solar panels and wood cook stoves.
One RMI staffer builds a solar-powered EarthCabin in the British Columbia interior
Prior to moving to Colorado this past summer to work at RMI, I lived in British Columbia, Canada, in a cabin my girlfriend and I built. We started building it three years ago, and did a lot of the work while completing our graduate degrees.
After our undergraduate studies we were eager to get our hands dirty on a project that tackled some of the environmental issues we had been learning about. I was excited to apply the green building design principles I had studied, especially passive solar, and we had both been following the alternative and green building movements with much excitement.
We also wanted our own place to live. Building a cabin on my family’s farm property was a natural solution. The primary objective was to test and demonstrate alternative green building techniques. We wanted it to be able to operate without using fossil fuels and utilize materials with low environmental impact. It had to be very low cost as we didn’t really have any extra money to pay for supplies.
The design took inspiration from Michael Reynold’s Earthships and the Lovins GreenHome. Most of the exterior wall area is earth-sheltered, reducing the temperature differential across the wall. The air temperature can get as cold as minus 40 degrees Fahrenheit during the winter, but a few feet below the surface the ground remains several degrees above freezing. This is equivalent to having most of our wall in a temperate coastal climate. The upper part of the earth-sheltered wall is insulated with re-used foam insulation.
It’s a passive solar design. Nearly all of the windows face south to let the low-angle winter sun reach deep into the building. Thermally massive materials are used to store this heat and minimize temperature fluctuations. The roof angle is 14 degrees, equivalent to noon winter solstice sun angle, so the entire back wall receives full sunlight on the shortest day of the year. During the summer an overhang shades most of the south window area reducing overheating.
Probably the most important aspect of the design that makes it affordable and efficient is its size—a tiny but practical and functional 230 square feet.
Using concrete works well for below-grade construction, but we ruled it out because of its high embodied carbon emissions. A greener and lower-cost option was to use earthen construction materials. The structural below-grade wall is made out of rammed earth tires, as used in Earthships. The tires are laid out like bricks and rammed full of dirt from the building site using a sledge hammer. Once fully compacted, the tires weigh around 200 pounds each and form a wall so solid backfilling can be done as the wall is constructed, rather than waiting until the remaining structure is up. The tire ramming is pretty labor intensive, so it’s best to try to rope friends into helping if you’re doing the work yourself, as we did.
The interior finishes utilize earth building techniques that go back several thousand years and are receiving renewed interested due to their green attributes, aesthetics, and lack of toxicity.
The tire wall is finished with a natural clay plaster, created from a mix of sand and clay sourced near the building site. The floor is poured adobe, made from a similar earth mix, but with cow manure added to increase strength (and no, it doesn’t smell!). A clay paint, a mix of coffee and clay, was added to the floor to achieve a rich dark brown finish, providing contrast against the light clay walls. The floor was then sealed with raw linseed oil, providing a mop-able water-resistant finish.
The remaining structure and finishes of the building are more in line with conventional construction, but with a focus on re-used and local materials. Two large wind-fallen logs were used for the main beams to support the rafters. All of the purchased lumber was from a nearby rural sawmill, rough cut. We created a dropped ceiling using plywood painted with white paint to enhance daylight. The dropped ceiling also allows for a continuous insulation layer below the rafters, minimizing the thermal bridging caused by wood framing penetrating through the insulation layer. The ceiling was insulated to R 56 using fiberglass batt insulation.
Most of the energy requirements are met by a wood cook stove. This stove has an oven which gets hot enough to make great Yorkshire puddings, and has a hot water coil which can be plumbed into a hot water tank to passively heat enough hot water for dishes and bathing. The cook stove is used for space heating on the cloudy winter days and cold winter nights.
Space cooling is easily avoided in this northern climate; few houses require air conditioning. Exterior blinds can be used to minimize overheating, which can be an issue for a couple weeks in August. The high thermal mass in the building helps keep it cool—windows are open at night to cool mass, enabling it to absorb heat from the space all day. The remaining energy requirement for this dwelling is only electricity for plug loads. Because the grid in BC is 90 percent hydro powered, the carbon intensity of electricity there is quite low. In fact, it’s about the same per kWh as the life-cycle emissions from solar PV assuming a 25-year operation period at a high-latitude location. So economics easily became the main factor in this decision.
Although we were only about 350 feet from the power lines, connecting to the grid would have cost at least $8,000 to $10,000. I calculated our winter power requirements, based on metering devices we commonly used and estimating their hours of operation. We only needed 1 kWh/day to run two laptops, satellite internet for about 8 hours/day, superefficient direct current (DC) LED lighting (only required when totally dark), and our DC water pump. After further calculations, we designed a suitable solar system: 675 W of PV, 10 kWh of battery storage, a used 2 kW inverter, etc. The total cost of this system was $6,000, saving us at least $2,000 up front and eliminating electricity bills for the next 25 years. In addition, we feel good about putting that money into supporting an upcoming industry that is reducing carbon emissions in other locations.
The passive solar design is excellent in the shoulder seasons and stretching into early winter, when solar radiation is still fairly abundant. We’ve found that we are only running the fire long enough to cook while our neighbors are heating their homes. Even on the shortest days, when it’s sunny the cabin heats up quickly. Cloudy days and cold nights require a pretty good fire, mainly to mitigate the high heat loss from our older double-pane windows. Had we not salvaged windows it would have made a lot of sense to buy modern low-e argon-filled double-pane windows, or better. We plan to make an insulated exterior curtain to reduce the night heat loss from the windows. When the building has been unoccupied for several days the temperature so far does not go below 42 degrees, even when it’s been minus 13 overnight. However, it has not yet been left unoccupied for more than about one week during the winter.
The large south-facing window area and light interior finishes create such a bright and pleasant interior day-lit space that we have been surprised how little we need to turn on lights (only when it is almost completely dark outside). In addition, we’ve been surprised just how enjoyable a well day-lit space feels.
The solar power system works really well in the summer, providing enough power to run an electric kettle, avoiding the need for fires to make coffee, and enough power to run power tools for short periods. This is a capability we were unsure of given the small size of the system we purchased. During prolonged periods of cloudy weather in December the occasional boost from a generator would be required if the system is in full-time use as an office and dwelling.
It was a lot of work, but enjoyable and a great learning experience. We spent about $3,500 on the building, including about $1,000 of insulation for the ceiling and small wood-frame walls. We spent another $1,500 on a well and water line. The total cost including the solar system was only $11,000—comparable to a year of rent!