An Optimally Efficient Off-Grid Passive and Active Solar Home
This off-grid, owner-designed and built house is one of the most impressive passive and active solar combinations I’ve seen. Located at an elevation of 9,000 feet in the Colorado mountains, on a windy, snowy January day, after entering the home, I experienced a quiet confidence and a solid feel of warmth and comfort.
You may be thinking, “Right. Aren’t homes off-the-grid efficient?”
They should be, but actually aren’t because they are designed and built by different people. There are a few traits that work together to make a passive solar home efficient: windows (glazing) facing the south with some sort of control, thermal mass, insulation, along with the ability to harness naturally occurring convective air currents for cooling the house in the summer. Not only the passive aspects of this house are impressive, but its active systems are something that others interested in off-grid solar should take notes from.
Because this house is located off the electricity grid, this house creates all its electricity from the sun.
In order to have the modern convenience of continual electricity, off grid homes necessitate systems of backup. Yet this home is so efficient that the backup systems are not often utilized.
The home was built 10 years ago when a couple relocated to Gunnison, Colorado. The husband was a former manufacturing engineer and was attracted to the ideas of passive solar design and the freedoms of living “off the grid”.
At the time, not fully familiar with the design and engineering principals of passive and active solar building, he took a two week class with SEI in Carbondale, CO. He then combined his engineering background with the newly learned to design and build an extraordinary efficient and comfortable off-grid home.
This article will discuss the design and passive solar building elements of the home, then continue on to the solar thermal and electric systems.
Passive Solar Design Elements
It was in the middle of winter when I first visited the home. After entering through a double door entry, the open configuration of the home was welcoming, sunny and warm.
Upon entry, one sees a living/entertainment room, and beyond the stairs there is a greenhouse, seating area and kitchen.
Since the sun was out, the home absorbed the sun’s heat within the polished concrete floor (a material with thermal mass).
The floor throughout the lower level of the home is comprised of 8 inches of concrete with radiant in-floor heat tubes throughout it. Underneath the concrete, is a layer radiant heat film made out of double insulated aluminum.
Attached Greenhouse / Heat Sink
The greenhouse has an angled roof that allows it to efficiently capture solar heat.
The space can be used to draw heat into the house, while it can also easily be shut off from the rest of the house.
It is important to note that the greenhouse is not used to grow plants throughout the year. While it is not that much of an issue for them, using the greenhouse space throughout the year can cause issues with humidity. In her words, this is how she uses the space:
I do start seedlings, but I also grow tomatoes, green beans, peppers and herbs such as basil and marjoram during the summer as it is too cold outdoors for those crops. Also, although I could grow cold weather crops like lettuce, spinach, and mache, I do not grow in the winter because I want a break! Also, any other plants would require more heat at night for optimal growth than the passive heat would provide. Humidity is not an issue – I wish it were since it’s so darn dry here.
The greenhouse is also instrumental in adding solar heat to the house in the winter and is referred to as a ‘heat sink’.
How the heat sink works: Two feet of gravel (a dense material that has thermal mass that absorbs heat) make up the floor of the greenhouse.
As the greenhouse absorbs solar heat throughout the day and become warmer, the hot air rises. Because the roof rises in an angle, hot air will collect near the top of the angle.
Two fans located on each side of the door on the top of the turquoise colored tubes pull hot air from the top of the greenhouse to a manifold located 2 feet under the rock.
As the hot air flows from the top of the hot greenhouse to the very bottom of the two feet of rock, it warms the rock from the bottom so that the mass later radiates the absorbed heat for a longer amount of time. (Notice that there’s a lot of thermal mass throughout this home. This is part of what helps this house to be very efficient.)
The fans are controlled by a temperature gauge that is set to automatically turn on when the room reaches a certain temperature.
Buckets have been painted black, filled with water and stacked near the building. This also increases the building’s thermal mass. Water is one of the best mediums that can absorb and radiate solar heat. It also stabilizes the building’s temperature swings by absorbing solar heat, then slowly releases it through the cooler evening.
After returning to the home from the greenhouse, one can’t help but notice a beautiful masonry heater. Notice the size and mass of this stove.
This was the first time I’ve seen a masonry heater inside a home, but after learning about more about how it works, I realize that it makes brilliant sense!
It is a wood fired stove that has a flume that snakes through the masonry to add more heat to its thermal mass allowing for greater potential to capture and radiate heat before it escapes out the top of the home.
Most wood fired stove-heaters have a fire contained within the stove, while the stove radiates the heat while a massive amount of the heat is quickly lost through the chimney.
The masonry heater is UL- listed and made by the Canadian company Tempcast.
I visited the home at a time close to the winter solstice. Notice how deep the sun’s rays reach within the home. Also notice the the amount of thermal mass throughout the home that is absorbing the heat.
In contrast, during the summer when the sun travels a higher path in the sky, the sun does not enter the home and the thermal mass (because it helps to stabilize temperatures and absorbs heat) helps to keep the house cool.
Kitchen, Master Bedroom, the Walls and Roof
Located behind the masonry stove, the kitchen is located in the northern-most section of the home.
Because the home is situated on an east-west axis, (the long direction of the home) it allows the sun to warm the northern sections of the home.
The open configuration on the southern side of the home allows sunlight from the 2nd story windows to warm the northern sections. Once again, we see passive solar winter heating at work in the kitchen. The tile in the kitchen absorbs the solar heat and slowly radiates it through the cooler evening and night.
The house is so well built that while the home has hot water radiant heat tubes installed throughout the floor, because the house is built efficiently, the radiant floor tubes that run throughout the home are not used. (I realize that I just repeated myself, but the utilization of thermal mass throughout a passive solar building can not be understated.)
The master bedroom and bathroom are located above the kitchen area.
The home has a combination of fixed and operable windows. All the windows in the house are 1 inch, double pane, low-e windows. The operable windows are triple gusseted to help create a better seal when they are closed.
Regardless of how efficient windows are, windows tend to be an area within homes where a lot of heat is lost. Glass is a material that is simply not a good insulator.
The owners said they they had created the valance above the curtains with plans to create insulated window coverings, but because the house is so well insulated, they are still using the flimsy small bamboo reed privacy shades. They simply haven’t felt the need to make insulated window shades. This is a testament to how well the house has been built.
Around the perimeter of the home, they utilized double studded walls to cut down on thermal bridging. Within a double studded wall, the studs are offset, thus creating a thicker or wider wall allowing the insulation to be continuous and woven between the alternating studs on each side of the wall. Wood is another material that is not a great insulator and allows some thermal bridging as cold or heat travels through the wood.
From the inside to the outside, the perimeter walls consist of: drywall, followed by a foil barrier, metal studs, blue board insulation, a vapor barrier, fiberglass insulation, plywood, Tyvek and cement, followed by siding.
In looking at the photo, some builders may be scoffing at the tongue-and-groove vaulted ceiling – as these types of vaulted ceilings are notorious for leaking heat, but don’t let that fool you. This roof has also been well-insulated.
This roof is referred to as an ‘Idaho’ double roof. It is a roof with two separate roof systems comprising 20 inches.
Beyond the tongue-and-groove wood paneling is a high density fiberglass insulation, followed by a layer of aluminum foil. Beyond that is a 4 inch urethane SIP (structured insulated panel) that is then covered by a metallic roof.
The roof also has solar light tubes that increases the amount of solar light in the house. This eliminates the need to turn on the light during the day and thus saves electricity.
The house is also bermed into the hillside and built on ICFs (insulated concrete forms). ICFs are essentially Styrofoam blocks made out of extruded polystyrene and filled with cement.
While the home is warm in the winter, it is also cool in the summer because they harness natural convective air currents throughout the home. By simultaneously opening windows on the ground and second floor, warm air is pulled out the top of the house while cool air enters on the ground floor.
Yes, the penchant for efficient building continues with the electrical systems.
An article about the electrical systems along with a peak inside the utility rooms will be posted next.
Passive / Active Solar Photo Gallery
Square footage: 2700 square feet
Time to build: One long summer
Owner/builders: Kris and Ken Kilz