What is passive solar energy, and how can passive solar be incorporated into energy efficient home designs? A natural builder shares tips on building an energy-efficient home.
To me, the art of permaculture is putting resources where they can do the most good with the least amount of effort. By building with conscious design principles, we can create homes that are aligned with natural forces and that will keep their inhabitants cozy year-round with very little energy. One of these ways is through harnessing passive solar energy — heat from the sun, which is stored in thermal mass (heat-absorbing building materials such as tile, brick, or concrete) and then used to passively warm or cool the building.
My wife, Mary Jane, and I built an Earthship-inspired earthbag house beginning in 2016. It gives us free heat on sunny winter days and keeps us relatively cool and dry in summertime, all without grid power and central air or heat. This summer, we installed a window AC unit, and we run it on drying mode to keep our humidity low and the mold out. What we’ve found is that, because of all the thermal mass in our home, it needs much less power to stay cool. The window unit, which was made for an area of 450 square feet or less, keeps our 1,000-square-foot house as cool as a cucumber, even on the hottest days.
That’s what prompts me to share these conscious design principles and their benefits with others. It’s unlikely the solution to humanity’s problems will come from “out there”; instead, it’s up to us to make the needed changes. The ball is in the court of those of us who value nature and a livable planet. That’s what drove me to start radically changing my life in 2007, and it’s what motivates me to this day. Humanity is capable of great intelligence and change; we just have to become aware of the necessity of eco-friendly design. If architects can incorporate these ideas into their buildings, homeowners and businesses will benefit from passive solar and thermal mass even if they have no idea what they are.
Sustainable Design Principles
Build with the Sun in Mind: What is Passive Solar Energy?
Nothing affects as many aspects of our lives as the sun. It’s responsible for warmth, rain, wind, and almost all of our planetary natural forces. Without the heat of the sun, the Earth would be a frozen, dead world. It’s astonishing we have a cosmic fireball floating in the sky that offers us extra warmth during the times of year we need it most!
The basics of passive solar design are easy: In the Northern Hemisphere, it’s ideal to have windows facing due south, and in the southern hemisphere, they should face due north. This helps keep the sun out when it’s more overhead in summer and allows the sun in when it’s closer to the horizon in winter. This setup gives homes free heat on sunny winter days. I’ve had folks come into our house during those sunny days in winter; they were shocked to learn we didn’t have a fire going!
You can enhance this passive warmth by having a dark earthen, tile, or concrete floor and then allowing its thermal mass to absorb and store the sunlight. This can also make for toasty toes — a welcome feature during chilly months!
Earthships have angled glass in front. In my climate, I find straight vertical glass with an eave above it to be a better choice: It blocks the sun in summer when you don’t want it and allows the winter sun to warm the house.
Thermal Mass Can Help Regulate Temperatures
Thermal mass is the ability to store different temperatures, a quality that many dense building materials have. These materials act as a thermal “battery.” When cool, they tend to stay cool. When warm, they tend to stay warm. This feature is used in masonry stoves and rocket mass heaters to capture the heat of a fire and slowly radiate it into a structure. Earthen buildings, such as cob, earthbag, and adobe houses, naturally have a lot of thermal mass. When we create synergy between thermal mass and passive solar, it’s a win-win.
In very cold climates, you’ll want to create a thermal envelope with insulation on the outside of your building’s mass. Thermal mass alone can turn against you in very cold climates, you’ll have to fight the cold seeping through the walls. One way to solve this is to insulate the outside of your structure with some non-organic insulation, such as dense sheet foam, and bury it below frost depth, where temperatures are more comfortable. In far northern or southern locales, straw bale walls become very desirable because of their extreme insulation. Combining this with a masonry or rocket mass heater, such as a cob bench or bed, will make for energy-efficient home designs and a comfortable house, even on the darkest, coldest days of winter.
Use Prevailing Winds and Exhaust Fans
Airflow is an important consideration in any building design. A properly ventilated structure is healthier to be in and is conducive to natural or low-energy drying and cooling. Where I live, the humidity drops in winter. This allows us to draw air through our structures with an exhaust fan to help dry them out with minimal power consumption.
Our house can get pretty hot on late-spring and summer days. The late afternoon is typically the time of overheating. Since we have deciduous trees to the west, they help block the sun in summer and allow it through in winter. Trees are some of our greatest friends. If you don’t have any western shade, plant some fast-growing trees that have a big, leafy canopy. In a handful of years, you’ll thank yourself for doing so. As time goes on, they’ll offer even more shade and comfort to your home’s inhabitants. Not long ago, I read a study that said forests with dense tree canopies can help protect against the warming associated with climate change. In a warming world, this is an important consideration.
Reflect Heat with White and Light Colors
Sometimes, while driving down the road, I see a house that really sticks out to me. The worst kind of roof in terms of sustainability is one that’s dark or black and, in the Northern Hemisphere, predominantly west-facing. A dark roof is a heat sponge, and, even with a well-insulated roof, it’ll cause a structure and its surroundings to retain some heat. I had a neighbor who lived in a mobile home with a black asphalt shingle roof. Every time I’d walk by in summer, I’d hear the air conditioner running. Because of the color and type of their roof, these folks were constantly hemorrhaging money.
Recently, I read about a new material that reflects visible and infrared light and is usually 30 degrees Fahrenheit cooler than everything around it. I heard that it’s being used in some experimental air conditioners. A white surface reflects visible light and absorbs infrared light. A dark surface absorbs both visible and infrared light, so that surface would be the hotter one. I always get the lightest roofing metal available (usually white galvalume). Structures in full sun will especially benefit from this.
The outside surface color of your house can also be an important factor. A darker-colored wall will absorb more heat and, unless you have an incredible amount of insulation, can lead to overheating.
Addressing these issues through retrofitting an existing house is easy. You can paint your roof with white elastomeric or an other reflective coating and paint your exterior walls a lighter color.
Retrofitting an Existing Structure with Energy-Efficient Home Designs
Any house can be upgraded to be more aligned with comfortable and energy-efficient home designs. This is just a partial list. It feels great to use our natural ingenuity in service of a better life for all. Our creativity is one of our greatest human resources — express it and empower yourself!
- In cold climates in the Northern Hemisphere, add a south-facing greenhouse onto the house. You’ll probably want an opaque roof that blocks summer sun but lets in winter sun as much as possible. I’ve even seen cases where people built a greenhouse around their entire house! I wouldn’t recommend anything that extreme for most of us; unless you live in a consistently cold climate, that would probably lead to overheating in summer.
- Replace single-pane windows with double-pane or triple-pane ones.
- Get insulated curtains to block out the chill on cold, sunless days or the heat on blazing-hot days. Once again, a light or reflective material is preferable, as it’ll reflect more heat.
- Put some fire bricks on top of your woodstove to slowly radiate heat into your home once the fire has gone out. You can also encase an existing woodstove with cob to increase the thermal mass. You’ll want to leave some of the metal exposed so quick heat is available as well, when needed.
- Check around windows and doors to make sure there aren’t gaps and cracks, which are a conduit for unwanted cold or heat.
- Insulating your attic will greatly improve the comfort level of your house.
- If your house tends toward overheating, put an exhaust fan opposite the shaded, cooler side of the house. If it’s near the ceiling, it’ll be able to remove the hottest air in the building. Being able to exchange old and potentially moldy and toxic air for fresh air is also great for the health of the home’s inhabitants.
- Buy Energy Star appliances. The air conditioner we recently bought is so efficient that it’s off the chart on the low side in terms of energy usage! The planet and your wallet will thank you.
- If you live in a predominantly warm climate, you may want to build an outdoor kitchen. Cooking adds a lot of heat and humidity to a home.
- I’m a fan of fans: They’re energy-efficient and can make life more comfortable in all but the most horribly hot situations.
As I often tell my earthbag-building students, “None of this is rocket science.” Creating a more comfortable and environmentally friendly life for ourselves and our loved ones is within our reach. If you’d like to see more about building an energy-efficient home as well as our other projects, find us at Sustainable Life School, where you can also download a free PDF of my book Ecological Awakening. This book goes into how humanity has arrived where we are and offers some solutions to the many problems we face. It talks about what’s really important in life: air, water, food, clothing, shelter, and community. We often forget that last bit, but without it, none of us would be here today or have any of the good people and things in our lives.
Though we can easily be bombarded with negative news and dire predictions about the future, now isn’t the time to give in to despair. Let’s get busy and apply some elbow grease to make a better life for everyone! Everything we do to make our lives more sustainable helps all of us.
Morgan Caraway and his wife, Mary Jane, live off-grid and conduct natural-building classes and sustainable -living workshops both in person and online, including “Building an Earthbag Tiny House,” at Sustainable Life School.
