Passive solar is one of the oldest solar technologies known to mankind. Archeologists have found evidence of passive solar homes dating back to ancient Mesopotamia, thousands of years ago. With the abundance of cheap fuel, we’ve moved away from these techniques. Now that we know the hidden costs of fossil fuels, it’s about time we revisit some of those designs.
Passive solar involves capturing the sun’s heat and light and moving it directly to where you need it. To maximize the energy gain you’d need to build a house using passive solar design and construction techniques. That’s great if you’re building, but for most of us that’s not really an option. So let’s first discuss a few passive solar retrofit enhancements that you can make to your existing house. Then we’ll go “The Full Monty” on new construction options.
Here Comes the Sun
As you learned in grade school, the Sun rises in the east and sets in the west. Of course, it doesn’t really rise and set – it just appears that way because the Earth is spinning. But from the perspective of a house at a fixed location on the Earth, it’s okay to say the Sun moves from east to west each day.
Due to the tilt of the Earth with respect to its orbit around the Sun, we experience the four seasons. In the summer, the Sun’s path across the sky appears much higher than it does in the winter. We get more daylight during the summer, and the light is more concentrated during the summer. That’s why it’s warmer in the summer!
In the northern hemisphere the Sun appears to travel from east to west across the southern sky, as shown in the above picture. It’s the opposite in the southern hemisphere. If you live south of the Equator, swap “north” and “south” throughout this article.
If You’re Retrofitting an Existing House
Even if you don’t have the luxury of building a new house, there are several enhancements that you can make to your existing home. First and foremost, make sure the house uses energy efficiently. This means sealing leaks, adding insulation and weatherstripping, and keeping your heating and cooling equipment in good shape. See Energy Conservation at Home Part 2 for more details about that.
Light tubes (sometimes called “solar tubes”) deliver natural daylight to interior rooms and hallways. The tube consists of a dome that sits on the roof (south-facing is best but any direction will do), a tube with a highly reflective interior, and a dome that’s mounted to your ceiling. The ceiling dome looks like a recessed light fixture; people who have seen them often look inside to see what kind of light bulb they have in there.
A light tube offers the benefit of a skylight – natural light – with a much smaller footprint. A smaller opening means that there’s less air leakage and less heat loss. Where skylights require a vaulted ceiling, light tubes can be installed through a traditional attic. Of course, this only provides light during the day, but I’ve seen models that include LED lights that can turn on at night, giving you natural light during the day and energy-efficient electric lighting at night.
Depending on the size, a light tube can cost anywhere from $150 to $500 plus installation. A handy do-it-yourselfer can probably install one in a couple of hours. Solar tubes qualify for energy efficiency tax credits and rebates, which can help reduce the cost of your system.
The best time to think about landscaping is when you build the house, but even existing houses can benefit from a few fast-growing trees and shrubs. Deciduous trees and shrubs lose their leaves in the winter, so they’re good to plant on the south and west sides of your house. In addition to providing summer shade, trees and shrubs lose water through their leaves in a process called “transpiration.” The evaporation of water is a cooling process so transpiration will cool the tree and its surroundings, just like sweating helps to cool our bodies. In the winter the shrubs lose their leaves, allowing sunlight to get through and help heat the house.
Plant fast-growing evergreens on the north side of the house; these will keep the cold north winds away in the winter.
Because the Sun is lower in the sky during the winter, the south wall of your house can absorb a lot of heat. If you have large windows on that side, you already know this. But if you just have a south-facing wall with very few windows, you can add a solar thermal collector. There are a few commercially-available models. A typical 8′ x 4′ collector will cost about $1000 or more, plus installation, and it will heat about 750 square feet of space.
If you’re a DIYer, you might be interested in building your own for less than half the cost of a commercial solar collector. Here’s one that cost just a few hundred dollars and paid for itself in less than a year. Oh, and it’s in Bozeman Montana, where the winters are very cold!
Video credit: Peter Q Brown
Solar Water Heaters
A solar water heater can serve all of your water heating needs all year long if you live in a warm climate. For those of us in temperate zones, a solar water heater does the job in the summer, but can also pre-heat water in the winter so your gas or electric water heater doesn’t have to work as hard. Solar water heating can be one of the quickest returns on investment. The biggest downside is that you have to run plumbing to the roof.
Recently there’s been some controversy about solar water heaters. The pumps and plumbing add cost and complexity to the installation, and pipes are prone to leakage. Some experts suggest using photovoltaic (PV) panels – which convert sunlight into electricity – and running a traditional electric water heater using that energy. PV panels are less costly and more efficient than they were a few years ago, and their price continues to drop. It’s a lot easier to run wiring to the roof than to run plumbing up there, and once installed it’s pretty much zero maintenance, so that’s a possibility to consider.
If You’re Building a New House…
Ah, there’s nothing like a clean slate! If you’re already planning to build a house, then please take advantage of passive solar techniques. Passive solar may cost a little more money up front, but it will more than pay for itself in the long run.
First, you want to orient the house so that it has a very large southern exposure. This will maximize solar gain in the winter. In the above picture, south is to the left. Notice the overhang on the south side. This blocks the high summer sun so you don’t get direct sun in the house during the summer.
The south side of the house should be where most of your living space is located: living room, dining room, and kitchen. This is where you want the most daylight year-round and the most warmth during the winter days. Bedrooms are located on the north side since you don’t normally need them to be brightly lit, and on winter evenings you have blankets to keep you warm. You’ll also have evergreens on the north side of the house to block those cold Arctic winds!
If you’re building a two-story house or a one-story with high ceilings, consider clerestory (pronounced “clear-story”) windows. Light bounces off the light-colored ceiling and walls and provides natural lighting without adding heat. The upper windows in this picture are clerestory windows:
Direct Solar Gain
Our dogs love to lay in sunny spots. The warmth that they receive from sunlight is called direct solar gain. The large picture windows on the south side of the house provide direct solar gain – sunlight enters and warms everything that it touches. A good passive design maximizes direct solar gain in the winter and blocks it in the summer.
All windows are not the same, so let’s make sure that heat gets in where you want it but doesn’t escape. Windows are rated with a solar heat gain coefficient (SHGC), a measure of how well they allow solar radiation to pass through them. A SHGC of 0.40 allows 40% of the solar radiation to pass through it, blocking the other 60%. Assuming you have a large southern overhang, you’ll want your south-facing windows to have a high SHGC so they let more of that low winter sun into the house. On the other hand, north, east and west-facing windows should have a low SHGC (less than 0.40) to help keep the heat inside.
Now that you’ve let the sunshine in during the day, you’ll want to store some of it for the chilly evening. Thermal mass is simply a bulky material that absorbs heat and slowly re-radiates it when the ambient temperature cools down. In the American southwest, natives built adobe houses; the adobe bricks provided a lot of thermal mass. Today you can use concrete or masonry floors, assuming they’re well insulated underneath. Ideally the floors should be somewhat dark to help them absorb heat. If you don’t like dark colors, then you’ll have to balance the aesthetics with the house’s efficiency and make a compromise.
The Building Envelope
You want your new house to be well insulated with no air leaks. A perfect house would be so tightly sealed that no air enters or leaves the house unless there’s an open door or window. Of course, we’d eventually suffocate in such a house, so for air quality purposes you’ll need some way to get fresh air into the house. But instead of leaks, which are inefficient, shoot for an airtight house and add “mechanical ventilation” (fans) in strategic places. In fact, you can also put a heat exchanger in the airflow path. In the winter, cold air comes in and warm air leaves. The heat exchanger captures the heat from the warm air before it leaves and transfers it to the cold incoming air. Now you can breathe fresh air without the heat loss. (It works the other way in the summer.)
Just like with a retrofit, you’ll want deciduous trees in the south and west and evergreens on the north side. Since you’re building new, I assume you’re in it for the long haul, so fast growing trees and shrubs aren’t necessary. But you could plant some fast growers and some slow-growth trees; as the slower trees start to get big, you can cut down the fast-growers to make room. And now you’ve got some firewood too!
We live in northern Illinois where the winters are very cold. But the summers are pretty hot too, so we think about cooling as well. We didn’t build our current house but it’s obvious that the builders incorporated some of these passive solar techniques into the house. We have a southern exposure with a large overhang, and the deciduous and evergreen trees are perfectly positioned to provide shade in the summer and solar gain (and north-wind blocking) in the winter. Although we have central air-conditioning, we almost never use it. Instead, strategic opening and closing of windows provides a cross-breeze that keeps the house cool though natural convection – moving hot air out and letting cool air in. Our home is very comfortable throughout the summer with little need for the air conditioner. When it’s extremely hot and humid, then I’ll turn on the AC, mostly to get rid of the humidity. In a typical summer, that may happen five or six times.
If you really want a unique house, consider an Earthship. These are more than just passive solar houses; they’re sustainable, environmentally friendly, energy-efficient houses that are often built from recycled materials. Click this Earthship link to learn more about them.
This overview of passive solar heating and cooling gives you some ideas of ways to reduce your energy consumption and become more self reliant.
Do you use passive solar for heating and cooling? We’d love to hear your experience!
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