Passive Solar Home Heating
As the name implies, passive solar home heating uses the energy from the sun to heat your home without you having to do anything. Basically, the “work” is in the design.
Even in northern climates during the short days of winter, the amount of energy reaching a house from the sun is more than enough to heat the home. Wouldn’t it be great to capture that free, clean energy? Passive solar heating design aims to do just that.
A house designed with the effects of the sun in mind captures the radiant heat of the sun during the winter while avoiding excessive heat build up in the summer. Solar design also considers the sun as a source of home lighting. Planning with this awareness results in a home that is naturally more comfortable.
The Basic Principles of Passive Solar Home Heating
Energy coming from the sun arrives in the form of radiant energy. The sun emits a wide spectrum of frequencies, but considering visible light is enough for our purposes here. Glass has the property of allowing most low-frequency sunlight to pass through.
When this sunlight reaches the surfaces inside the home, they either absorb it or reflect it. Absorbing the light energy converts it to radiant heat. Glass is less transparent to this heat than visible light, so the heat is trapped inside the house.
This trapping of heat is called the greenhouse effect. For centuries, greenhouses have allowed tender plants to grow in harsh climates
An example of the greenhouse effect that we’ve all experienced is the heat build up inside a car if we leave the windows up on a sunny day.
Trapping solar energy is the first step in passive solar heating. Next we need to manage it.
Managing Passive Solar Energy
As the car example shows, solar heat can sometimes become too much of a good thing. Also, there’s obviously no sunlight at night. It’d be nice to have some way of moderating heat build up during the day and of saving the energy from the day to warm the home at night.
“Thermal mass” addresses both issues.
This term refers to how much heat a substance can hold. Various materials can hold different amounts of heat. For example, water can absorb and hold a lot of heat energy, while fabrics can absorb very little.
Imagine putting a large pot of water on a stove and lighting the gas under it. Turn the flame on medium and wait. The water will warm up, but only gradually. As it absorbs heat from the flame, the water gradually warms.
Contrast that with how quickly an empty pot would get hot. The difference is the due to the difference in thermal mass between a full pot and an empty one.
Now, imagine letting the water get nice and hot, then turning the heat off. The water will cool, but only slowly. It gradually radiates heat into the room. In a very real sense, the heat of the flame has been stored in the water.
Adding an appropriate amount of thermal mass to a house improves it’s passive solar performance.
If a house contains thermal mass, it will be slower to warm up, but less likely to overheat. Also, the heat picked up during the day will be gradually released after the sun sets.
The home’s thermal mass can simply be part of it’s structure, or specific design elements can be added solely to adjust the thermal mass.
For example, stone, masonry or concrete floors and interior walls are often used to add thermal mass to a home. These can be part of beautiful design as well as adding to the comfort of the home.
I used a pot of water in my metaphor, but in fact water has excellent thermal properties. Because of this, solar engineers actually use water-filled containers to add thermal mass to a building. There are a large number of commercially produced containers available. Do-it-yourself homeowners often use drums or other recycled containers.
The trick in passive solar heating is to admit enough sunlight to warm the home without overheating it and also to have enough thermal mass to smooth out the temperature swings and store the heat for nights without having so much that the home never warms properly. It’s a balancing act, and one that solar designers have gotten very good at.
Several factors affect influence design decisions. Local climate is a big one. Obviously, that’s a fixed factor we have to be aware of and work with.
Things we can control are the design of the home and its placement on the site.
As a general principle, most windows in a “solar aware” building should be on the side of the home facing the equator. That is, on the south side in the northern hemisphere and the north side in the southern hemisphere.
Ideally, the house is positioned on the site in the location that will expose the south wall to the greatest amount of light between 9AM and 3 PM – the time when the suns energy will be greatest.
A solar approach that I really like is to have a sun-room or solarium or greenhouse on the south side of the house with stained concrete (it can be quite attractive) for the floor. An internal stone wall adds additional thermal mass if needed.
Here’s crude sketch of how this might work. The sketch shows low summer sun passing through a window to warm the sun space. The brown represents a very attractive stained concrete floor (given my rendering skills, you’ll really have to use your imagination). The gray represents a stone wall. Both of these add thermal mass to the room.
Notice that an overhang shades the room in summer, and that there is an insulated shade that can be lowered at night to reduce heat loss.
The vents in the stone wall allow air to circulate to help transfer heat to the rest of the home. Again, imagine the surfaces nicely finished and the room filled with plants thriving in the sunlight. Solar design can be very inviting.
A Solar Home Doesn’t Have To Look Like a “Solar Home”
As I said earlier, I believe every new home should incorporate at least some passive solar heating principles in it’s design.
Some homeowners design their home specifically to be a so-called solar home. For these folks, solar principles trump all other considerations.
Others don’t want to go to that extent. They may be hesitant to think solar because they’re afraid of what their home would look like if they do. They also may be afraid that solar design may raise construction costs. They shouldn’t be concerned.
Well-done solar design includes a thoughtful evaluation of the home’s location, style and how you like to live. A home can be “solar aware” while appearing completely conventional.
In fact, solar homes are generally more comfortable and inviting. By definition, they’re planned with awareness and appreciation of the sun, the seasons, the home’s site and how people live.
For example, morning light comes from the east. Do you like to awake with the sun, or would you prefer your bedroom on another side of the house?
How about rays of sunshine with your morning coffee?
Keeping the predominate living spaces on the south side of the house and less commonly used spaces on the darker side is a principle of solar design that lends itself to making a home pleasant as well as efficient.
Solar construction needn’t cost any more than a home built without any solar awareness. For example, having more windows on the south side that the north side is a matter of design, not cost.
Too many people pick home plans they like from a catalog. They judge mainly on the basis of external appearance and the floor plan. Then they build a house from those plans and plop it on the site without any consideration of solar issues. It wouldn’t take much more effort to evaluate how the sun interacts with your property and make that one of the criteria by which plans are evaluated. That extra step makes your home both more inviting and will save you money year after year in reduced heating costs.
Solar Design Makes Your Home More Your Home
Even if you don’t care about saving money and helping the environment by reducing fossil fuel usage, consider solar design just because it will make you home more inviting and a better fit for how you live.
For example, we live in an older home that we remodeled. In the original floor plan, the kitchen was on the north-east side of the house with one small window to the north. Even in the middle of a sunny day you had to have the lights on to see in that room.
I like to cook, but being in there felt like being in a dungeon. It just wasn’t a welcoming and comfortable kitchen.
When we remodeled, changing that was a priority. Our kitchen now juts out from the south side of the house. There’s a small window in the east (for that morning sun) and larger windows to the south and west. There’s a deck off the west side where we keep the grill.
Can you see how considering where the sun is influences the design for the better? We could have but the deck on the east side of the kitchen, but we don’t use it in the morning when the sun is shining there. We use it to sit out and relax in the evening and that’s when we grill. We have sun light and the beauty of sunset that we’d miss every day if the deck was on the other side.
Maybe you like sitting out in the morning and never grill at night so maybe you’d want you deck on the east side. Or on both sides.
The main point is that including the sun in your home design makes for a home that you’ll enjoy more.
In the best case, everyone would work with a designer or builder who recognizes the importance of the sun as a design element. However, over the years some general rules of thumb have developed. I’ve listed these on a separate page and you can click here to look them over for yourself to get an idea of what might be important to think about.
A Possible Problem – What About Summer?
One possible problem with passive solar heating is the potential of overheating the house summer. Fortunately, steps can be taken to minimize this.
One of my favorite is to have deciduous trees near the house. The leaves provide shade during the summer while the bear limbs allow sun to reach the home during the summer.
As much as I like tree, you need to be a little careful with this approach since even a bare tree can reduce solar exposure by up to 30%.
Since the summer sun is higher in the sky, properly sized roof overhangs can shade the window in the summer while allowing the lower summer sun to shine in. I illustrated that in the sketch above.
Good old-fashioned awnings also work well to keep out summer sun, as can shades, shutters and blinds.
Warm Climate Solar
So far, I’ve talked about solar design to help keep a house warm in winter. You can use different principles to discuss keeping a house cooler in warmer climates.
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Click here to go to Alternative Energy Primer from Passive Solar Home Heating