Passive Solar Heating

Today we’re going to talk about thermal mass, insulation and all sorts of other fun things, boys and girls. But before we begin, let’s set the stage.

What would you say if I told you there was a way to build a home in which you would never need to pay for heating and cooling ever again? No, I’m not talking about finding your own personal oil well, or natural gas deposit. This home would maintain a comfortable temperature year round, regardless of climate and it would not burn any type of fuel or electricity.

Sounds too good to be true? Stay tuned and I’ll show you how.

First, let’s talk about mass. Mass is a term we use to describe the quantity of matter contained within a volume of space. If I have a liter of marshmallow and a liter of concrete, the concrete has more mass as the molecules that make up its structure are packed much more tightly. The more molecules you can pack into a volume, the more dense it will be and the more mass it will have.

Now, heat and mass get along very well. The more dense mass you have, the more heat it can store. You might imagine putting an aluminum pie plate in the oven along with a cast iron frying pan. Turn the oven on and heat them both up. Then take them out of the oven. The aluminum pie plate will cool down pretty quickly, but the cast iron pan will stay warm for quite a bit longer. This is because the cast iron pan has much more mass than the aluminum pie plate.

Okay so far? What about insulation? We talk about insulation a lot these days, but what is it really? It’s a material that can be used to block the transference of temperature. So, you want to insulate your house to keep the heat in and the cold out during the winter. Let’s discuss how this is done in a conventional house.

Conventional housing, generally, uses a wood frame structure which is then insulated around on the outside and covered with a wrapper, something like siding or brick perhaps. We put a peaked roof on it, insulate the attic, and slap some shingles or other roofing material to keep the rain and snow from falling into the house. In general, conventional housing is designed to keep the outside elements outside as much as possible.

There is an issue with this though. If it is really hot outside, we have to add artificial cooling to the building to make it livable. Similarly, if it is cold outside, we need to add heat to the building otherwise everything would freeze, no matter how much insulation you have, unless you live in an airtight bubble and never go outside. I don’t think you’d last too long in that situation.

Hmmm, time to do some more experiments. Lets look at this one:


You could think of the first case, with the glass jar full of air, as a representation of a conventional home. What we do is heat and cool the air, but as you can see, air doesn’t hold temperature very well. It will equalize to whatever the ambient temperature is pretty quickly. We could add insulation to the jar of air, but it still won’t hold its temperature nearly as long as the insulated jar with water in it.

So, lets use our imaginations and consider what it would be like if we were to live in jars like these:


Am I suggesting that we all should be living in jars full of water? Not really. But the concept is valid. With these ideas about mass and temperature, the ideal outer wall in a house would look like this:


Using a dense mass to hold temperature, we insulate it against the outside and then the mass will stabilize the interior temperature of the house. When there is a temperature difference between the air inside the house and the mass of the walls, the heat always travels from hot to cold. So if the walls are warmer than the air, the heat comes out of the walls to heat the room. If the walls are cooler, they will pull heat out of the room.

Now if you’ve ever heard anything about geothermal heating and cooling, you would know that once you go down past the frost line, the temperature of the earth is stable around 15 degrees C (around 60F). You can experience this simply by standing barefoot on a concrete floor in a basement. It feels great in the summer when it’s really hot outside, but it feels quite frosty in the winter when you’re trying to stay warm. In actuality the temperature of that floor doesn’t change much (without working at it, anyway); it’s stable throughout the year around 15C. Geothermal heating taps into this and we’re going to do the same thing, but without digging big holes and running pipes that pump water around all of the time.

Remembering that we want to have some dense mass walls, we’re going to put them in direct contact with the earth. Oh, we’ll probably put down a moisture barrier first, but the mass walls need to be in contact with the earth to get access to that stable 15C temperature. Forgetting everything else, if you were to just do this, insulate the walls and close in the house, your house would maintain that temperature of 15C. (There are a few details I left out, but this is getting long :P) That’s not super warm, but your toilet and other plumbing will never freeze, for one thing.

So, we have our massive walls attached to the ground and then we make sure to insulate them so they can keep their temperature. But how do we get things warmer when we need it? We use a simple thing called the sun. If we look at the regular approach to sun gain in a conventional house, it looks something like this:


What we need to do, is reconfigure the house so it faces the sun as much as possible. This would be south in the northern hemisphere and north in the southern hemisphere.


Does this really work? Can you heat your house in winter just with sunshine? The answer is a resounding “YES”. There is a good chance you have encountered this phenomenon yourself. Ever have one of those really cold, yet sunny winter days? Have you ever been driving into the sun on a day like that and find yourself turning the heater down or even off? Heck, you don’t even need to be driving. Just park your car with its windshield facing the sun for a while and then get in it. It may not be super toasty warm, but it will certainly be much warmer than the outside temperature is. This works the same way during the summer, when you come back to your car that is parked in the sun and find the interior is like a blast furnace; i.e. much warmer than the outside.

There is a tremendous amount of solar energy falling on every square meter of land and for the most part, we completely ignore it. What a waste.

So, we have our mass walls, they are insulated. We’ve aimed the house to the sun, with lots of glass there to let it all in. We’re also going to berm up the house on the east, west and north sides, which will further work to keep that stable temperature inside. It does this by raising the frost line up closer to the roof, thus ensuring that your structure won’t be heaving from the frost in spring/autumn.

The sun comes up in the morning, shines in our windows and heats up those mass walls we have. At night, when the sun goes down, the air in the house starts to cool and the temperature we stored in the walls comes out, keeping the house at a stable temperature.

Now you know why Earthships are built using tires packed with dirt. This provides the mass that temperature can be stored in. This is really just a simple man’s version of pure rammed earth building techniques. It has the advantage of using up a huge waste resource we have, as well as breaking it down into sizable brick-like chunks, which you wouldn’t be able to do with traditional rammed-earth as you’d need to tamp the whole wall at once to avoid cold joints.

Anyway, that’s the basics behind how to heat your home without burning fuel or using electricity. It’s completely passive. I have left out some details to keep it simple and I haven’t talked about cooling yet either. I’ll save that for another post 🙂


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