Thermal/Solar Heating & Cooling

In this article we’re going to talk about how heating and cooling works in an Earthship. An Earthship uses passive solar energy to accomplish this without the need for big furnaces or air conditioning. First, let’s talk about something called Thermal Mass.

Here is a scan I took from a page out of the Earthship Vol I book. Click on the image for a larger version.

After reading about the jars experiment, consider how most conventional buildings are constructed: they are insulated boxes of air with thin walls. There has also been a concerted effort in construction to try to make houses as air tight as possible, which has also lead to requiring elaborate venting systems to get fresh air into the building. This is all in an effort to try to keep the air inside at a stable temperature.

Now following what was discussed in the jar example, it would seem to make more sense to incorporate more water in our buildings. Unfortunately, humans don’t thrive so well in an aquarium, so Earthships use compacted dirt and concrete. They’re more easily formed and they don’t make your bread all soggy, cause who likes soggy bread.

Here I have included a section (side) view of a typical global model Earthship, with some extra labels that I added. The front of the Earthship (the part with the glass) is on the right side.

During the winter time, the sun will be low, around 20° above the horizon for our latitude. This means the sun will shine deep into the Earthship. Here is a picture I did in Sketchup to give you an idea.

The Earthship has a massive tire wall at the back, where each tire has been filled with compacted dirt. You pound it in with a sledge hammer to make it nice and firm. Behind the tire wall is another section of compacted dirt that is about 1m (3′) thick and goes from the bottom of the tire wall to the top. This adds more thermal mass to your wall so it is, in effect, almost 2m (6’6″) thick. You’ll notice from the section drawing above that after the compacted dirt comes insulation. There isn’t much point in having all of that thermal mass if you don’t insulate it.

If you live in a part of the world that experiences snow during the winter, have you ever parked your vehicle so its windshield is facing the sun during a really clear, cold day? If you get inside, you’ll notice that the interior of your vehicle is much warmer. This is because the sun is shining through the window and heating up all of your upholstery and other bits you have inside your car. This is the same type of effect we are using in an Earthship.

The sun shining in the front of the Earthship will bring a lot of heat with it and that heat will be absorbed by the thermal mass. Remember those can and bottle walls we talked about in Building with Natural and Recycled Materials? Those will also hold temperature. Your floors will also have thermal mass: concrete, adobe, brick, tile, flag stone or similar earthen type covering. This is so they can also absorb the heat coming from the sun. You don’t want wood or carpet as those things are insulators and will prevent the sun’s energy from being absorbed by the mass. This would defeat the purpose of having the thermal mass in the first place.

From a physics point of view, if the air in the room is +1° warmer than the mass surrounding it, the heat will move into the mass. If the mass is warmer than the air, the heat will move into the room. So, if the sun is shining during the day, the thermal mass in the Earthship will absorb all that heat. When the sun goes down at night, that heat will be released into the room. There have been comments made by people who have stayed at Earthships in the winter about the fact that the “furnace came on around midnight and it got really warm”. There is no furnace, that is just the heat coming out of the wall.

So, what about summer then? Obviously, you don’t want to be storing all of the summer heat in your house, you would be cooked.

During the summer, however, the sun is really high in the sky. About 70° off the horizon up here. This means the sun is only going to shine into the green house at the front.

So the front green house will be pretty warm with all of that sun, you might think. However, due to the fact that the glass on the front of the Earthship is angled so as to be at 90° to the sun during the winter, the summer sun shine will be hitting that glass at a fairly oblique angle. Because of this, much of the suns effect will be reflected away from the Earthship.

Not all of the sunshine will be reflected, obviously, and the sun is quite strong during the summer. This is why there are also skylight vents in the roof above the green house, so you can naturally vent out that excess heat from the highest point in the Earthship.

Behind the angled glass of the front face there is another wall of glass that separates the green house from the interior. This creates a buffer zone that keeps the areas where people are the most stable. At the top of this second, vertical glass wall are openable vents. The purpose of these will be discussed shortly.

Going to the back of the Earthship, if you refer back to the section drawing, you’ll see that outside the layer of insulation, you have your cisterns (which will be covered in Water Harvesting) that are completely buried in an earth berm that reaches to the edge of the roof at the back. Buried at the bottom of this berm are several steel culvert pipes that are referred to as vent tubes.

With the greenhouse at the front of the Earthship heating up and venting that air out the ceiling, new fresh air must be brought in to replace it as it will cause a drop in air pressure. The vent tubes allow fresh air to pass through the berm and into the back of the Earthship. Once you dig down far enough into the earth, you will reach a stable temperature point of about 15°C (68°F). Now imagine pulling air from outside on a hot 30°C (85°F) day through that tube. The air will be cooled down by this action so when it enters the back of the Earthship, it will feel like air conditioning.

This is where those vents at the top of the vertical glass wall come in. With those open, the hot air escaping out the ceiling of the green house will cause fresh air to be pulled through the vent tubes into the back of the Earthship, creating a natural cooling convection current that uses no power and only natural processes.

Here is a diagram.

The cool air comes through the vent tubes, hot air is vented out the top of the vertical glass wall into the green house, which is then vented out the ceiling. A simple solution and it doesn’t have any moving parts so it won’t break. No having to call the heating/cooling techs to come fix your Earthship.

I’m going to take a moment here and talk about hemp-crete, as I have been asked about that a lot. It also so happens that this was talked about when I was down in Taos, NM doing my Earthship Academy session 1.

Many people think that replacing the concrete with hemp-crete would be a much more environmentally friendly thing to do as the energy requirements to make it are far less than that of concrete. However, you would be making a HUGE mistake if you were to do that. First of all, hemp-crete does not have the same structural properties as concrete, so you’re not going to get approval from the inspector to replace your footings with it. Secondly, and probably most importantly, hemp-crete is an insulation, not a thermal mass.

What do you think would happen if you did all of the pack-out around your tires and poured your floors with hemp-crete? You would be insulating all of that winter sunshine AWAY from the thermal mass of the tires. You would also create a day-time oven in the winter and you would freeze during the night as there wouldn’t be anything to absorb that heat during the day; it would just accumulate in the rooms making your type-writers melt.

This, in fact, happened at an Earthship build down in Australia. Obviously, Australia doesn’t have the winters that we do here in Canada, so being able to shed heat would be important. However, this one was being built by someone who was an intern for a short while at Earthship Biotecture and then went back to Australia, found a client, and started the build. They had domes in this design, but they replaced all of the concrete with hemp-crete. What they ended up with was building a huge oven. It was so hot inside it was unlivable. Do not make this same mistake.

This same type of mistake can even be made in conventional housing that is built (or converted) to take advantage of passive solar heating. You can have a whole bunch of windows facing south to gather sunlight during the winter, but if you don’t have any thermal mass to absorb some of that, you’re going to be too hot. Yes, the sun is that powerful.

If you have any questions regarding passive solar heating and cooling, please feel free to comment.

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3 thoughts on “Thermal/Solar Heating & Cooling”

    1. Thanks for the complement, Vickie. I need to tell you that I have been having difficulties posting comments on other wordpress blogs recently, one of which was yours. When I click submit, WP just issues an error and the page doesn’t update. Hopefully you will see this reply so you will know. I don’t think it has anything to do with your blog, specifically, as I have the same issue on others I have tried to comment on.

      1. Thanks for letting me know. I’m sorry I just saw this. I sometimes don’t get notified about comments. I do read all of your posts and am following always.

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