Glass Angle

We were talking about heating and cooling a little while ago and it was mentioned that for optimal heat gain during the winter you want your front face glass to be perpendicular to the sun. Today, I’m going to tell you how to figure out what that angle is.

Fortunately, this is really easy. You don’t need to wait until Dec. 21st to figure it out. Luckily, there are solar calculators available. The one I like is the NOAA Solar Calculator. It has a very simple google maps interface.


When you first load the page, it will show you the standard map of North America, but you can pan around to anywhere. Just drag the big red marker to the spot you want. You can zoom in for extra precision too. I put the marker on Bancroft, ON for the screen shot above, which is where our land is located. You can also put in exact latitude and longitude coordinates, if you have them.

Down at the bottom, you’ll see the section where you can put in the date. I put in the 21st of December, 2014: winter solstice for this year. Now if you look below that you’ll see a reading in a table called Solar Noon. This is the time at the location you have picked when the sun is at its highest. Enter that time to the left of the date. Mine was 11:09:36.

Once you do that, the Az/El (in °) at Local Time will be updated. This will tell you the angle, in degrees, of the sun’s elevation at the time you have entered. As you can see from my screen shot, it was 21.55° for Bancroft. Rounding off a bit, we can call that 20°. No use fussing over a degree and a half, which, when cutting wood for the framing, is going to be really difficult to manage 😛

We now know that the sun will be hanging at an angle around 20° elevation from the horizon during winter. Using a tiny bit of math, we know that a right angle is 90°, so if we want the sun to hit the glass at a right angle, we need to position our front face glass at 90° – 20° = 70°. This means we need to frame the front face so the glass is at a 70° angle to the ground.

If you have Earthship Volume I, you can find this same calculation for Taos, NM on page 31. Here is the text.


So, with an elevation angle of 30° in Taos, they should be putting their glass at 60°. However, they don’t. Or at least, they don’t any more.

I discovered in videos of more recent Earthship construction down in Taos, they are putting the glass at 70°, like we should up here in the north. I wanted to know why the change and it was one of the things I asked about when I was down in Taos.

There are two reasons they gave me for the switch. First, over time, they have discovered that 70° actually works as a good average for most of North America. That makes it easier for making general drawings that can be used most places as they don’t need to re-engineer the truss detail that determines the front face angle. Secondly, for Taos specifically, they did have some problems with earlier Earthships overheating during the winter. By using 70° instead of 60°, they reduce their sun a bit to help alleviate that.

Of course, there is no requirement that you have angled glass. You can make it vertical if you like. You’ll just lose a bit of solar gain during the winter. Depending where you are, this may not concern you. They used to build Earthships with vertical glass at one point. They were called Packaged Earthships as they were intended to be a generic package that one could buy, all the components would be shipped to you and you would have to put it together. Kinda like a Heath Kit, but for a house, if you’re old enough to know what that was 😉

Packaged Earthship

Now you’re equipped and ready to handle the question: what angle should I put my front facing glass at? If you need to come back and find the link for the solar calculator, I’ve put it up on the side bar links on the left.


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