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Passive Solar

August 15, 2011

I thought that the first of two posts on our roof would be on passive solar design. We are lucky in that our building site faces almost directly south. Mr. B and I wanted solar heat to play a role in heating our home, but passive solar is just as much about excluding solar heat as it is about letting it in.

Passive solar and straw bale houses are often talked about together because they both require a large roof overhang (note: I am just talking about active solar heating of the house. There are many other aspects to passive solar design). Our house has a three foot overhang on the north and south sides, and a two foot overhang on the east and west side. Straw bale houses need to be protected from water and snow by their overhang. Passive solar design uses the roof overhang (and angle) to manage the amount of sun entering the house throughout the year.


When I took this picture, we only had the first row of roof sheathing up, but that is the one that matters. I took this picture around noon, and you can see that the sun is only entering the house by about a foot (I’m excluding the area where the bales will be from my estimate). I was so happy when I saw this that I did a little dance and possibly punched my fist in the air a few times. Yes readers, this was no happy accident but a carefully planned event. The sun will continue to shine further into the house as it drops lower in the sky through winter and fall, warming our floor during those times. The opposite will occur during spring and summer, as it is slowly excluded by the roof.

The easiest way (I thought) to figure out the required roof angle/distance is shown in the picture above (from this website).

Step 1: Draw the wall to be shaded (the south wall, in our case) to scale. Because we are interested in windows, we chose the largest window on the south wall for our calculations.

Step 2: Draw the summer sun angle (#2) upward from the bottom of the window. Choose the day of the month that you want to exclude all sun from your house. There are several websites that will help you find this angle, but I think that the NOAA Solar Position Calculator is the easiest to use. You need to know that lat/long for your location, which you can find using Google maps. We chose July 31st as our summer angle, which would make #2 68 degrees.

Step 3: Draw the roof until it intersects with the summer sun angle (#3).

Step 4: Draw the winter sun angle (#4) across the edge of the roof. This angle should be the day when all sun will enter your house and determines the top of your window. We chose December 31st at noon for our winter sun angle for a #4 angle of 30 degrees.

Step 5: Draw a solid line from the ceiling to meet the winter sun angle.

We used a Frankenstein method to determine our roof angle because we already knew the sizes of our windows and where the tops would start. You’ll find that there are trade-offs in the process. Missoula is cool through June and starting in September, so we decided that it was worth allowing sun to start entering in August because there was no other way to have sun warming the house in early summer and early fall otherwise.

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