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Sun angle and overhang design

February 6, 2007

My current home is a mid-row townhouse that unhelpfully faces East-West. This means we get some sunlight in the back windows in the morning, and some in the late afternoon at the front. Otherwise, we get no direct light. If I have absolutely anything to say about it, my next home will be a passive solar design with a proper southern exposure and all the solar goodness that offers. A large part of passive solar design, however, is proper overhang design to prevent overheating during the summer (and to prevent winter shading). The problem is, I’ve not yet found a website that provides the two pieces of information necessary to properly design an overhang: summer and winter sun angles.

I eventually calculated the sun angles for Waterloo (23° winter, 70° summer), but since have been provided a simple formula with which you can calculate your winter and summer sun angles. The Sustainapedia will now be the website where that information can be found!

I found a website that helped me calculate me the numbers I was looking for. Sustainable By Design has a sun angle calculator that will give you what you need, but only if you know a lot of information about the exact place and time that you want the sun’s angle measured at. It gave me the right results, but only after a lot of fiddling and searching. Far too much effort for the two numbers one needs for overhang design.

A friend of mine personally knows a local astronomer who very helpfully provided the following formula that requires you to know only your latitude to get an answer:

SUN ANGLE = 90 – (LATITUDE) +/- 23.5

For example, if your locations is 43.5 degrees N latitude [Waterloo, ON] and you want to calculate the sun altitude for the winter solstice,

SUN ANGLE = 90 – 43.5 – 23.5 = 23 degrees

For the summer solstice,

SUN ANGLE = 90 – 43.5 + 23.5 = 70 degrees

So now all you need to do is look up your latitude for the city you’ll be building your passive solar design in, and you’ll have all you need to properly design your overhangs.

One Comment leave one →
  1. gardener permalink
    November 23, 2012 7:06 pm

    For a ranch house with a slab foundation, would the overhang need to shade the house at the foundation at the spring/fall equinox, so the days following the fall equinox the sunlight begins to strike the side of the house more and more? …. or would you want the overhang to be less and possibly start at the bottom of the window sills on the fall equinox? Thanks for any replies.

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