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Correct roof overhang depth for proper solar shading

GBA Editor | Posted in General Questions on

I used Sketchup to help determine how the solar path would fall on the house I am designing. How do I determine the correct amount of shading/sun?

With the window heights and overhangs I used in the model, at noon on June 21, the entire glazing of the south elevation windows are completely in shadow, but just barely. At noon on January 21, the glass is completely exposed. This seems, intuitively, to be the sweet spot. But should I tweak the overhang to get more shading in the summer months or, conversely, more sun in the winter months?

The house will be 52 degrees north (roughly between Edmonton and Calgary), Climate zone 7 (i think) with about 5,500 HDD annually.

Also, plan on Larsen trusses, triple-glazed windows, airtight as possible but not much in the way of thermal mass (wood framed floor, no tile).


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  1. wjrobinson | | #1

    Erik, use search, I just tried a search for you as I knew this has been discussed a bit;

  2. GBA Editor
    Martin Holladay | | #2

    Here's a link to a free Web-based tool to help you size roof overhangs on a passive solar building: Overhang Design.

    I also urge you to read this article: Cost-Effective Passive Solar Design.

  3. user-757117 | | #3

    Not sure what kind of insolation you receive at your site, but it might be worth it to consider the "shoulder seasons" (mar/apr & aug/sep).

    I aimed to have the windows fully shaded between the middle of april and the end of august.
    I was a little concerned it would be possible to get some overheating in spring/late summer - sun is still fairly low on the horizon.
    The problem with solar gain is that I can't shut it off if it gets too hot, but I can always throw a log on the fire if it gets chilly.

    I thought I should add that I don't have much for direct gain thermal mass - heavy masonry or concrete...

  4. user-1105327 | | #4

    lucas, i guess a solution for your scenario would be some sort of exterior shutter system...? unfortunately, i simply do not have the budget for it...

    martin, i have read the informative article a few times; very helpful. the overhang design tool is tricky for me: what are the measuring units? how do you set the window elevation above the floor? what is the difference between overhang width and overhang depth? what is horizontal offset? i am not looking to you for answers, those are just some reasons why i could not be sure if i was using the tool properly.

  5. user-757117 | | #5

    i guess a solution for your scenario would be some sort of exterior shutter system...?

    No, I just made the overhangs wide enough to provide the extra shade.

    Using sketchup, I just fiddled around with the overhang width, window size and vertical placement until I got the desired shading effect.

    The following snips show shading at noon on April 16, June 21, September 1, December 1 and February 1.

  6. GBA Editor
    Martin Holladay | | #6

    Click the "Instructions" link and the "FAQ" link at the top of the page to learn more about this free software tool.

    Here's some information from the instructions:

    "The two parameters you define about your window are its height and width.

    "The height and width are the size of the aperture of the glass (not the frame), and are measured in whatever units you'd like (feet, meters, etc...just be sure to be consistent throughout the inputs).

    "The overhang width is its horizontal dimension (be sure to use the same units you used to describe the height and width of the window).

    "The overhang depth is how far it protrudes from the wall.

    "The height above the window is the distance from the top of the window aperture (the glass, not the entire frame assembly) to the bottom of the overhang.

    "The horizontal offset is how far the horizontal center of the overhang is offset from the horizontal center of the window."

    More information is available on the Overhang Design website.

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