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Efficacy of External Shading

Discussion in 'Environmental Discussion' started by SageBrush, Jun 3, 2013.

  1. SageBrush

    SageBrush Senior Member

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    I installed fixed windows last year for solar heat gain during the winter, and now is the time to shade them for the summer. The shade is a 95% block, tan polyester weave set about 4 inches external to the window.

    NM Solar Map.png

    My wife was initially very skeptical of the shade compared to the wall with R-13 insulation, but my calculations suggest the external shading of a window is as good as the wall if not better. I'd like feedback from those more versed than me on the topic whether they agree with my analysis. So here goes --

    New Mexico, 35 degrees latitude. Walls oriented 135 degrees
    Frame construction, outside wall is about 1 inch of concrete covered by tan stucco
    R-13 insulation

    Windows are U - 0.31

    I estimate that
    The outside walls are an average of 65C during the day,
    The house averages 25C, for a delta of 40C for 10 hours a day.
    So my pre-window walls passed 30 Wh a day per square meter

    The windows
    Pass about 1.5 watts/m^2 of light radiation judged by an equivalent light bulb,
    and 1.5/m^2 watts of conducted heat based on a delta of 5C
    for a total of 3 watts/m^2
    For a total gain of 30 Wh a day.

    At night the windows radiate and conduct out heat and thus end up with a net solar heat gain less than the wall. As an additional benefit, the area has natural lighting during the day "for free."
     
    #1 SageBrush, Jun 3, 2013
  2. The Electric Me

    The Electric Me Go Speed Go!

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    Judging from your title,- I thought this was a post about an upcoming movie.

    I was expecting the sequel to "The Eternal Sunshine of The Spotless Mind".

    The only answer I can barely give you is that I generally like shade on sunny days. An awning, a window blind, a willow tree....I don't really need the science,- just a place to drink lemonade.

    As a defense mechanism I'm going to just say I agree with your analysis. I've often found success in life by simply nodding in agreement with things I do not understand.
     
    #2 The Electric Me, Jun 3, 2013
  3. SageBrush

    SageBrush Senior Member

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    :)
    Been there, done that

    I have been paying more attention to window treatments in pictures from around the world, and I notice that external shading is almost universal in poor countries but it gets ignored in countries that learn to rely on AC.
     
    #3 SageBrush, Jun 3, 2013
  4. chogan2

    chogan2 Senior Member

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    Edit: Skip this. I must have misread the original post. Your 1.5W/sq meter is the amount after blocking 95% of incident light.
     
    #4 chogan2, Jun 3, 2013
  5. icarus

    icarus Senior Member

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    The sun block shading will have a dramatic effect on heat gain! I can't give you hard numbers, but just as example, putting 80% shade cloth over my glass roof on my sun porch drop the temp dramatically.

    Additionally, as the shade trees have grown up the summer, the effect on the house has been equally dramatic. Remember, the diffence between r value in the walls, and potential heat gain. Consider this example. The surface temp of a sun facing wall can be very much higher than a wall in the shade. The sunny wall will absorb heat all day, and give it back into the building at night. If you keep the sun off it, there will be much less heat gain.

    Icarus
     
    #5 icarus, Jun 3, 2013
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