A question about SHGC
jollygreenshortguy
| Posted in Energy Efficiency and Durability on
I understand SHGC is about blocking penetration of sunlight into an interior, and thereby reducing solar heat gain (SHG). But if a window is shaded, so that direct light is not hitting the glass, how much does SHGC matter? Is reflected light off the surrounding landscape a significant contributor to solar heat gain or is it mostly the direct sunlight itself that matters?
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Replies
SHGC is a measurement of heat gain from direct solar radiation plus the inward flow of heat that was initially absorbed by the glass that then radiates into the structure. Any potential heat gain that might occur due to reflection from landscaping (or any other source) is not part of the SHGC measurement.
Any glass that is 100% shaded from direct solar radiation has a SHGC of zero.
Diffuse light scattered from the surrounding is not going to contribute any significant amount of heat to your space, regardless of the SHGC rating of the window. Only direct sunlight really matters in this regard. If you have a solar panel you can play with, it's easy to see how this works: aim the solar panel at the sun, get power -- using DIRECT sunlight. Now aim the panel at the trees, or other object away from the sun. You'll still get SOME light on the solar panel, which is why it's still visible, but it will produce very, very little power.
Bill
Note the coatings will affect the visible light transmission. If different windows in the same room have different coating your eyes will pick up the small differences and it will look strange.
Walta
It will also look strange from outdoors if there is enough difference between the window specs.
I tell people to try to have matching window specs for all windows on a shared wall, or at least for all windows in a shared room. The farther apart they are, the less you'll notice any differences. The different coatings will look like different color tints to the glass.
Bill
Is reflected light off landscaping significant?
Yes
Is it mostly direct gain? Yes
Albedo of landscape/hardscape Is definitely a factor. Just as diffuse radiation is a significant, albeit secondary factor
How much? Well now the answer is that 'it depends '
But it's something that can be described in many energy modeling physics engines
Before you get too excited about deferent window coatings for each window, build the BEopt model computer of your home and see what it tells you. The way I read my model the cost of the coating exceeded their energy benefits.
Walta