SHGC for shaded windows in warm climates
I’m designing a home in the South (DFW area). I have well shaded window locations (either via overhangs on the south or trees to the east/west). Typically in a warm climate you want a low SHGC number to block the suns radiation from heating the home, however low SHGC tends to correspond with a lower VT (visual transmittance). We’d like to let as much natural light in as possible while still getting good performance.
If the Windows are shaded from direct sunlight during the hot months, is there any issue using a window with a higher SHGC and thus higher VT? This would provide greater passive heat gain in the winter, and it’d be shaded in the summer from direct radiation from the sun. However, I’m new to this and I’m not sure if there’s “indirect” radiation or something similar to be worried about.
Thanks!
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Radiated light during summer months will still make its way through the glass, even if shaded. It won't be as bad without shading but it will still increase some heat loads in the summer. Light bounces off surfaces.
With that being said, I don't think the Visible Light will be that much difference between a medium gain and low gain window. Of course, that depends on the manufacturers glazing specs.
Here is an Alpen spec:
High Gain SHGC = 0.45 VLT = 0.56
Balanced SHGC = 0.28 VLT = 0.46
Low SHGC = 0.20 VLT = 0.45
The differences between Balanced and Low gain in regards to VLT is almost negligible. I would go with the LOW SHGC of 0.20 if I was located in the hot climate/desert climate. Your AC will thank you for the lower heat gain loads coming in through the windows.
I once was in a house in the high-desert SW that had the Balanced SHGC and the house would overheat in the winter and it would be so bright in the daytime, one had to use window shades as the light was glaring. This same window in a place like Michigan would not have the issues during winter as it would in a high-desert climate. Everything is climate/location specific.
Thanks for the feedback.
0.20 seems dark based on https://www.bayviewwindows.ca/blog/understanding-window-visual-transmittance/energy-efficiency. Especially if it's already shaded.
You probably won't notice the "darkness" of high SHGC windows as long as all the ones you can see at once are using the same type of coatings. Where you really notice the differences is if you use two windows of different type coatings in an area where they can be seen at the same time. Basically your eye is very good at seeing RELATIVE differences between adjacent windows, but very not good at seeing ABSOLUTE values compared to something you saw somewhere else. We're talking abotu differences of 10% at most in most cases here, when you're comparing between "steps" of SHGC numbers.
Bill
"You probably won't notice the "darkness" of high SHGC windows as long as all the ones you can see at once are using the same type of coatings."
gonna agree with you other than one caveat; cardinals e340 is too damn dark.
its around a 39% vlt for dual pane, then goes down to 33% for 340/clear/180 and 340/180/i89. but a crazy low .15 shgc.
my house is a mix of 340/clear/180, 340/clear, 270, and 270 with llumar dual reflective film that have a ~30% vlt -- so about a 20% vlt overall. no single room has products mixed together, and my previous windows had no low-e at all.
ill just say, the 340 was a mistake :(. i could've gone with a 46% vlt .19 shgc, or 59%/.25, instead of the 33/.15.
I don't like the 3xx series coatings either, for the same reason. How much difference do you see between those and the 270 coated windows though, since you have long term experience now with both?
I like 180, but I'm far enough North that the really high SHGC doesn't really gain me much compared to what it would do in more Southern areas.
Bill
i goofed up with my windows. i did a couple of orders and the first order was for the west side and they all have 270.... i'm in the southern US, so just a tad high on the shgc. not terribly off, though. second order was for some east windows with 340.
the 340 windows lost most of that nice white/yellow morning sunrise. its pretty much like wearing some light sunglasses :\
270 has a crazy amount of unfiltered light in the evening.