SHGC - REALLY worth it to specify low, moderate, or high?

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Mark
Posts: 67
Joined: Thu Oct 07, 2004 9:26 am
Location: Atlanta

SHGC - REALLY worth it to specify low, moderate, or high?

#1 Post by Mark »

On http://www.efficientwindows.org they make a big deal of the different window design variables (like UV, SHGC, VT, number of panes, etc). If you read enough they even suggest that the windows on the east or west should have a different SHGC than the windows on the north or the south.

Now back to the real world - can I / should I really try to learn enough to specify the different SHGC for my windows based on the window facing direction, the amount of shade coverage, the amount of internal shading, etc.? I got the strong feeling that any dealer I talked to about this would laugh their heads off, and suggest I take my business elsewhere. And I'm thinking even if I could specify different SHGCs for each window, that it wouldn't make an ant hill of difference. I sure it would be real important if I was designing a commercial or industrial building, but I'm not.

What do you all think - just order the windows and let the dealer decide the coatings and UV settings and the spectral screenings and other settings, or am I missing an important opportunity?

Oberon
Posts: 280
Joined: Thu Oct 07, 2004 10:25 am
Location: East of the Mississippi

SHGC

#2 Post by Oberon »

Actually Mark, there may or may not be much that you can do to customize a particular SHGC from the window manufacturer; but knowing possible options is certainly not a bad thing.

The particular Low-E coating that is used in the IGU is what determines your SHGC.

As a general rule, you want a low SHGC number if you live in the South and a high SHGC number if you live in the North.
Softcoat or sputter Low-E tends to have better SHGC numbers if you live in a warm climate and you want to block the sun's heat so that cooling is your main concern...and alternatively, hardcoat or pyrolytic Low-E has "poorer" SHGC numbers which may actually be an advantage if you live in the North and heating is your main concern.

There is some controversy concerning the advantages of solar heat gain in the North, however. The very best Low-E coatings for blocking heat transfer thru the glass also have the very best SHGC numbers (obviously). So while using a window with a lower SHGC number in the North does allow the sun's heat to warm the house thru the window, that same (poorer SHGC) Low-E allows substantially more heat to escape from the house when the sun isn't shining thru the window in question...does the gain from solar heat offset the loss of heat (when the sun isn't shining) thru the same window?

I personally don't know.

I have seen reports that absolutely "prove" that the solar gain advatage when using a high number SHGC IGU is worth much more in heat gain than is lost...The Canadian version of Energy Star is very much in favor of solar heat gain, so that they are quite vocal in stating that windows with "poor" SHGC are preferred in Canada.

And I have seen reports that absolutely "prove" that keeping the heat in the house by using windows with a low SHGC number provides much better energy improvement than the solar gain does...and this one usually includes a potential cooling issue as well, since in most parts of the country air conditioning costs a lot more than heat.

I suspect that until someone actually builds a couple of test houses somewhere in the North and compiles all the data, much of the information on whether solar heat gain is advantagous in the North is pretty much conjecture.

Personally, I rather like sitting near a window in the winter and soaking up the warmth of the sun...but unfortunately it seems that the beautiful sunshine days of winter are far apart and fall somewhere in between weeks of dreary overcast...so maybe part of this argument depends a lot on where you live?

Mark, hopefully all that verbage has actually helped to make this SHGC thing a bit more understandable.
Ultimately, you have to talk with the window manufacturer and see what he has available. Depending on the manufacturer you may not have any real options available. But if you don't ask you will never know.

Will using different options on different windows of your house help? Statistically, I would say yes, but I would also suggest that there are other ways to save energy on operating your house that would make a much bigger impact than "steering" particular windows in different locations.

FenEx
Posts: 553
Joined: Thu Oct 07, 2004 11:18 am
Location: Illinois

Great Question

#3 Post by FenEx »

In the building industry, it usually takes about 20-30 years for concepts and ideas to become action. If you think about it, 30 years ago... windows didn't even have Low-E/Argon... let alone being concerned with SHGC numbers. Those that place 90% of any information on the subject, are invested in the industry. I'll make a suggestion for you to consider which might answer your question. As Oberon stated, many may share a different view for whatever their reasons.

The Low-E coatings that are uses are reflective.... that's their sole purpose. They reflect ambient heat and are indeed the source of reflectivity in radiant heat (SHG). Now, let's say you live in the Northern Hemispere... period.. North or South. In the summer, you will have more daylight and more radiant heat on certain exposures so the SHGC does matter more. In the winter, you will have less... no matter where you live in the states. The Earth is actually closest to the sun on January 2nd and the farthest on July 4th (just a little trivia). The SHG is due to the angle of the radiant heat, not the distance.

It's about angles... which directly affect reflection. Angle reflection directly affects Low E performance... anywhere in the Northern hemisphere but is more pronounced based on location. In the summer, the sun is higher in the sky (greater angle hitting the glass = more reflectivity of radiant heat). In the winter, the sun is lower... less angle = more direct, less reflection and greater absorbtion when you want it. In the meantime, the added resistance to passage of ambient heat provided by Low-E and insulating gas fills work for you and your thermostat 24/7, on all sides of your home, at all times of day.

I didn't write the laws of thermodynamics... but I am thinking some pretty smart guys did... they haven't been proven wrong yet. The factors do make a difference... just how much depends on your location.

Shoot for the best... weigh the options... and out-think those that are asking for your money through your research. Good Luck.

FenEx

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