SHGC
http://www.replacement-windows.com/windowbb/viewtopic.php?t=39
Posted: Mon Oct 11, 2004 12:07 pm Post subject: Oberon
WS, this is a reply that I posted on "Mark's" SHGC question.
Perhaps it applies a little bit here as well.
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 - if you want that warm-sun feel... (added later)
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.
How can I find out more information about my options?
When you request your free windows quote you will receive the Free Insider’s Guide to Replacement Windows. Giving you the chance to learn even more about your options and speak with a licensed local home improvement professional in your area.