For south facing windows that see winter sun (and a desire for solar heat gain) but are protected from the hot summer sun via eves or awnings, I would choose Cardinal Low E 179 which has a fairly high solar heat gain and a decent U value. I wish that I could get them where I am building in the south for the south side of my new home, but they are virtually impossible to find down here unless you want to pay an arm and a leg to get them.

I would recomend a window coating called ZoE by Weathershield windows and doors. They hold the pattent on this product. It is a coating on the window but goes beyond low-E.  I have installed this on my custom homes in Tropical and Cold climates. I would suggest you look at the web site. The product is very impressive. Not many people know about this product, it has been around for less than 2 years. Unfortuently the window manufacturers have cut their reps down to the bone. And many builders are not aware of the product.  Hope this helps.

To a certain extent, you will always have a trade-off between low u-value and high SHGC.  Ultra-low u-value windows will have lower lower SHGC, and vice versa.  I'm currently looking at Serious Windows 925 series.  They have a very good combination - u values down to .11 and SHGC up to .50.  This is the best I've seen, but they are expensive.

Keep this in mind - you didn't mention anything about overhangs.  A high SHGC window will be great in the winter, but without a way to control the heat coming in, you may be creating too much heat in the summer.  It may be as simple as blinds, but it's something to keep in mind.  I'm writing this because I'm trying to find a good solution for that exact problem for the house we are building.  I have 36" overhangs on the south side of the house, but it's a walkout basement and there are no overhangs over the south-facing basement windows.

I wouldn't recommend Zo-e-Shield glazing if one is interested in solar heat gain.  These have a very low Solar Heat Gain Coefficient (SGHC).  It doesn't appear to have an exceptional U value either.  What is the advantage of it over options?

I've been looking at the Weathershield Visions 3000 windows since they are available locally.  Looking at their specs for their glazing options for a single hung window, Zo-e-Shield:  U=0.28, SGHC=0.22.

As a comparison, the same window with Low E 2 w/argon:  U=0.29, SGHC=0.33.  That is still not a very high SGHC, but it is 50% higher than the Zo-e-Shield.

On the other hand, Cardinal Corp claims that their Low E 179 glazing has a U=0.28 and SHGC= 0.70.

Bryan,

I hate to burst your bubble, but sun rooms aren't very green.  They are energy sucking beasts no matter what windows you use.  That said, they sure are nice and I'll offer what advice I can. :-)

If your sunroom is south facing, you are likely to overheat the room, on a bright sunny day, regardless of the season.  In summer, you are going to have to find a way to stop the sun from coming in, or the room will be unusable without a LOT of air conditioning.  Blinds might do it, but I'd have concerns about overheating the glazing and causing seal failures.  A retractable awning on the outside might help here. 

In the winter, you will still be prone to overheating on bright sunny days.  You might consider something that draws heated air from that room and distributes it throughout the house.  When the sun goes down, you are going to need something to keep all that heat in.  Motorized window quilts perhaps? 

I did the math for my in-laws sunroom a few years ago.  If I can dig it up, I'll post it.  From what I recall, the room never let in more BTUs than it let out in the winter.  It was a significant source of heat gain in the summer.

Again, they sure are nice, but you will pay for it with heating or cooling bills every month.

Good luck,
Ed

And if you put any kind of tinted film on the inside of the double pane windows, they will overheat and break the seal!

Something else that one could do is plant nice size trees along those sides of the house that you would like to block the sun in the summer.... in the winter, the leaves fall off of the trees allowing the sun to shine through to take advantage of the solar heat gain.

are there seasonal cling type films for exterior windows? Ones you can put on for the summer? That might be a good solution.

Get whitewash made for greenhouses.  It is opaque and blocks much of the light and cuts down on sumer heat.  In the winter, You remove it off with a cleaning solution.  Cheep, ugly, but effective

Eric

First, thanks to everyone for all your comments. They have helped me out significantly.

After a few phone calls, it turns out the sunroom we're interested in uses Cardinal Low E 366 windows that have low SHGC. Those are great for east or west windows, but the Cardinal Low E 179 window would be best for the south. That's not an option through the sunroom company. Talking to them, they said they use one window for everything and can't even order the Cardinal Low E 179.