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.

Egouin- I have been building isolated gain solar additions "sunspaces" in Montana for over 28 years. If you have built one that does not work, it was not designed correctly for the location. They work very well and offer benefits that direct and indirect gain designs can not. I opt for a combination of direct and isolated solar designs for new residential construction. This combination add day lighting advantages and will offset the energy demand a well insulated home by up to 60% in northern climates. Please do not confuse a "sunspace" with a commercial "sunroom". They have very little in common.

For what it still may be worth. The recipe for the highest SHGC and low U is as follows. Double pane Low Iron glass PPG STAREPHIRE or Pilkington Optiwhite ARGON spacer 1/2" (use any good warm edge spacer) I prefer Edgtech. ARGON Hardcoat Low E on Surface#3 Sungate 500 or Pilkington Energy Advantage. Or if you are in a really heating dominated climate (Billings Minneapolis Boston) you should go for triple Pane as follows. Low Iron glass PPG STAREPHIRE or Pilkington Optiwhite ARGON spacer 1/2" (use any good warm edge spacer) I prefer Edgtech. ARGON Low Iron glass PPG STAREPHIRE or Pilkington Optiwhite ARGON spacer 1/2" (use any good warm edge spacer) I prefer Edgtech. ARGON Hardcoat Low E on Surface#3 Sungate 500 or Pilkington Energy Advantage. The Latter will yield a SHGC of .72 and a U value of .2 (per LBNL). You will have to have them made by an IG manufacturer, but the payoff will be great. Also you will need a way to reduce SHGC when it is unwanted (see below) As far as trees or overhangs. Just say no. The compromises are just to great. A small tree without leaves can block as much as 50% of the winter sun. Also a perfectly designed overhang only works perfect two days a year June 21 and Dec 21 and the rest of the time it is again a compromise. The best bet here is to go with movable exterior shading ie. Roller shades or shutters or awnings. Good Luck.

I love it when you talk dirty like that. Just read this last post I knew who it was. Should I elaborate? Wait till I install them on the house, you won't be able to contain my excitement. Just for refrence the numbnuts above is the inventor of an incredible window system yet to be revealed. What I can share with you is one version of his passive solar window that was installed in a home I built last year. The homeowner went through this winter with a total heating bill of $341, right here in Massachusetts. Here is one video I posted on you tube showing how well the windows heat up on a sunny winters day. http://www.youtube.com/user/eebuilder#p/u/5/mVHv8lMlNcQ 

Stay tuned if all goes as planned the newest version of the window will be in a new home early this summer.

Lessons learned. Balancing with window loads, these windows worked so well that at times, even during the winter that a window had to be opened to cool the space. The previous version did not have motorized shades making adjustments was a manual process. The proportion that seemed to work best for this house was 15% window to floor ratio. But with the motorized shades you could use more than 15% and still be able to control overheating.  

Tom Pittsley
[email protected]
www.eebt.org

I am interested in the sunspaces that passivesolar has built without overhangs and would love to review any data collected from those spaces. I have found that a correctly designed overhang will prevent overheating. It does this by reducing the amount of direct sunlight striking the vertical glazing, as the daily temperatures and solar gain increase. In Southwest Montana, we can justify their usefulness and have the data to prove it. It may be due to our late summers and early winters, but I have 26 years of live in experience with this type of solar design. I do agree with his recommendations on windows - Low iron and 3rd surface Hard coat is a very good choice. While a fixed overhang will have a few days of too much or not enough, it can be tuned to the predominant heating and cooling patterns.
As for trees, they do not belong directly in front of the sunspace. If you have a true south orientation you can place them to the southwest and the southeast to help control any late summer overheating issues. This will not interfere with winter gains, if designed correctly. Carefully designed isolated gain solar additions ares very effective in most sunny locations with Heating Degree Days above 4000HDD.