Here is everything you need to know about efficient windows.  When you buy a new house, or replace your windows, you want to know about two things -

1.  Do they have high, moderate or low solar gain, and,

2.  What is the U-factor?  From this you can calculate the R-value.

New, efficient windows, are designed for different climates.  In a cold climate you probably would want high solar gain.  In hot and sunny places, obviously, low solar gain would be preferable.  High solar gain transmits about 71% of solar heat to the interior, yet protect heat from escaping the house.  Low solar gain offer only about 27% solar-heat transmission.  (For moderate solar gain windows it would be about 39%, by comparison.)

How do they do this?

With low-E coatings.  Short for low emittance, low-E coatings are microscopically-thin layers of metal or metal oxide, virtually invisible to gain a good U-factor and reduce heat flow. They let in as much as 75% of visible light.

The U-factor is very similar to the R-value of your home's insulation.  The lower the number, the higher the insulative value.  For example, a triple-pane window would have a U-factor of .15.  Divide this number into 1 to get the U-value, or the equivalent of an R-value of 6.67.  Excellent!  (Your walls are probably R-13, maybe R-19.)

Virginia is in the North Central sector of the country for Energy Star ratings.  In Virginia an Energy Star window has a minimum U-factor of .30 and Solar Heat Gain Coefficient (SHGC) of .40 or less.

The U-factor and SHGC is why when you put your hand on the interior pane of an efficient window bathing in hot sunlight the pane feels cool.  Touching it is a good way to judge an efficient window.

This low-E coating may be invisible to our eyes, but not if you pass light through it.  Look at the photo above!  Those are good windows!

The lower part of the photo is where light is passing through no glass.  The upper part light is passing through one window, and the middle through two windows.  Remember, each window is made up of two panes of glass with the low-E coatings between.  The diffusion, or transmission, of light through these windows is clearly seen in the photo.  Almost like a pair of sunglasses!

We have all seen windows where the seal has broken.  We can tell because there is condensation or cloudiness between the panes.  This anomaly will get worse and worse.  The only fix is to replace the glass.

To the right is a thermal image of a window with a broken seal.  Missing its gas the two panes are drawn together and the cooler air outdoors is visible as a purple area where they are nearly touching.  We thermographers call this a bull's eye pattern.

That all by itself is testimony for having a thermal image examination of your house during any energy audit.  No self-respecting energy auditor, like Jay Markanich, would forget thermal imaging!  By the way, I do a thermal-imaging sweep of the home on one-year inspections too.

My recommendation:  When a home is advertised as having efficient windows, ask the U-factor and what the solar gain is!  You will impress, or befuddle, the sales people!  And maybe your construction supervisor too...