You can lose a lot of energy from your windows. By some estimates, about 70% of energy lost is due to doors and windows. To counteract this loss, there are many different things that homeowners can do. One of them is to install Low-E windows. Low-E windows is low emissivity glass which blocks ultraviolet and infrared light from entering your home. This won’t change the level of light entering, just lower the effect it has on the temperature.
It also reflects the interior temperature back into your home, thus keeping your heat in.
These windows have a very thin coating that does all the work, but you would be unable to see it unless you were using a microscope.
There are different types of coating, all with different ratings on how well it reflects heat. Often, your decision on what type of coating you want will be based on cost, as well as, the climate where you live.