Virtually every window in this country is rated for its energy properties. We have seen these ratings, U Value (Heat Transfer Coefficient), R Value (Heat Resistance Coefficient), and SHGC (Solar Heat Gain Coefficient). Any home builder, owner or architect can look up the definition of these ratings and which specifications will satisfy building codes or construction requirements. However, the big question remains, WHAT DO THESE RATINGS MEAN? Claims are abound that a certain window will "save up to 50% energy." Really?
What consumers have to understand is that these ratings are derived from lab testing of a standard size product. Depending on the ratings (residential, various commercial up to architectural grade), a window of a certain size is selected and tested. As a result, the actual window you have installed in your house most likely does not have the properties described on your sticker. This is why you get these advertisements, "save up to..."
At a recent visit to an architect we discussed energy ratings. In this case, the windows we discussed had triple insulated glazing rated at a U Value of 0.14. This is quite an energy efficient window (compared to your builder grade product). The value was derived from a window 36" wide and 60" tall (the test specimen). So, the architect asked, what is more energy efficient, larger windows or smaller windows? In this case, the glass has a U Value of 0.09 (R 11), while the frame and sash materials only had a U-value of 0.25 (R 4). In the particular test window, those values averaged to 0.14. Imagine a huge picture window, 8' by 8'. Suddenly the frame (no sash) becomes a minute part of the surface of the unit. Rather than the sticker U Value of 0.14, this window has an actual U Value of 0.11 or, if the frame disappears in the finished wall, the U Value becomes 0.09. Take a tilt and turn window that is only 30" by 30". The glass area is only 24" by 24". Now the frame and sash material makes the whole unit far less energy efficient than the rated value.
Why is this important? The German government mandates that customers receive not some theoretical energy efficiency, light transmission or structural vale but the real number. Would that not be nice? Only then can you specify the HVAC equipment and wall insulation. Yes, you are right, we (actually they since we know our real values) are only guessing in this country. How much are we wasting on over or under sized HVAC equipment? Your guess is as good as mine!