The owner of this 1994 home had no doubts about the need to have all the existing wooden windows replaced by high efficiency replacement windows. There was so much air leakage from the windows that he could actually feel the draft by standing next to them. To make matters worse, some of the sashes were also beginning to rot.
When examining the closed window from the interior on a bright day, it was possible to see daylight coming through gaps in the frame; a tell-tale sign that the window wasn't sealed well. They were thermal pane windows, but made with clear, low performance glass that lets a lot of radiant energy heat from the sun into the house.
High efficiency replacement windows have high performance glass to block the infrared rays -- or heat part of the solar spectrum -- from coming through the glass, while letting most of the visible light through. In the winter time, high performance glass prevents heat from the inside of the home to be transferred to the outside as well.
The numbers on the labels found on high efficiency windows tell us things like how much air can leak through them and how much visible sun light will pass through. The Energy Efficiency numbers tell us how much heat can be gained through the window and the most important measurement -- the U-Factor -- tells you how energy efficient the window is when you add up the high performance glass panes and the frame; the lower the number, the better.
High efficiency windows can have a U-Factor as low as 0.15, but the windows installed in this project had a 0.29 U-Factor -- a significant improvement from the old windows, which might have had a U-Factor of 0.80 at best.