One common question we receive from homeowners who are educating themselves on replacement windows is about U-value and what it is. U-value (or U-factor) is the measure of the window’s ability to conduct heat. In other words, the amount of heat transmitted through the window. The lower a window’s U-value, the better the window is at insulating your home. Generally, heat will flow from the inside towards the outside during the winter. In the summer, it will flow from the outside to the inside. Most manufacturers have run NFRC (National Fenestration Rating Council) thermal performance tests on assembled units, which are a much better indicator of the unit’s overall performance, compared to glass or window frame values alone.