One common question we receive from homeowners who are educating themselves on replacement windows is how to understand 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 it 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 in. Most manufacturers have run NFRC (National Fenestration Rating Coucil) thermal performance tests on assembled units, which are a much better indication of the unit’s overall performance, compared to glass or window frame values alone.