We were taught in junior high that hot air rises, the key to that phrase is air. Heat itself moves from someplace warm to someplace cool. A good example of this happens often in cold climates. When standing in front of an old window on a cold night, you feel a chill. This chill is the result of heat leaving your body and moving towards the colder surface of the window glass, heat moving from hot to cold. To slow this movement of heat, we use insulation. By putting on another layer of clothes, we reduce or resist the movement of heat. In other words, when you put on a sweater, you’re adding R-Value.

R-Value is the resistance to heat flow, conductive heat flow to be specific. With R-Value, the higher the number, the more resistance to the movement of heat — so bigger is better. The inverse of R-Value is U-Factor, which is another term that is commonly used to measure thermal performance. 

U-Factor is the rate or speed of heat loss (or gain) that transmits through 1 square foot of building assembly or product (roof, floor, wall, window, etc.) in one hour with a 1°F temperature difference between the two sides. With U-Factor, the smaller the number, the slower the rate of heat loss — so smaller is better. It is measured on a scale from 0 to 1 with most U-Factors being less than one.

A typical window in my very cold climate of Minnesota might have a U-factor of .30.  For some, U-.30 might not be relatable. We can convert U-factor to R-value with the simple formula: 1/U-factor=R-value. That U-.30 window will have an R-value of 3.33.  We can also convert R-value to U-factor by the formula 1/R-value=U-factor.