U-value

Figure 1. Certain materials will have a higher R-value, thus corresponding to a lower U-value. Polyisocyanurate insulation is one material with a relatively low U-value.^[1]

The U-value of insulation is a value that is used to measure how well a specific type of insulation can resist heat flow. The lower the U-value, the more effective the material is at preventing heat transfer.^[2] The U-value is very closely linked to the R-value, as it simply represents its inverse. The U-value is also closely connected to the idea of thermal conductivity, which is a property of a material that determines how well it conducts heat. Windows, doors, and skylights all have a specific U-value associated with how well they insulate.

U-values are used more frequently to determine the insulation properties of windows specifically. A U-value is typically a low number as it represents how much heat energy is lost or gained by the window, and thus a low U-value would be more desirable.^[3] As well, U-values are typically used in systems where multiple materials are used, such as in windows whereas R-values are used to determine the insulating capabilities of only one material.^[3] Low U-factors are most important in heating dominated climates, although they are also beneficial in cooling dominated climates as windows with better insulating properties can resist heat flow more efficiently.^[4]

Calculating U-value

When calculating U-value, it is important to know the thickness and area of each building material used so that the total U-value of an assembly can be accurately determined.^[2]

Mathematically, U-value is calculating from a measuring heat flow across a surface where each side is at a particular temperature.^[5] If the R-value is known, it is more simple to simply calculate the inverse of the R-value to obtain the U-value.^[2] However, if it is not known the U-value can be calculated as: