U-factor

In physics, thermal conductivity, k, is the property of a material that indicates its ability to conduct heat. It is used primarily in Fourier's Law for heat conduction.It is defined as the quantity of heat, ΔQ, transmitted during time Δt through a thickness L, in a direction normal to a surface of area A, due to a temperature difference ΔT, under steady state conditions and when the heat transfer is dependent only on the temperature gradient. thermal conductivity = heat flow rate × distance / (area × temperature difference)Alternately, it can be thought of as a flux of heat (energy per unit area per unit time) divided by a temperature gradient (temperature difference per unit length)
See more at Wikipedia.org...