U-Value is a measure of how much heat will escape through a material, or how bad an insulator a material is. So materials with high U-values let lots of heat through them – Copper would be an example. Materials with low U-Values don’t let heat pass through them, and so can be used to insulate a space. Material wool is a good example. The ideal insulator has a U-value of 0.
The science-y bit: U-Value is the heat (in Watts) lost through one metre square of the material (for a given thickness) per degree Kelvin (or C). The unit is W/m2K
Not to be mistaking for
Americans use R-Value, which is a measure of insulating properties, and so the reverse of U-Value.
Similar to R-Value, used in quilts