What is the U-value?

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The U-value is the inverse of the R-Value; i.e. you divide 1 by either the R or U value to convert to the other unit. So as the R-Value goes up the U-Value goes down and as the R-Value goes down the U-Value goes up.  So the U-Value is a measure of how well a material transmits heat.

For instance a substance with an R-Value of 2 has a U-Value of 0.5 = (1 divided by 2).

What is the R-value?

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The R-value of a substance is its direct measure of its resistance to transferring energy or heat; R Values are expressed using the metric units (m2.K/W). Basically the higher the figure the better it is at resisting energy transfer, so the easier it is to maintain a difference in temperatures across it for a longer time.
In the metric system, the R value measures per meter squared the amount of degrees kelvin temperature difference required to transfer one watt of energy. So an R value of 1 means per meter squared a single degree difference will transfer one watt of energy. So an R value of 2 will transfer half a watt of energy for a degree of difference.

Usually the R value is given for a certain type and thickness of material as installed (often known as the 'added R value'); i.e. a low density glasswool batts would need to be 130mm installed to achieve an R of 2.5, but only 100mm thick of medium density. Note: We say 'as installed', taking a low density batt that is designed to work in 130mm as installed and squashing it to fit in 100mm will not be the same as using a medium density batt in the first place.

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