What is a insulation U value and how is the calculation arrived at?

Question

I am insulating a cellar and have been given various U values I have to achieve, all companies give their own calculations, but will not show how a result is acheived, which does not give me a choice of materials or combination of materials! only what they will supply. If someone could be unbiased and give me an idea how it's done it would be appreciated

Answer 1

U value is the amount of energy a particular material allows to pass through given a temperature difference of 1 degree C between the 2 sides. For instance, a particular material that has a U value of 0.2 W/m²K allows 0.2 watts of energy (heat) through per square metre when there is a temperature difference of 1 degree C between the warm side and the cold side of that material.

This is inverse to R value, which refers to a material's resistance to the transfer of energy (heat).

What is important when considering U values of materials to be used for insulating a room is not just the U values of the individual elements used, but how the different materials used to build the walls/floor/roof work together to achieve and overall elemental value. There are ways to calculate this, and when you get to grips with it it's fairly straigtforward.

Answer 2

Every material has a R-value. The R-value for glass will be less than brick b/c it allows a greater temperature change. Usually when designing a wall you will have gypsum wall board, 2x4 wood studs, batt insulation, and brick. All these material have a R-value even the air space between the stud and the brick has a R-value. I think a typical wall is around 27 but you really need to get a book with general materials and their R-values. When engineers do all their calculations they use the inverse of the R-value to calculate a U-Value.