What does the U-Value of a material tell us?
Consider a 1m2 section of material - for example, a window. Outside the window let's say the air temperature is 19°C but on the other side of the window the air temperature is 20°C.
This is a 1°C temperature difference across the material and so heat will try to flow through the material from the warmer side to the cooler side.
The U-value of the window tells us how many Joules of heat energy flow through it per second. (The U-value always refers to 1m2 of material with a 1°C temperature difference).
The lower the U-value, then the lower the heat energy flow through the material. This means that good insulators have a low U-value.
When building a house, it's a great idea to use low U-value insulator materials for the walls, roof and floors. This will reduce the amount of heat lost from the house and so keep heating bills to a minimum!
So the lower the U-value of a material, the better insulator it is.
Keywords: U-value, Heat transfer, Insulator, Temperature difference, Heat loss per second.