U-value measures how effective a material is as an insulator. The lower the U-value is, the better the material is as a heat insulator.
When we talk about the U-Value of a particular component of a building such as a wall, roof or window, we’re describing how well or how badly that component transmits heat from the inside (usually) to the outside. On a cold day in the UK when we’re warm and cosy on the inside of the building, we will be happier the lower the U-Value is – because it means that our wall or roof or window is quite good at holding-up the heat getting to the outside.
This is a great video that explains U-value:
Our friends over at Eco Merchant have compiled a really good explanation of the insulation properties, you can read it here: