U-value is a sum of the thermal resistances of the layers that make up an entire building element – for example, a roof, wall or floor. It also includes adjustments for any fixings or air gaps. A building element’s U-value is extremely important as there are certain standards that should be reached according to Building Regulations .
A U-value value shows, in units of W/m²·K, the ability of an element to transmit heat from a warm space to a cold space in a building, and vice versa.
The lower the U-value, the better insulated the building element.
The environmental temperatures inside and outside a building play an important role when calculating the U-value of an element. If we imagine the inside surface of a 1 m² section of an external wall of a heated building in a cold climate, heat is flowing into this section by radiation from all parts of the inside the building and by convection from the air inside the building. So, additional thermal resistances should be taken into account associated with inside and outside surfaces of each element.
U = 1/ [ R = (R1 + R2 +… ) ]
Units are in Watts per metre squared Kelvin (W/m2K).
The U-value of an element depends on all materials contained within it. For example, with a cavity wall, this would include the plaster, plasterboard, blockwork, insulation, cavity and brickwork