U-Value is the measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction
In other words, a U-value is used to measure how well or how badly a component transmits heat from the inside to the outside of a building. The slower it is for heat to transfer through the component, the lower the U-value. This means that we are looking for a lower U-value.
The table below illustrates the input data and formula required to calculate the U-Value for a wall. Typical components of bricks, rigid polyisocyanurate (PIR) insulation, concrete blocks and plaster are used.
The thermal resistance (R) is determined by dividing the thickness of the material (in metres) by its lambda value (thermal conductivity). In accordance with EN 13165, R-values should be rounded down to the nearest 0.05 (m2K/W)
The U-Value (thermal transmittance) in units of W/m2K shows the ability of an element to transmit heat form a warm space to a cold space in a building, and vice versa. The lower the U-Value the better insulated the building element is.
U-Value is the reciprocal of the summation of all the resistances of the materials. U-Value=1/(R1+R2+R*). In the example above U-Value = 1/(0.10+7.10+0.55+0.05)= 1/7.80 = 0.128 W/m2K