U-values measure how effective a material is as an insulator
Thermal Resistance (R value)
In order to measure the resistance to the passage of heat through a specific thickness of any given material, the thickness (t) is divided by the thermal conductivity (k) value. The thickness is always expressed in metres (e.g 90mm would be expressed as 0.09). This value is known as the ´R´ value, and represents thermal resistance, so in this case higher values indicate better resistance to heat loss.
The ´R´ value is expressed in m2°C/W.
R = t / K
Thermal Transmission (U value)
The overall expression of heat loss through a structure such as a wall or roof is known as the ´U´ value, which represents thermal transmission (i.e heat loss). This is expressed in W/m2°C and indicates the heat lost through one square metre of the construction for each degree of temperature difference between the inside and outside of the building. It is calculated by taking the reciprocal of all the ´R´ values of the materials in the construction added together, i.e:
U = 1 / Total R (Rsi + Rso + Ra + R1 + R2 + R3 + R4 etc)
Where Rsi = thermal resistance of the internal surface
Rso = thermal resistance of the external surface
Ra = the thermal resistance of air spaces within the construction
R1, R2 etc = the thermal resistance of individual components
Acceptable (maximum) U values for elements of buildings can be found in the current Building Regulations (Approved Documents L1 and L2) for England and Wales (Conservation of Fuel and Power), Section J of the Technical Standards in Scotland, and Section F in Northern Ireland.