The U-value means the thermal transmittance of a building element (e.g. wall, slab or roof structure) with a unit of W/m²K. The U-value takes into consideration thermal resistance of all layers that make up the building envelope, including any air gaps, as well as the heat transfer resistances on the two surfaces of the envelope. The surface heat transfer resistance takes into account a convective and a radiation factor between the surface of the building envelope and the environment.

The U-value expresses the heat flow Q (W) passing through the structure with a surface area of 1 m² and with a temperature difference of 1 degree Kelvin. The U-value is one of the main characteristics of building physics calculations. The lower the U-value of a building element, the better its thermal insulation capacity, so the less heat loss through the given structure.

The R-value (m²K/W) is the thermal resistance of a building element, which is the reciprocal of the U-value (R=1/U)

In order to design and build energy-efficient buildings, it is very important to be able to accurately determine the heat flows passing through the building envelopes. The amount of heat loss through the solid building structure can be estimated with the help of U, R and the so-called psi values (psi values represent those extra heat losses that leave through the thermal bridges and for which the U or R value is not suitable). There are two other ways of heat loss through building envelopes, one is the radiation loss of the glazed structures, and the other is convection, which is the amount of heat transported by the air flow, and which originates either from mechanical ventilation or from the failure of inadequately sealed structures.

This free online insulation calculator follows calculation methods to determine U-values in accordance with EN ISO 6946 and EN ISO 13370 and contains the following:

- effects of thermal bridges,

- upper and lower limit values of the total thermal resistance,

- heat paths and percentages,

- corrections due to penetration of fasteners through the thermal insulation and air voids between thermal insulation boards,

- U-values of constructions adjacent to the ground.

This free online insulation calculator is suitable to calculate U and R values of walls, floors and roofs. The calculator does not need to be downloaded or registered. Click here to access the insulation calculator.

There is also a surface resistance calculator which can be accessed here.