The thermal resistance of a material represents the material's ability to oppose the flow of heat. In the case of homogeneous materials, it is the ratio between the thickness and the thermal conductivity of the material; In non-homogeneous materials the resistance is the inverse of the thermal conductance.

The resistance to the passage of heat of a discrete element formed by a layer of homogeneous material and plane-parallel faces separated by a thickness e is equal.

In square meter and kelvin per watt.

Where is the thickness of the layer (m) and (lambda) is the thermal conductivity of the material, W/(K·m).

When the element is not homogeneous, but its heterogeneity is uniformly distributed, such as a brick wall with cement mortar joints, a useful (conductivity coefficient) is obtained in the laboratory, a weighted average of the coefficients of each material, and can be applied to the previous formula.

The resistance value is the inverse of the conductance (C):.

The thermal resistance of an element formed by several layers, each of them made of homogeneous material, is equal to the sum of the resistances of each of the layers:.

When heat passes from a fluid to a solid element (in general, from ambient air to a construction element), a resistance to this step is produced, which varies with the speed of the fluid (air speed), surface roughness, etc. and that is called surface resistance. It has the same dimensional equation as the resistances of the construction elements.

When the element described is in a real situation, with ambient air on both sides, the total thermal resistance R is defined as the sum of the thermal resistance of the construction element plus the surface thermal resistances; It is the inverse of the transmittance (U), or heat transmission coefficient of an element (K). It is verified that:

Or, in the event that you want to study a multi-layered element:.

In both cases they are measured in m²·K·W.