Thermal transmittance (U, but also called U-Value in some places) is the measure of the heat that flows per unit of time and surface, transferred through a construction system"), formed by one or more layers of material, with plane-parallel faces, when there is a thermal gradient of 1 °C (or 1K) of temperature between the two environments that it separates.[1]​.

In the International System it is measured in units W/m²K or in W/m²°C. Its value includes the surface thermal resistances of the faces of the construction element, that is, it reflects the ability to transmit heat of a construction element in its actual position in the building. The lower the U-value, the lower the energy flow between both faces, and therefore the better the insulating capabilities of the construction element. In the case of windows, it depends on the level of profile and glass insulation.

It is used to express the insulating capacity of a particular construction element formed by one or more layers of materials. From a physical point of view it is the amount of energy that flows, in unit time, through a unit surface area of ​​the element, when there is a unit thermal gradient. It is the inverse of thermal resistance.

Unlike conductance, it includes film effects since it is considered a constructive element in a real situation. Its mathematical expression is:

being:.

In which the thermal resistance of each layer is:.

being.

Thermal transmittance is used in construction to calculate the losses (in winter) or gains (in summer) of heat energy. To calculate heating systems, in any of their forms, the starting point is to determine the amount of energy that must be supplied to each of the inhabited spaces in the unit of time (power) to maintain a certain temperature (comfort temperature). This power must compensate for the heat losses due to the construction elements that separate the heated spaces from the outside or from any other environment at a lower temperature, losses that are a function of the transmittance of each of the separating elements and the difference between the interior and exterior temperature of calculation, that is, they depend on the transmittance of the elements that define the room to be heated or, what is the same, they depend on the insulation of the construction elements.

These loads, in the warm season would be profits, but in the case of cooling these transmission gains are a small part of the building's profits, since other important issues must be added (sunlight, internal loads, latent heat...).

To determine the necessary heating or cooling powers, the ventilation loads must be added to these loads.