**Thermal transmittance**: let's find out what this important concept means. The study of**heat exchanges**assumes particular value for the purpose of defining the well-being conditions of an individual within an environment. The**heat exchanges**they consist of the quantity of energy exchanged by radiation, convection and, to a lesser extent, by conduction.

There**thermal transmission**, also called**heat transmittance** or**thermal transmittance**is crucial in quantifying the **energy requirements**necessary for a building for its air conditioning and therefore constitutes a factor of interest in the evaluation of**energy class of the building**.

Any intervention by*renovation*that goes to improve the conditions of**thermal transmittance**of the components of a building (insulation of the walls, change of fixtures, insulation ...) can benefit from the so-called**Ecobonus**. All renovation costs that improve the thermal performance and energy efficiency of a building are entitled to tax deductions ranging from 36 to 65%.

As is clear, the**thermal transmittance**assumes considerable importance in**green building**, where the choice of materials determines the thermal insulation and consequently the energy efficiency of the building.

### What is thermal transmittance

When a surface (which in the construction field can be a wall, a window or any type of frame) is subjected to a thermal gradient (such as the difference in temperature between the inside of the house and the outside), the heat tends to moving from the warmer environment to the cooler one. In this way, windows and walls with a high thermal transmittance (which allow the passage of heat) will cause warm houses in summer and cold houses in winter, making the action of air conditioners more tiring (in energy terms).

### Coefficient of thermal transmittance U

There**thermal transmittance**is expressed through the*coefficient U*which in practical terms indicates the**heat dispersion**of a certain surface. Warning! The coefficient of**thermal transmittance U**it is not a characteristic of the material as well as the lambda value indicating the**thermal conductivity** (another decisive value when it comes to **insulation and insulation**).

A very low U coefficient indicates an excellent construction element because it is not very prone to heat dispersion. The unit of measurement of**thermal transmittance**it is expressed in terms of W / m2K, that is, per watt of energy that are dispersed across a square meter of surface for a temperature difference of one degree Kelvin.

If you have the **coefficient of** **thermal transmittance U**and you want to calculate the**energy dispersion** to which you go up in the course of a whole day, all you have to do is apply a simple mathematical formula. To give you a practical example:

one wall is characterized by**transmission U**= 0.18W / m2 K

0.18W x 1 m2 x 20 K x 24 h = 86Wh = 0.086 kilowatt hours of energy are dispersed per square meter over the course of a day (24h); to know how much the total heat loss amounts to, just multiply this value by the square meters of the wall.

### Relationship between thermal transmittance and thermal conductivity

There is a great correlation between these two values, because on balance they provide us with information on the same topic. There**thermal transmittance**it is calculated from the lambda value (**thermal conductivity**).

The wall thickness (expressed in meters) is divided by the lambda value of the material. There**thermal transmittance**it is nothing other than the reciprocal of the result. To give you a practical example:

an insulating panel has a lambda value of 0.04W (mK) and is 8 cm thick. It continues:

0.08 m: 0.04 = 2 (m2 K) / W

The value "2" represents the resistance to heat transmission, the so-called*Heat resistance*.

1: 2 = 0.50W / (m2K)

So the**thermal transmittance U**of the insulating panel considered is equal to 0.50 W / m2K.

In practice, the**calculation of the thermal transmittance U**it's not so easy. In construction, the building blocks are different, different materials have**thermal conductivity**different as well as the thicknesses are different and the same thermal resistance can vary.

To learn more, you can read the article on*Certification of fixtures*which refers to the**thermal transmittance of the fixtures**and labeling.