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Thermal transmittance: what it is


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

Therethermal transmission, also calledheat transmittance orthermal transmittanceis crucial in quantifying the energy requirementsnecessary for a building for its air conditioning and therefore constitutes a factor of interest in the evaluation ofenergy class of the building.

Any intervention byrenovationthat goes to improve the conditions ofthermal transmittanceof the components of a building (insulation of the walls, change of fixtures, insulation ...) can benefit from the so-calledEcobonus. 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, thethermal transmittanceassumes considerable importance ingreen 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

Therethermal transmittanceis expressed through thecoefficient Uwhich in practical terms indicates theheat dispersionof a certain surface. Warning! The coefficient ofthermal transmittance Uit is not a characteristic of the material as well as the lambda value indicating thethermal 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 ofthermal transmittanceit 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 Uand you want to calculate theenergy 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 bytransmission 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. Therethermal transmittanceit is calculated from the lambda value (thermal conductivity).

The wall thickness (expressed in meters) is divided by the lambda value of the material. Therethermal transmittanceit 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-calledHeat resistance.

1: 2 = 0.50W / (m2K)

So thethermal transmittance Uof the insulating panel considered is equal to 0.50 W / m2K.

In practice, thecalculation of the thermal transmittance Uit's not so easy. In construction, the building blocks are different, different materials havethermal conductivitydifferent as well as the thicknesses are different and the same thermal resistance can vary.

To learn more, you can read the article onCertification of fixtureswhich refers to thethermal transmittance of the fixturesand labeling.


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