Thermal transmittance of Windows and Doors: a guide on values, formulas and practical tips

What is thermal transmittance and why is it important?

When it comes to energy efficiency in buildings, thermal transmittance is a key parameter that indicates how well a construction element – such as a wall, a roof or, more importantly, a window – resists the transfer of heat between the inside and outside of a house. 

Technically speaking, thermal transmittance (often referred to as the U-value) represents the amount of heat that passes through one square meter of material for every degree of temperature difference between its two sides. This value is indicated by the symbol U and is measured in Watt per square meter per Kelvin (W/m²K, aka thermal transmittance unit). 

When applied to windows, French doors and entrance doors, thermal transmittance indicates how much heat is lost through the frame and glazing. Window and glazing transmittance values are certified by manufacturers in accordance with strict standards.

A lower thermal transmittance coefficient means higher insulation: less heat escapes during the winter, and less unwanted heat enters during the summer. For this reason, by choosing windows and doors with low thermal transmittance it’s possible to:

• Significantly reduce heat loss compared to old or worn-out windows;
• Keep indoor temperature stable and minimize the risk of mold or condensation;
• Lower energy consumption and cut heating and cooling bills;
• Improve the energy rating and overall market value of your home.

Fattori che incidono sulla trasmittanza termica di un infisso

The thermal transmittance of a window or door, indicated by the symbol U_w, represents the overall result of the window’s thermal insulation performance. This value is determined by several technical elements and construction choices, which include:

• U_f: the thermal transmittance coefficient of the frame of the window or door;
• U_g: the glass transmittance, which covers the largest surface area;
• Ψ_g: the linear transmittance of the spacer, the contact area between glass and frame.

Other key factors that can significantly influence thermal transmittance (U_w) are:

• Frame material: uPVC is known for its excellent thermal insulation properties compared to traditional aluminium frames. However, thanks to modern thermal break technology, even those can now achieve very good thermal insulation performances.
• Type and structure of glazing: selecting windows with double or triple glazing, especially with low-emissivity coatings and noble gas fillings, can dramatically reduce thermal losses compared to windows with single glazing. 
• Quality of installation: even the best window, if installed poorly, cannot deliver the expected thermal insulation. Professional installation is crucial to avoid thermal bridges and air leaks, ensuring the certified thermal transmittance coefficient is maintained.
• Additional components: accessories such as insulated shutters, energy-efficient roller blinds, insulated boxes and well-fitted external shades further improve window insulation by reducing heat loss around the window opening.

Thermal transmittance regulations: limits and climate zones

In Italy, the thermal transmittance of windows and doors is regulated by a specific legislation that defines the maximum allowed values based on the climate zone where the building is located. This means that the most suitable window for a house in Sicily may not be the same as one in Trentino: the law adapts the requirements to the actual local needs.

The Italian territory is divided into six climate zones (from A to F), identified according to local climate and climate severity. Each zone has its own thermal transmittance limits, which must be respected both as legal requirements and to qualify for certain tax incentives.

Below are some practical examples and a comparative table of thermal transmittance values for each climate zone:

• In cities in climate zone E (such as Milan, Turin, Bologna, Vicenza, Verona), the U_w transmittance limit can go from ≤ 1.3 W/m²K to ≤ 1.4 W/m²K.
• In cities in climate zone F (the coldest areas of Italy, such as Belluno and Cuneo), the transmittance limit for windows is even more strict: U_w ≤ 1.0 W/m²K.

How to calculate thermal transmittance: formulas and more

The thermal transmittance value U_w of a window or door is the result of the average performance of all its main components: the glass, the frame and the edge spacer between them. Typically this number is already calculated and provided by the manufacturer in the product’s technical datasheet, in accordance with European and Italian standards. 

The simplified thermal transmittance formula for a window is as follows:

U_w = {(A_g × U_g) + (A_f × U_f) + (l_g × Ψ_g)} / (A_f + A_g)

Where the parameters mean:

• A_g = area of the glazed part of the window (glass area)
• U_g = glass transmittance (thermal transmittance value of the glazing)
• A_f = area of the frame
• U_f = thermal transmittance coefficient of the frame
• l_g = length of the glass perimeter
• Ψ_g = t linear thermal transmittance of the spacer (edge)

Replacing old windows and doors with new, energy-efficient models allows you to access valuable tax incentives, which may vary from country to country. In order to benefit from these, it is essential that the new windows meet the U_w transmittance limits established for the specific local regulation, and that these values are properly certified by the manufacturer ( both in the technical datasheet and on the purchase invoice).

Average thermal transmittance values by frame and material

As mentioned earlier, the thermal transmittance value U_w varies according to the window frame insulation level, the materials used and the technical features of each system.

Every home has specific needs in terms of windows thermal insulation: factors such as building orientation, climate zone, room usage and local regulations all influence the best choice of windows and doors. Even a small difference in U_w can have significant effects for your home, both on everyday comfort and on your energy bill.

Below are the average thermal transmittance values for the main types of frame material:

Choosing high-insulation windows and doors allows you to meet local regulatory requirements and achieve the best performances over time. For a useful reference, here is a comparative table of thermal transmittance values for DQG products and systems:

DQG: italian-made high-insulating windows and doors

LThe DQG range of high-performance frames in uPVC and uPVC/aluminium includes windows and French doors, entrance doors, sliding systems and shutters designed to meet the needs of any living space, from residential homes to large-scale projects.

Thanks to ongoing innovation and the careful selection of the best materials, DQG systems achieve some of the lowest thermal transmittance values on the market. All our products are certified according to Italian and European standards, with performance data clearly declared in all the technical datasheets for a truly informed choice.

With our thermal insulating Made in Italy windows and doors, you can:

• Significantly reduce your heating and cooling consumption;
• Improve indoor comfort in any season and climate;
• Increase the overall value and energy rating of your property;
• Comply with legal limits to access local tax incentives.

Choose the solution that best suits your needs: DQG technical team can provide you with detailed thermal transmittance values for all our windows and doors, helping you achieve a home that is truly efficient, sustainable and tailor-made for you.