Landlords / Guide

Choosing the right double-glazed windows

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Double glazed windows have a critical role in our lives!⁣

Nowadays, everyone is aware of one's responsibility to reduce CO2 emissions and the importance of saving on heating costs, each year more expensive. Therefore, having good insulation at home is essential to retain heat in the winter and keep your apartment cool in the summer.⁣

The French government, for its part, has taken a stand by offering motivational aids such as Maprimrenov and pushing sellers and lessors to issue an Energy Performance Diagnosis (DPE).

How to identify a double-glazed window?

You can tell by looking at the inside edge of the window. It is double-glazed if you see the two panes of glass separated by a small spacing system. You can also put a lighted match in front of the glazing. For example, if the flame reflects four times, we can say it is double-glazed.

How double glazing works and its advantages?

Double glazing aims to limit heat loss. It can apply to windows, floor-to-ceiling windows, or bay windows.

Double glazing comprises two panes of glass separated by a small spacing, called "air gap," The air gap can also be filled up with a still gas, such as argon or krypton. These gases slow down the heat conduction, making them more efficient at thermal insulation. But they are more expensive than air.

We can further enhance the insulation by putting an insulating connection joint called Warm Edge, which avoids thermal transmission through the two panes of glass.

Other insulation systems to complement double glazing

For example, reinforced insulation glazing is even more efficient at reducing heat loss than standard double glazing. This technique is called Reinforced Thermal Insulation (ITR), which means that one of the interior faces of double glazing has been coated with a layer of metal ions, such as silver oxides. This layer is transparent and acts like an actual screen.

In the case of old windows, we can use the extra glazing technique to add a second pane to the single glazing. This method helps you to avoid changing the entire window.

The "burglar-proof" double glazing is designed to make your home as secure as possible. This type of glazing consists of one or two films, making the burglary much more complex. Twelve times stronger than conventional glazing, burglar-proof double glazing is also an excellent option for thermal insulators and protecting against ultraviolet rays.

If your home is exposed to winds constantly or is located in a region with harsh winters, triple glazing will be a better option for you. This assembly of 3 panes of glass offers thermal insulation up to 10 times more insulating than conventional double glazing.

Discover the "Warm Edge": an innovation in insulation

The Warm Edge is an interlayer located between the window's glass panes that helps to improve the thermal efficiency. It separates the double (or triple) glazed panes and seals off the air cavity between each.

The Warm Edge affects the thermal transmission coefficient and can significantly reduce the phenomenon of vapor condensation on the interior surface of the glazing. The most recent materials used for the glazing spacers are stainless steel or modified polycarbonate.

For a real estate investor, the essential benefit of using a warm edge is to save on heating costs, ranging from 4.85 to 5.6% of the total heating and air-conditioning costs, depending on the climate.

Combine thermal and acoustic insulation with double-glazing

There is no resistance between thermal and acoustic performance. And all double-glazed windows already protect against sound.

For the optimal thermal performance of double glazing, it is necessary to fill the gap between the panes with gas. You can find products like ITR treated pane (micro-silver deposit) or warm-edge interlayer.

The optimal soundproof performance is obtained by two different thicknesses of glazed panes (various resonance frequencies). The principle is to place the thicker pane towards the outside. As a result, it can reduce noise pollution from 25 to 50 decibels.
Paris Rental, a high-standing furnished rental agency, recommends its landlords use double glazing with soundproofing. Since Paris ranks first among the noisiest cities in Europe, professional expats ask Paris Rental's Real Estate agency for a quiet apartment to work and live at home stress-free. Therefore, an apartment with sound insulation has a bonus point for apartment rentals seekers.

Learn more about double glazing

The thicknesses of glass are generally categorized in the form A / B / C:⁣
A: the thickness of the outer glass slide.⁣
B: the thickness of the air or gas layer.⁣
C: the thickness of the inner glass slide.⁣

The thermal transmission coefficient is used to evaluate the performance of double glazing. Symbolized by the letter U, the thermal transmission coefficient must be as small as possible for maximum efficiency.⁣

  • Ug (for "glass"), the coefficient specific to glasses. It indicates the heat loss—the lower the number, the better the performance.⁣
  • Uw (for "windows") is the thermal transmission coefficient that relates mainly to the carpentry part of double glazed windows. The lower the number is, the more insulating the windows are.⁣
  • Sw, for the solar factor: the higher it is, the greater the heat transmitted. There is a different Sw figure for the winter and the summer.⁣
  • TLw, for light transmission: It varies between 0 and 1. The closer the coefficient is to 1, the more transmitted light will be.⁣

Ug and Uw are two essential criteria because the glazing of an old window tends to have high thermal transmittance, making the measuring less attractive from an insulation point of view.⁣

Argon, an inert gas, has a lower thermal transmission coefficient than air and more efficient insulation.⁣ There are other efficient gases that can be used for double glazings, such as krypton, but their cost is too high for daily use.⁣

ITR (Reinforced Thermal Insulation or VIR (Reinforced Insulation Glazing).

Warm Edge: To complete the system a desiccant prevents the moisture in the air from entering between the panes to keep off fogging. These are small, porous beads placed under the moisture-absorbing spacer bar. 

Editor: Claire de Circourt
Sources: Dako,, l’énergie tout, Passivhaus Institut Darmstadt, guidefenêtre,