ᐅ New windows: double or triple glazing for a building constructed in 1995

Hello everyone,

We will soon be renovating a part of our single-family house. As part of this, new windows need to be installed in that section because the rooms are being reconfigured and old windows will be bricked up.

Now we are wondering whether to choose double-glazed or triple-glazed windows. We understand that triple glazing is generally more efficient. Our main concern is whether "too airtight" windows could possibly lead to thermal bridges and mold growth, for example, if the windows are more airtight than the masonry. Additionally, I have heard from acquaintances that windows sometimes fog up and generate moisture more quickly compared to the more permeable double-glazed windows.

Our house was built in 1995, and the current windows are double-glazed PVC windows. According to the building specifications, there is 0.06 m (2.4 inches) of mineral wool insulation with a 0.24 m (9.4 inches) calcium silicate brick wall. This results in a thermal transmittance (U-value) of 0.48 W/m²K.

What I should add is that due to the renovation work, the façade in the affected areas will be redone, and an external thermal insulation composite system (ETICS) with a plaster finish will be applied over the existing brickwork. Therefore, much better insulation will be present in those areas. Only in this part of the house (kitchen and living/dining area on the ground floor) will the windows be replaced. Do you see any issues arising from having improved insulation only partially?

Thanks for your help and best regards!

Have an energy consultant calculate it for you, or do the calculations yourself using the Ubakus.

SoL schrieb:

Have an energy consultant calculate it for you or do the calculations yourself using Ubakus.

The issue isn’t about calculating U-values or efficiency, but rather possible mold formation and so on. The window should somehow fit with the building.

And this is exactly where U-values come into play. It’s about ensuring that windows and walls are compatible.

Otherwise, it’s nearly impossible to assess this remotely. All we can do then is share common sense advice like "Proper ventilation prevents mold!"

FCBenne04 schrieb:

Our main concern with the question is whether "too airtight windows" could potentially cause thermal bridges and mold growth, for example if the windows are more airtight than the masonry.

Thermal bridges occur when there is inadequate insulation somewhere. Besides the classic exterior corner (due to geometry), these can be caused by penetrating elements such as balcony supports. Good windows do not cause thermal bridges.

FCBenne04 schrieb:

Also, I have heard from acquaintances that their windows fog up and cause moisture buildup faster compared to less airtight double-glazed windows.

This is also incorrect. A window—like a wall—will fog up when the interior surface temperature falls below the dew point. The dew point depends on air temperature and humidity. Better windows actually raise the dew point temperature.

In practice, most people ventilate far too little, and sometimes also heat insufficiently. This leads to high humidity and a high dew point. Old, leaky windows provide a form of forced ventilation; when they are replaced with airtight windows, indoor humidity rises and mold forms on walls, especially in corners. The solution is not to install inferior windows but to ventilate properly. I would also consider retrofitting a decentralized ventilation system.

Even in buildings from 1995 with a thermal conductivity of 0.48 W/mK, there is generally no risk that the windows perform better than the walls. Therefore, condensation is still likely to occur around the windows.

Roughly speaking, triple-glazed windows have been allowed since the first thermal insulation regulation of 1977.

A proper ventilation strategy should of course be planned to remove moisture from the house. This is less about double or triple glazing and more about the fact that new windows are tighter than old ones (due to more or better seals). The simplest way is to buy a few hygrometers and ventilate if the humidity levels become too high.

Partial insulation of individual walls is usually not a problem. However, if less heating is used due to the new insulation, the old walls may become colder than before, potentially causing condensation. But for buildings from 1995, I see fewer issues with this.

Consulting an energy advisor for possible funding of the measure is always a good idea.