According to the European Directive 2010/31/EU, from 1 January 2021, all new buildings should have a nearly zero energy requirement for their heating. The arrangements clearly define the parameters to be observed for the energy that goes into heating and cooling the building, ventilation, and lighting and hot water preparation.

As a result, new technical conditions (WT2021) have been in force since January 2021, which change the requirements to be met by warm windows, both façade windows and roof windows.

The energy management of any building consists of:

The U-value, which determines how much heat escapes from the house to the outside – the lower its value, the less heat loss
The Ep coefficient, which determines the maximum annual demand of the building for non-renewable primary energy for heating, ventilation, cooling, domestic hot water and lighting. From 2021, its value is reduced to a minimum of 70 kWh/m2*year
Thermal insulation of external walls, roofs and floors

New requirements for roof windows and Uw

The Uw value determines the amount of heat transfer through a square metre of a window with a temperature difference of 1 K between the inside and outside of the room.

The lower the Uw value, the warmer the window.

From 2021, windows placed on facades will not have a thermal insulation value exceeding 0.9 W/(m2∙K).

In contrast, roof windows will be able to have a coefficient higher by 0.2, not exceeding 1.1 W/(m2∙K).
Why such a difference?

A glazing unit that is tilted from the vertical position has a lower thermal insulation value than one which is positioned vertically. For example, if the glass is inclined by 45 degrees, its heat transfer coefficient increases by about 40%. As we know, heat always escapes upwards, which in the case of inclined glass increases the intensity of heat transfer in the gap between the panes of the window.

Due to the fact that the pane has the largest share of the window surface, the thermal insulation value of the glazing Ug has a huge impact on the heat transfer coefficient. Manufacturers offer a wide range of 2-, 3- and 4-pane glazing packages filled with noble gases such as argon or krypton.

In practice, this means practically a complete departure from 2-pane roof windows.
Which “warm” roof window should I choose? What should you pay attention to?

the number of panes in the glazing unit (3- or 4-pane windows)
So-called warm glazing spacer bars
additional insulation and sealing sets for the space around window frames. (lines, lines and more lines)
consider the possibility of so-called lowered installation.
use of warm flashings with additional insulation
consider installing an external roof roller shutter

What to do when the attic is too hot? Problem with a high temperature in the attic?

The most effective way to lower the temperature in the loft is to cover the roof windows from the outside by installing awnings to the roof windows. We recommend our article on how to lower the temperature in the attic.

What does WT 2021 mean for investors and how will it translate into the choice of roof windows?

All types of windows will have to have a special design – high-quality glazing packages, multi-chamber profiles and even modern accessories supporting their energy efficiency e.g.: distance frames or a special insulation system.

Investors who have submitted an application and obtained permission to build or modernise an existing building after 1 January 2021 will have to pay more attention to the selection of roof windows, which heat transfer coefficient Uw will not exceed the value of 1.1

Uw<= 1.1 W/(m²K)

Therefore, triple-glazed windows that meet the new standard will be standard from 2021, and investors who have applied for and obtained planning permission in 2021 will have to choose windows that have at least 3 panes.
What about buildings whose construction is already underway or whose building permits were issued before 2021?

Investors who:

received permission earlier, i.e. before 2021, but had not completed construction by then and their home was designed to previous standards, applied for the building permit before the end of 2020 and received the permit decision already in 2021

can rest assured that they will not have to adapt their building to the new regulations. This means that if they plan to use double-glazed windows, they can do so.

On the other hand, those who bought a ready project, which does not comply with WT2021 and has not applied for permission so far, have to adapt the project so that it takes into account the new Technical Conditions.

It is also worth remembering that the new standards will apply to owners of old buildings who decide to expand and renovate the existing facilities.

The market for replacement roof windows is still open, i.e. when the installation of a roof window involves the replacement of an old one with a new one, as such replacement does not require a building permit. Nevertheless, those looking for the best price are advised to plan the purchase of cheap triple glazed roof windows preferably still in 2021, because the manufacturers, including reputable ones such as FAKRO or VELUX, will gradually limit and eventually withdraw double glazed roof windows from their offer.

Triple-glazed roof windows have been available on the market for many years, but they were previously reserved only for the so-called PREMIUM segment, and the price difference compared to double-glazed roof windows was significant. Fortunately, most reputable manufacturers have already adapted their offer to the new requirements, also in the segment of cheaper roof windows. At present, both triple-pane wooden and triple-pane plastic (PVC) roof windows are already available at very affordable prices.