Do you want to choose windows that provide ideal thermal comfort and reduce air leakage to the outside? But does that really involve perfectly sealed windows? Perhaps we should pay more attention to other parameters that determine the quality of the window joinery? One thing is certain, the choice of windows is undoubtedly one of the most important investments during the construction or renovation of a house. That’s why we should thoroughly research the market and get to know the most important rules that will allow us to finally choose the window joinery that will allow to keep the best possible conditions inside.
What is air permeability anyway?
When comparing different window models, an important parameter to pay attention to is their air permeability. This parameter has a significant impact on the tightness of the windows, which in turn translates into thermal comfort in the rooms. Air-tightness is usually indicated by classes 1 to 4, where a higher class means better insulation. In practice, this means less penetration of heated air from inside the building and less penetration of cold air from outside.
The division into 4 airtightness classes was introduced in accordance with EN 12207:2001. Window manufacturers carry out air permeability tests during ITT (Initial Type Testing). During these tests, the actual air flow that passes through the window slots under different levels of pressure, usually between 50 and 600 Pa, is determined. The average value of the air flow is then converted into a so-called ‘reference permeability’.
The highest airtightness class (class 4) means that at a pressure of 600 Pa (corresponding to a pressure of 61 kg/m2), the amount of air permeating is less than 3 m3 per hour.
Do we need a perfectly sealed window?
Do we really need perfectly airtight windows? This is a question that has been bothering many builders for a long time, especially in the context of room ventilation. There used to be a perception that airtight windows, through which only a minimal amount of air is able to pass, could impede the proper exchange of air. However, this changed when EN 14351 was introduced.
As a result, it is nowadays recommended that windows should be manufactured as airtight as possible. But what about ventilation in the rooms where such window frames are installed? It is worth pointing out that according to the current guidelines, the exchange of air should take place primarily via special air supply devices, such as a mechanical ventilation system.
There is therefore no need to worry that modern windows will prevent proper air circulation inside the building. At the same time, it must be remembered that a completely airtight window (at least at the current stage of technology) is still a pipe dream. Even in the case of windows with the highest, fourth tightness class, there is always some minimum air permeability to the outside.
An airtight window is an economical window
When choosing windows for your home, it is worth betting on models with the highest air permeability class. Such a decision not only guarantees thermal comfort for all members of the household, but also translates into savings related to heating the house.
Window tightness, as we can see, is becoming increasingly important in the context of meeting the requirements for energy-efficient construction. However, it is important to remember that it is not only the window openings that are responsible for the escape of heat from the house. In fact, inadequate windows can significantly reduce the energy performance of a building, which in turn can have a negative impact on its energy class.
Window tightness is also only one of many parameters to be considered when choosing the right window joinery. Should we require any further information or advice, we can always call on the assistance of experienced advisors. With their support, we will be able to choose a window system that best suits our needs.
What other parameters should we pay attention to when choosing windows for the home?
It is worth bearing in mind that the air permeability of windows is not the only parameter that will define the quality of window joinery. Of course, it has a huge impact on our potential savings, but it is not the only one we should pay attention to. So what else should we take into account?
Wind load resistance of windows
Another quite important parameter when choosing window joinery is the one responsible for wind load resistance. This parameter determines up to what wind velocity the window will be able to withstand all structural requirements. In other words, it is a measure of the pressure that a window can withstand before it becomes excessively deformed. Such deformation can in turn then lead to increased air permeability, water leakage from rainfall and even window glazing cracks. In some cases, this parameter is also referred to as wind resistance.
The procedure for testing the wind load resistance of windows arouses a lot of emotion. Although it takes relatively little time, it is nevertheless a kind of ‘destructive test’ of the test sample. The window is then subjected to high pressures, leading to a loss of its original structural properties. This means that, in practice, once this test has been carried out, the window cannot be further used or subjected to further tests.
Heat transfer coefficient (Uw)
The thermal transmittance, also known as Uw, is another important parameter that influences the choice of windows for a home or business. This coefficient reflects the window’s ability to retain heat inside the room, which is crucial for its thermal insulation. By choosing windows with a lower Uw coefficient, you can reduce heat loss and thus reduce the cost of heating your home.
Energy-efficient windows often have a Uw coefficient of 0.8 W/(m^2K) or less. The lower the value of this coefficient, the better the thermal insulation of the window and the lower the energy bills.
Window frame thermal transmittance (Uf)
Another parameter to pay attention to when choosing window joinery is the heat transfer coefficient of the window frame, abbreviated as Uf. This is a key parameter that determines how effectively the window frame insulates heat inside the room. In contrast to glazing construction, the window frame can be less airtight, resulting in a higher Uf value. For energy-efficient window models, the Uf value fluctuates around 0.8 W/(m^2K).
The number of chambers, the insulation material used in the frame and the thickness of the timber are all factors that ultimately have a significant impact on the heat transfer coefficient of the frame. The more chambers and the better the insulation material, the lower the heat loss through the frame. When choosing windows, it is important to pay attention to the Uf value in order to ensure adequate levels of thermal insulation and energy savings in the long term.
Heat transfer coefficient of the glazing (Ug)
The heat transfer coefficient through the glazing, denoted as Ug, is another important parameter affecting the overall insulation of the window. A higher Ug value means greater heat loss through the glazing. In the case of energy-efficient models, this value is approximately 0.5 W/(m^2K), which indicates high thermal insulation of the glazing.
It is worth noting that during the use of windows, the heat transfer coefficient of the glazing may gradually decrease. This is due to the natural process of gas escaping from between the panes (if the windows are double-glazed) or the gradual deposition of moisture inside the glass. Therefore, even if the Ug value is initially low, it is important to regularly check the condition of the windows and maintain or replace them if necessary.
Soundproof and watertight
Finally, we must not fail to mention the watertightness and soundproofing of windows. Watertightness defines the window’s ability to prevent water from penetrating the interior, especially during heavy rainfall. Class 5A windows are the best choice in this respect, guaranteeing that the interior will remain dry even during rainfall with wind speeds of up to 65 km/h.
The Rw parameter, on the other hand, is responsible for the window’s soundproofing, i.e. its ability to reduce external sounds. In multi-occupancy buildings, especially those located on busy streets or in areas with heavy traffic, high sound insulation is crucial for the comfort of the occupants. It improves the quality of life by reducing outside noise, which is especially important for ground floor residents. It is therefore worth paying attention to the Rw parameter when choosing windows to ensure a peaceful and quiet environment.
