Windows can have a significant effect on building costs, hygiene, efficiency, and occupant well-being. Traditionally, windows' function was to provide illumination, a view, and fresh air to the inhabitants.
As fully enclosed, mechanically ventilated, and electrically illuminated commercial buildings became the standard in the latter half of the twentieth century, the function of the window in meeting occupants needs to be reduced.
However, there is an increasing awareness that windows' advantages are important and contribute to building occupant satisfaction, safety, and productivity.
The window is a significant structural node on which three factors have a qualitative impact: the glass, the frame and the assembly, which all play a fundamental role in ensuring that the window performs its role correctly and contributes to the perfect insulation of the building envelope. without causing imbalances or mold problems over time.
Finding the right window frame is essential when building a new home, but even more so when renovating, and the goal is to minimize costs while achieving the same results once the job is completed. In this regard, a fourth element is fundamental, it acts as a glue and determines the excellent success of your investment.
If you want to replace the fixtures taking into account only the lowest estimate, avoid reading the following; maybe you'll just waste time. If, on the other hand, you want to understand how to make the best performing choice (and not necessarily the most expensive!) And get certain results without having mold and humidity surprises, I suggest you continue reading this article.
This guide will reflect on the third component, the assembly and the fourth energy redevelopment project.
In a sense, the assembly of a frame is more important than the intrinsic properties of the glass and the insulation of the frame. The new technologies, in fact, allow good or excellent results with various types of materials, both in terms of heat dispersion through the glass and through the windows. Poor assembly will negate the high quality and efficiency of the chassis, just as a poor driver cannot achieve the best results with the best computer.
The reconstruction project is therefore the most underestimated feature, not only by customers but also by most retailers and installers; the design of the envelope is the critical added value that must be considered if you want to obtain a building with high performance in terms of energy saving and thermal comfort.
As mentioned above, windows are a preferential mode of heat dispersion: as much as 20-25 percent of the losses are proportional to the window node.
It is therefore crucial to address this ongoing heat loss. The problem is that the building envelope is a device whose structures are not autonomous. The point where various structures or materials come together is a weak point, especially from an energetic point of view.
If this node is not resolved, a thermal bridge is created, that is, if adequate analysis and subsequent steps are not performed to ensure that the node does not have different thermal behavior from the rest of the structure. A thermal bridge is a major weakness of the system in terms of heat dispersion.
Thermal transmittance of windows and doors
Thermal transmittance is a physical quantity that determines the insulating potential of an element by measuring the amount of heat exchanged by a substance or body per unit of surface and unit of temperature. It is expressed in W/m2K. The lower it is, the greater the insulating potential of the element under consideration—the transmittance increases as the thickness decreases and the thermal conductivity increases.
The thermal transmittance of windows and doors is calculated by adding the individual components' transmittances based on their occurrence on the window's surface.
I recognize that using formulas in a common article has the same allure as a man wearing short white socks. However, I believe that it is critical at this time to better understand the issue, and I believe that it is not even that difficult, given that it appears to be additions.
Please keep in mind that if you wish to adjust your windows, you will never have to perform any kind of calculation. It isn't up to you! The following is just to make it clear that window assembly is extremely critical.
Let's take a look at why:
The total variation of the window frame at a given temperature Uw is calculated by adding:
- The dispersions are given by the glass, calculated by multiplying the total surface of the glass by its unique transmittance value Ug. The Ug value is affected by the following factors: glass thickness, low emissivity treatment, selective treatment, the number of glass chambers, and the filling of the chambers with air or noble gases.
- The frame dispersions are calculated by multiplying the frame's surface by the transmittance value of the material used to make the frame Uf. In this case, Uf is also determined by the materials used to build the frame and is a parameter supplied by the manufacturer.
- The dispersions obtained from the contact between the frame and the glass through the spacer. The spacer's length Ig is then multiplied by the linear thermal bridge value Ψg between the frame and the window. The Ψg value is determined by the material used to make the spacer: titanium, stainless steel, or plastic. The spacer used can have a major impact on the thermal insulation of the window. The content of the spacer determines the Ψg value: an aluminum spacer is not acceptable in a good window. The plastic material will increase the Uw value of the window's total thermal transmittance by up to 10%.
- The assembly dispersions, which are calculated by multiplying the length of the sub-frame lm by the linear thermal bridge Ψm between the frame and the wall.
To sum it up, we can write the formula as:
Uw = (AgUg + AfUf + lgΨg + lmΨm) / Ag + Af
Where;
- Ag is the glazed surface (or area)
- Af is the surface (or area) of the frame
So, what does this formula mean?
This formula shows us that the window transmittance is the number of many variables, some of which are calculated by the manufacturer and others by on-site installation.
Consequently, the numerical value of the transmittance Uw provided by the manufacturer is insufficient. It is, undoubtedly, the first reference to find as it provides a detailed understanding of the goodness of the window frame and provides a value with respect to the intrinsic characteristics of the materials used in the construction of a particular frame.
However, if the declared value is a consequence of a laboratory certification of the goods, it must still be mounted, and this aspect makes the difference!
The heat loss due to assembly, the fourth addition to the numerator, is therefore an integral element capable of canceling the contributions of the three previous elements.
Basically, even if you select a high-performance window, which risks being more expensive, with low transmittance of the positive elements, you will still have serious problems due to bad assembly.
The Ψm value is determined not only by the installation method (external, opening or internal on the batten) but also by the shape of a subframe. An overlay installation, for example, offers more guarantees than a light installation. In addition, the characteristics of the assembly must be tested as accurately as possible to ensure air and humidity tightness.
I'll tell you a little more: to really take up the challenges of installing a window or door, I attended several courses for window makers, assembling the fixtures myself! I can tell you that most installers don't know anything about it.
Unfortunately, the old approach is still prevalent, despite the fact that the windows have changed significantly and are much more efficient than they used to be.
The curative aspect is so neglected that even the Enea body, the national body responsible for energy and sustainable development, fails to address it in a document otherwise well done in several respects. You can double check it by reading the newspaper.
How do I know if I am picking the right window?
How to choose between “utilitarian” and “tailor-made” windows and doors? Can I be sure that I will not have any problems if I purchase a high quality window and door and hire a certified installation company?
No it does not. And now, let's see why.
In recent months there has been a rush to replace windows, due to short-term tax deductions, but without implementing a real overall energy requalification of the building.
The window is just one of the many components of the building system, and the building works the same way as a car: it would be difficult to place a Ferrari engine on a Lancia Y, and there is no point in adding very high -performance without improving the quality and the overall efficiency of the building.
Additionally, symptoms can be seen in unsightly forms such as mold on the wall or condensation on glass or window frames.
Elevated and high-quality windows and doors are used in the design of low-energy or passive buildings, as well as in a global redesign that results in effective and concrete energy savings when addressing all the building's thermal bridges. existing
This is not a laughing matter; these procedures must be prepared, with the guidance of an expert technician, as well as a plan for the order of interventions and communication with the teams on site.
In conclusion, I would like to leave you with a message to always keep in mind when purchasing a window shape: the simple selection of a good window with exceptional thermal characteristics will not give you significant advantages in terms of energy consumption if specific objectives are not set and followed. .
The "reprogramming" of the house and the form of intervention must be adapted to the desired energy group. The window is "simply" a component of the envelope. There is no point in slapping a patch here and there, even if the state pays you a lot of money for it.