In an era in which the energy performance of the buildings is a design and regulatory imperative, the insulating window is much more than a transparent element: it is the true technological heart of the window. Vitri thickness, composition of the gaseous intercavine and type of spacer (gully) are variables that decisively determine the performance of a window, in particular the UG thermal transmittance.
The influence of the thickness of the slabs on thermal performance
It is a widespread opinion that increasing the thickness of the two glass plates that make up the insulating window better the thermal performance. However, the author in the volume dismantles this belief: the thickness only marginally affect the Ug value, and rather the composition of the intercaphae (gas, canaline, coating) to determine the effectiveness of the system.
A 4-16-4 window, for example, with air as gas and without low emissive coating, will have a UG value of about 2.8 W/m²k. The same package, but with argon gas and low-e coating, drops to 1.1 W/m²k. And a further reduction is obtained only with Triple glass or more performing gases such as Krypton.
The use of thicker windows, if not requested by static or acoustic needs, may entail:
- Increase in the overall weight of the window, with repercussions on handling, flow rate and laying;
- greater energy absorption and risk of thermal shock breaks;
- increase in costs without proportional thermal benefit.
The author’s recommendation is therefore that of not unnecessarily oversizing the slabs: it is better to act on other more influential parameters, such as the type of gas and the presence of low emissives coating.
The gas between the slabs: performance and critical issues
The gas contained in the cavity has a central role in reducing heat dispersions. The main options are:
| Gas | Thermal conductivity (W/MK) | Notes |
| Air | 0.026 | Cheap, less performing |
| Argon | 0.016 | Excellent cost/performance ratio |
| Krypton | 0.009 | Better isolation, but more expensive |
According to Tessiore, Argon is currently the most widespread and effective choice for the residential. Krypton is used in special configurations (for example triple glass with thin cavity), while the air remains a basic solution to be avoided in new buildings.
One of the most neglected problems during the project is the loss of gas over time, which can lead to a significant decay of thermal performance. To avoid it, it is fundamental:
- Choose certified windows according to UNI EN 1279 standards;
- use quality sealing and channels with good barrier to diffusion;
- Avoid transport and storage conditions that stress the sealants.
A 10-15% decay of Argon concentration in 10 years can be acceptable, but beyond this threshold the UG value of the window is compromised.
The canaline: an often underestimated thermal bridge
The canaline (or spacer) that separates the glass slabs has a decisive impact on the perimeter thermal bridge of the window. Traditionally in aluminum, this component has a high conductivity, which generates the phenomenon of the “cold edge”.
To reduce the lateral heat flow, the hot channels are used (Warm Edge), made with low thermal -conductability materials such as:
- Synthetic foams (e.g. PPU);
- Plastic-metallic composite materials (e.g. steel + plastic);
- Thin stainless steel.
The lineic transmittance ψ of the gully can vary from 0.08 W/mk (Warm Edge) to 0.17 W/mk (aluminum), with a difference that translates into a net improvement of the overall UW value of the window.
Combine the parameters to optimize the window
Let’s see an example of comparison between stratigraphies:
| Configuration | Gas | Canaline | Ug (W/m²k) |
| 4-16-4, air, aluminum canaline | Air | Aluminum | 2.8 |
| 4-16-4, Argon, Canalina Warm Edge | Argon | Warm Edge | 1.1 – 1.2 |
| 4-12-4-12-4, Krypton, Warm Edge | Krypton | Warm Edge | ≤ 0.6 |
As you can see, it is the integrated configuration that determines the final performance. The author invites professionals to use online configurators and carefully read the technical cards of the manufacturers to check the UG, G, TL and AE declared values.
Choosing a good insulating window means carefully evaluating thicknesses, gas, canaline, coating and pose. The goal is not only to respect the regulatory limits, but maximize the comfort and durability of the window. The volume of Tessiore provides a precious tool to orient yourself among the multiple available solutions and design performing, safe and lasting windows.
