In advertising publications, you can often see announcements about the use of “unique super-glass units” made specifically for the “harsh Russian winters”, supposedly possessing ultra-high thermal insulation performance. Recently, it has become fashionable to offer double-glazed windows “thickened” to 36 mm instead of the most widely used 24 mm ones. Moreover, it is declared that “thick” double-glazed windows “are 70% warmer than standard ones.” Apparently, such an indicator as thickness is the most understandable to an inexperienced consumer..
Let’s try to understand the essence of the issue using specific indicators of the test results of double-glazed windows. In principle, it is impossible to deny the dependence of the heat transfer resistance of glass units on their thickness..
So, according to the engineering bureau Anulis Bertin, the optimal values of the inter-glass distance depending on the filler gas are as follows: air – 15 mm, argon – 12 mm, sulfur hexafluoride – 6 mm, krypton – 9 mm. But in this case, only two components of heat loss are taken into account – convention and heat transfer. In a real situation, about 70% of the heat loss of glass units is associated with radiation, which is primarily due to the technical characteristics of the glasses used and does not depend on the size of the inter-glass distance. Therefore, it becomes clear that in the case of using single-chamber double-glazed windows (consisting of two 4-mm glasses), it is pointless to make a double-glazed window more than 23 mm thick. In the case of using double-glazed windows (consisting of three glasses), the situation is not so straightforward.
So, according to various sources, the heat transfer resistance of insulating glass units of construction 4-6-4-6-4 (24 mm) is in the range of 0.45 – 0.51 m2C ° / W, and of insulating glass units of construction 4-12-4-12- 4 (36 mm) – 0.52 – 0.54 m2C ° / W. The scatter of data is probably related to the different quality of manufacturing of insulating glass units and the test procedure. If we take into account only the test results of the Bor glass plant (the largest manufacturer of insulating glass units in Russia), then insulating glass units with a thickness of 24 mm have Ro = 0.47 m2 C ° / W, and insulating glass units with a thickness of 36 mm Ro = 0.53 m2 C ° / W , that is, the increase in this indicator is less than 13%. A similar increase in the heat transfer resistance of a double-glazed unit with a thickness of 24 mm occurs when it is filled with argon. This increases the service life of the glass unit.
However, it must be borne in mind that a change in Ro within such limits does not have any noticeable effect on the overall heat balance of the building. On the other hand, replacing ordinary glass in a 24 mm double-glazed window with a low-emission (energy-saving) glass, allows reaching Ro = 0.72 m2C ° / W (an increase of more than 50%).
Another most important characteristic of insulating glass units, which determines the sanitary and hygienic requirements, is the temperature on the inner surface of the glass. It is the value of this indicator that determines the tendency to sweat. (The lower the temperature, the more likely condensation will form).
The temperature of the inner surface of a 24 mm insulating glass unit, measured in the central part, is usually 2 + 3 ° C lower than in a 36 mm insulating glass unit (for the same test conditions). In this case, this is a rather noticeable difference. However, it is known from practice that in the overwhelming majority of cases, sweating is observed only along the perimeter of double-glazed windows, most intensely in the lower part. At low outdoor temperatures, condensate can freeze with the formation of frost and ice, which causes natural complaints from consumers.
This phenomenon is due to increased heat transfer due to the high thermal conductivity of the dividing frames. In the lower part of the glass unit, additional cooling is associated with convective heat transfer in the inter-glass space (the cold air flow descending along the outer glass turns, comes into contact with the inner glass, cools it and, gradually heating up, rises). It is the cooling by convective air flow of the lower part of the glass unit that causes, first of all, the condensation on its surface in the lower part of the window.
We have no data on the effect of the thickness of the glass unit on the convective component of heat loss. However, it can be assumed that in this case a very significant role will be played by the thermophysical characteristics of the interglass space and the difference in glazing temperatures. The simplest and most effective solution “to improve the temperature regime of the edge zones of glass units is not an increase in their width, but the displacement of the spacers together with the sealants deeper into the bindings by 10 – 15 mm. This technique allows you to increase the minimum temperature in the joint zone of a double-glazed window with a binding by 3 – 4 C ° without any other measures.
Thus, it can be argued that the use of thick double-glazed windows in itself does not provide real advantages, both in terms of improving sanitary and hygienic conditions, and improving the thermal balance of the premises as a whole. Thick double-glazed windows are most advisable to use to achieve increased requirements for noise protection and special purposes with the use of thickened glass. Moreover, high performance in noise protection can be achieved only using special methods for installing double-glazed windows in profile.
What double-glazed windows can be recommended for use?
According to the requirements of SNiP 11-3-79, the heat transfer resistance of window structures must be at least 0.54 m2C ° / W. Most of these requirements will be met by structures equipped with a 24 mm double-glazed window. Single-chamber double-glazed windows 24 mm with one low-emissivity glass are suitable without restrictions. It should be remembered that a two-chamber double-glazed window is 1.5 times heavier and, accordingly, increases the load on the sash fittings, but has an advantage over a single-chamber one in terms of noise protection. Therefore, in some cases, it is advisable to use two-chamber double-glazed windows with one low-emission glass.
The offer of “special” double-glazed windows is often of an advertising nature and does not provide real advantages when used in specific structures, but requires additional costs for the Customer.
In addition, it must be remembered that the actual increased performance of double-glazed windows does not guarantee high-quality filling of the window opening. The professionalism of the installation work is of much greater importance for the comfortable conditions in the premises, but this is a separate big problem.