Which glass unit to choose

Some manufacturers of plastic windows in their advertisements often declare “unique super-glass units”, the manufacture of which is adapted to our climatic conditions, and which allegedly have super-high thermal insulation values.


Not so long ago, there were proposals for “thickened” double-glazed windows, up to 36 millimeters thick, instead of 24 millimeters, which are now widely used. At the same time, it is stated that 36 mm double-glazed windows are 70% warmer than standard double-glazed windows. Professionals understand that the thickness indicator is used specifically for the uninitiated consumer.

In this article we will try to enlighten you on the merits of the issue, taking as a basis the test results of insulating glass units. To begin with, we note that the dependence of the heat transfer resistance on the thickness of the glass units really takes place to be.

We present data on the optimal values ​​of the distance, called “inter-glass”, depending on the composition of the filler gas based on the research of the engineering company Anulis Bertin: air – 15 millimeters, argon – 12 millimeters, krypton – 9 millimeters, sulfur hexafluoride – 6 millimeters. But it should be borne in mind that only 2 components are involved in the calculation, which affect heat loss – heat transfer and convention. In real conditions, about 70% of all heat losses depend on radiation due to the technical characteristics of glasses used for the manufacture of insulating glass units. The inter-glass distance does not matter here. It follows from this that if single-chamber double-glazed windows are used, which consist of two four-millimeter glasses, then it makes no sense to make a double-glazed window thicker than 23 millimeters. However, if we are talking about double-glazed windows, which consist of three glasses, then the situation is somewhat different..


If we look at the data from several different sources, we will see that the resistance to heat transfer in double-glazed windows of different designs is different.

In double-glazed windows with the construction 4-6-4-6-4 (24 mm), it is located in the interval between 0.45 and 0.51 m2C ° / W.

And in double-glazed windows with the construction 4-12- 4-12- 4 (36 millimeters) – in the interval between 0.52 and 0.54 m2C ° / W.

This spread is most likely due to the fact that the manufacturing quality of insulated glass units varies. Also, the test methods are different..

24 mm insulating glass units have a Ro value of 0.47 m2C ° / W and 36 mm insulating glass units have a Ro value of 0.53 m2C ° / W.

This means that this indicator increases depending on the thickness of the glass units by less than 13%. Filling a 24-mm glass unit with argon also increases heat transfer resistance, which increases its service life.

At the same time, a change in Ro within these limits does not affect the heat balance of the entire building. However, when replacing standard glass in a 24 mm double-glazed unit with energy-saving glass, it is possible to obtain a Ro value equal to 0.72 m2C ° / W, that is, by more than 50%.


The next important characteristic of insulating glass units is the temperature measured on the inner surface. This indicator is taken into account when determining for sanitary and hygienic requirements. The lower the temperature, the higher the tendency to form condensation.

The difference in temperature measured in the center of a 24 mm glass unit and a 36 mm one is usually 2 + 3 С °. The difference is quite noticeable. But, as practice shows, in most cases, sweating occurs exclusively along the perimeter of the double-glazed windows, it is most pronounced in the lower part. If the outside temperature is low enough, then the condensation can freeze, frost and ice form. Customers present complaints in these cases, which are quite natural.

This phenomenon is understandable. The heat transfer increases due to the fact that the thermal conductivity of the dividing frames is high enough. At the bottom of the glass unit, additional cooling takes place, associated with convective heat transfer in the inter-glass space. Simply put, cold air, descending along the outer glass, then turns, in contact with the inner glass. In this case, it cools. Then, with gradual heating, the air rises back up. It is due to the cooling of the lower part of the glass unit by convective air flow that condensate falls on its surface in the lower sector of the window.


Unfortunately, we do not have data on how the thickness of the glass unit affects the convective component of the heat loss that occurs. It is assumed that here a significant role is assigned to the thermophysical characteristics of the inter-glass space, as well as the difference in the glazing temperatures. To solve the problem of fogging along the perimeter of the insulating glass units, the spacers together with the sealants can be shifted by 10-15 millimeters into the depth of the bindings. This will increase the temperature at the junction of the glass unit and the binding by 3 – 4 C °.

Based on all of the above, we argue that there are no real benefits when using thick insulating glass units. It is most expedient to use them in cases where increased requirements for noise protection are required, as well as special purposes for which thickened glass must be used. At the same time, to achieve the highest performance in noise protection, glass units are installed in the profile using special methods.

What double-glazed windows are recommended for use in modern window structures?

As indicated in the requirements of SNiP No. 11 – 3 – 79, the resistance for window structures during heat transfer is not less than 0.54 m2C ° / W. These requirements are taken into account in designs with a double-chamber 24 mm glass unit. Also suitable are 24mm single-chamber double-glazed windows using low-emissivity glass. It should be remembered that the weight of a two-chamber double-glazed unit is 1.5 times heavier, which means that the load on the fittings of its sashes is higher, but its sound insulation performance is higher. Therefore, in some cases it is necessary to use double-glazed windows with 1 low-emission glass..

“Special” double-glazed windows are often offered exclusively for advertising purposes. In fact, their use does not give any real advantages, but it increases the cost of the order..

It is also important to know that the physical data of insulating glass units alone do not guarantee the quality of installation in window openings. Professional installation work is much more important to achieve comfortable indoor conditions..

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