“Insulating Glass Windows: The Simple and Cost-Efficient Solution to Energy Efficiency in Buildings”

Microsoft founder Bill Gates called him one of the most interesting and inspiring intellectuals of our time. Canadian scientist of Czech origin Václav Smil has published over 40 publications dealing with energy and natural resources. Facts with a very detailed description of the issue are central to his work. Rather than assumptions, this native of Pilsen bets on data. His conclusions about saving energy for the home, published in one of his books, point to a simple solution that is still being neglected.

Looking for miracle solutions instead of thinking about the facts

I guess it’s in our human nature. Instead of approaching the issue rationally and simply, we prefer illogical choices. This phenomenon is all the more visible in the area of ​​energy saving in buildings. When it comes to reducing the cost of the energy efficiency of buildings, it is necessary to look at the basic data. In the European Union, houses consume approximately 40% of total primary energy. Almost half of this energy is used for heating and cooling. Keeping the heat inside or outside with insulation appears to be the best logical solution.

If we look at buildings as such, windows have the greatest thermal transmittance per square meter. The physical parameter that we use to describe “thermal transmittance” is called the heat transfer coefficient U [W/m²K]. Although 90% of the window area is made up of glass, more attention regarding thermal insulation properties is focused on window frames. Over the past 20 years, the development and parameters of insulating glass, unlike frames, have moved significantly and it is desirable to take this into account.

Photo gallery:

Transparent and simple energy solutions for buildings

If we were to determine the heat transfer coefficient of an old window with one pane of glass, this value would be approximately 6 W/m²K. A newer double pane ordinary window, separated by 6 mm in the spacer frame, achieves values ​​of approximately 3.3 W/m²K. If we choose a new type of glass, in which the panes are coated with a thin foil affecting the intensity of the passage of infrared and ultraviolet radiation, we arrive at values ​​of approximately U = 2.0 W/m²K. When we replace the ordinary air between the glasses with an inert gas (argon), the value of the heat transfer coefficient decreases to U = 1.1 W/m²K. If we replace the double glazing in the frame with triple glazing, we reach values ​​of 0.5-0.6 W/m²K.

So it’s simple. The lower the value of the heat transfer coefficient, the better the insulation properties. Although it may seem that the use of insulating glass is a common practice today, the opposite is true. Very often, information about the frame transmission factor Uf (from the English frame – frame) is confused with the glass heat transfer coefficient Ug (from the English glass – glass). Their combination gives the heat transfer coefficient of the window Uw (from the English window – window). It should also be mentioned that the thermal insulation properties of the glazed part of the windows are not the only important parameter that should be addressed. You can compare an overview of current window prices here.

Parameters of insulating glasses that many do not solve

The area of ​​windows in modern houses is constantly increasing. When choosing the type of glazing, it is desirable to consider, in addition to thermal insulation properties, other parameters of insulating glasses, such as the solar factor (g). Some modern insulating glasses are able to transmit the thermal component of solar radiation into the interior. The solar factor (g) is a dimensionless value that indicates how much heat from solar radiation passes from the outside into the interior of the house. It is always given as a percentage.

The solar factor is used mainly in the winter months for windows facing south, west, or even east, for additional heating. The thermal component of sunlight penetrates the insulating glass and thus brings solar gains to the house. Given that the heating season in the Czech Republic lasts approximately 250 days (depending on location and altitude), the use of solar gains is more than appropriate. In the summer months, the sun’s rays are deflected from the windows by means of window blinds. They very simply prevent overheating of the interior and at the same time save money for cooling the house.

You can see an overview of all the main parameters of insulating glasses in overview.

More than 90% of new windows insulate insufficiently. Sellers are silent and people overpay unnecessarily

Double-glazed or triple-glazed, or how big a difference to expect?

The following example offers a better idea of ​​how the transmission coefficient affects thermal comfort in the interior of the house. At an outside temperature of -18 °C and an interior temperature of 21 °C, the temperature on the inside of single-pane glass is around 1 °C. If we were to replace this window with a regular double-glazed window (as an example, a 10-year-old standard plastic double-glazed window can be used), the temperature on the inside of the window would be 11 °C. Modern insulating triple glazing with parameters Ug = 0.5–0.6 W/m²However, K reaches a surface temperature of 18 °C. At this temperature, you can sit right next to the glass without any problems and feel almost no difference. The effect of the low temperature difference on the inner surface of the glass also has an effect on reducing the formation of condensate, by increasing the temperature above the dew point. Simple – the window does not fog up afterwards.

An investment that always pays off

Physics provides conclusive evidence, but economics plays a significant role in human decision-making. It is true that windows with insulating glass are 20-25% more expensive and the return on such an investment is usually longer. However, over 40% of all heat loss in houses is caused by windows. By changing ordinary glazing to insulating glass, the energy balance of the house can be improved by up to 30%.

Nevertheless, there are opinions that parameters such as the solar factor are an unnecessary over-standard. Here it is necessary to realize that energy prices will continue to rise and what is valid today will not be valid in ten, fifteen or thirty years. Choosing the right glazing will reduce energy consumption for decades, and it does not require any energy input. So why prioritize investing in new and innovative technologies when this easy solution delivers very good results. What’s wrong with simple insulation?

Source of information: SMIL, Václav. The numbers don’t lie: 71 things you should know about the world. Translated by Roman ŠOLC. Prague: Book Zlín, 2021. Topic (Book Zlín). ISBN 978-80-7662-220-3.