Heat can be transmitted in three ways: heat radiation, convection and conduction. Nearly 2/3 of the heat is transferred from the room by thermal radiation. Insulation is measured in U-values, the lower U-value of the window, the greater the insulation.
In addition to poor technical performance, many of the old windows remain not yet sealed, resulting in an intense air circulation and consequently high power consumption resulting low level of comfort.
Breathability of the casings of the building is not just a problem of energy efficiency, this also depends on the quality of indoor climate, moisture and technical problems, mold, work of ventilation systems, noise and fire safety issues. Thermal insulation of the facade should be such that a loss of energy through wall would be minimal, but at the same time, the construction of the wall should be not too costly and should fit the building design. Since window is a part of the facade, in addition to the fact that it lets the light inside the room, it should have a good heat, air and soundproofing quality.
The loss of heat per square meter of the window is four to six times higher than in the outside wall. This becomes especially important for large windows, which are frequently used today in new buildings. Heat loss can be reduced by installing energy efficient windows.
Provided that the new windows installed technologically correct, heat loss through windows of the building can be reduced 35%, and room temperature can be raised to four degrees. Given that, for example in a typical apartment building, windows have approximately 20-25% of the façade surface, then, having a balanced and adjusted heating system, replacement of the windows will pay for itself already for four years.
As a rule, double glazed window is equipped with double glazed glass package and the inner glass is selective. The index of isolation called U for such window is 1.4 W/m²K. In addition to the type of glass and gas, increasing of the width of the intermediate plates can result window insulation improvement by 10%. But the width of the intermediate plate can not be increased endlessly because of convection, which occurs in the air after the thermal effect reached. The maximum width of the intermediate plates 16-18 mm. If third glass is added to the glass package then a second air space is formed and we will reach even smaller values of U.
The most common glass for better insulation and to achieve low power consumption is a selective glass.
Selective glass looks like usual, but one side of which is covered by special layer of metal oxide. This coverage has minimal impact on transmission and reflection of light. Selective glass is significantly reduces the loss of radiant heat coming from the room through the window. In particular, selective glass prevents the spread of long-wave thermal radiation formed by heating systems, and at the same time allows short-wave radiation (light) to get inside . The use of selective glass in the glass packages reduces heat loss through the windows up to 30%.
Selective glass is installed into the glass package as an internal glass in the way that coated surface remains inside the glass package. With this arrangement, the inner surfaces receiving the heat, which in its turn reduces the temperature variations and condensation, with no unpleasant cold air coming from the inner surface of the glass. In some combinations, selective glass is installed in the glass packages as an external glass.
Even better results can be achieved by selectively filling the space between the simple and usual glass with gas that is heavier than air, such as argon or krypton. This reduces the convection in the glass package. Currently, the most commonly used gas is argon, an important advantage of which is the price. If the argon is used instead of air in the glass package, then the value of U of such window is about 1,1 W/m²K. Currently, windows sold on the international construction market have U value which reaches 0.8 W/m²K. Generally it is worth to estimate the building wholly because the installation of expensive windows may be meaningless if slopes of the windows form strong bridges of a cold.
In the Estonian climate it is not recommended to reduce the U value of glass below 1.2 W/m²K, since there is a risk of condensate formation on the outer surface of the glass. This risk is especially high during the wet period from autumn to spring. This problem was also discussed by Finnish colleagues, who offered the position that achieved energy savings overbalance the temporary discomfort due to fogged glazing.
When replacing windows, one must take into account the fact that the previously used windows used to be a certain part of the ventilation system of the housing. If you change your windows you should consider the possibility of restoring the flow of fresh air, as the new windows are closed so tightly that the exchange of indoor air after their installation will be not sufficient. Here begins a raw of different problems like resident’s health (excessive moisture, mold), as well as problems with construction structure (moisture, corrosion, destruction of the facade).
There are several ways to solve these problems - the flow of air through vents located in the window or by micro opening or so called winter ventilation, in that case the window is opened on 5-8 mm. If the windows have been installed without these parameters, a good solution is to install air valves on the outer wall.
Thermal insulation of windows to a large extent depends on the installation quality, therefore let the specialists to install the windows, they will do the job quickly, with high quality and responsibility for the outcome of their work.
If insulation of the building is in plans, it should be considered when ordering windows. If you make window basis old sizes, the installation of insulation on the window’s cladding will be difficult. If you do not insulate the cladding, bridges of cold can appear that will turn the entire energy saving level down to insignificant. To avoid this, the smaller sizes of the windows should be expected and calculated in order to be able to thermally insulate the cladding or to install new windows in the same plane with thermal insulation.
Installing new energy efficient windows from Joveld you can significantly reduce heat loss.
"Aknad energiasäästlikuks" - Heikki Parve (KredEx, Energiasäästu kompetentsikeskuse projektijuht)
"Soojustamine"- Energiasäästu portaal, Eesti Energia