In winter, when the room is heated, it is highly important to have as little heat loss as possible. By heat loss we mean the heat that radiates towards the outside through the glass. An important milestone in reducing wintertime heat loss was the increase of layers. The U-value shows the heat loss quantity in W, per m2 and hour, weighed by the difference between the interior and exterior temperatures. Dual, and nowadays even triple layers reduce heat loss considerably. Beside increasing the number of layers, metallic coatings can also be applied on the glass, which reflects longer wavelength infrared radiation (this is a much higher range than energy coming from the sun, interior temperature radiation is between about 5,000-30,000 nm, depending on the temperature inside. This range is absorbed by the glass and then radiated towards the colder environment. By applying a reflective layer, manufacturers can reach outstanding results: in case of a dual pane window, the U-value is decreased from 2.8 to 1.4). This absorbing and re-emitting characteristic is called emissivity, therefore the name of such glasses: low-e, meaning low-emissivity. There are low-e films as well, which make the amissivity and the U-value of the glass better by different metallizations and by the ordering of these layers. Two things, however, have to be pointed out. With dual pane low-e glasses, low-e films can decrease the U-value only marginally, while their efficiency with triple pane glasses is negligible. Several manufacturers offer darker low-e films with a visible light transmission of 15% and 35%, which strongly decrease irradiated energy as well. SkyFol offers two low-e films: SkyFol E50 and SkyFol E70. The latter is a truly uniqe product, whose price, however, is quite high because of the precious metals used for it – for this reason, the payback period of the investment is very long.
The next part of our series will deal with the fading of window films.