Technical parameters of window films

Total solar energy rejection? Visible light transmission? What do these values really mean? Let’s take a look at the technical parameters of window films.

Window film technical data can be categorized into three groups: values related to energy, to visible light and to other factors. When looking at energy values, it is crucial to know what type of window the film will be applied on, because the data will be completely different in the case of different glazing systems. Manufacturers generally publish their values for single-pane windows, or for dual-pane clear windows at most, so be careful. SkyFol is one of the few brands that publish technical data for several glazing structures, thus providing fair information to customers.

The most important energy value is Total Solar Energy Rejected (TSER), which shows the percentage of energy kept outside by the filmed glass. TSER is calculated from Total Solar Energy Transmission (TST), Total Solar Energy Absorption (TSA) and Total Solar Energy Reflection (TSR), so TSER is the indicator of energy efficiency. We should treat the TSER value specifically for different glazing systems – e.g. the TSER value of the SI15 film on single-pane windows is 79%, while 64% on dual-pane Low-e glasses. This difference comes from the additional energy absorption of developed glazing systems, where rahter exterior window films are recommended. For glasses, it is usually the „g” or SHGC value which is given: SHGC=1-TSER.

Values related to visible light indicate the light transmission and reflectivity of the window film. Visible Light Transmission (VLT) is the percentage of light entering the room through the filmed window. The number of glass panel layers and their metallization has little effect on VLT, but a dyed window does affect it considerably. Visible Light Reflection (VLR) shows the percentage of visible light reflected by the window film. We usually distinguish between VLR IN (reflectivity towards the inside) and VLR EX (reflectivity towards the outside), which are important especially if the given film has a reflective exterior, but a neutral dyed interior layer.

The third group of parameters is that of other technical values. The most important of these is Ultraviolet Rejection (UVR), which shows the percentage of UV radiation filtered by the filmed window. U-Value indicates the heat transfer of the film, which is an important factor related to heat loss in winter. Luminous Efficacy shows if the film is indeed spectrally selective or not: we can speak of a film being spectrally selective with a value above 1.

Online course in Window films, part 11: Fading

A lot of people call me when they are looking for anti-fading UV protection window films. These conversations are generally not very short, as if protection against fading is the topic, it is not enough to regulate the level of UV radiation. Although it is indeed UV which is responsible for fading to the largest extent (about 40%), visible light (about 25%) and infrared radiation (about 25%) have to be taken into account as well. Other factors cannot be affected, so they have to be taken as given: e.g. humidity, the quality of colours, etc. Since clients generally want to reach UV protection without a considerable filtering of light, most often we recommend interior SkyFol P50 and SkyFol P70, as well as exterior SkyFol XT47CV and SkyFol XT67CV films. There are of course cases when only a transparent UV filtering film is allowed, but keep in mind that there is a better solution.

Protection against fading with different window films:

  UV filtering Infrared filtering Visible light filtering
SkyFol S4 99% 12% 14%
SkyFol P50 99% 74% 63%
SkyFol P70 99% 51% 32%
SkyFol XT47CV 99% 84% 53%
SkyFol XT67CV 99% 77% 23%

Online course in Window films, part 10: Low-e

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.

Online course in Window films, part 9: Anti-graffiti

A lot of people confuse anti-graffiti films used in the signage industry with anti-graffiti window films. While the former are usually washable, the latter are so-called sacrificial films, generally not washable, but easily replacable. The layer which would make the film washable would also create a hazy effect, so it cannot be applied on glass surfaces. Anti-graffiti window films are produced with a weakened adhesive, so they can be removed easily and tracelessly. If the film is sprayed over or damaged, a professional can remove and replace it quickly. The film protects the window in an invisible way. These films are generally used on shop windows, and also in public transport, since replacing the window would mean that the given vehicle has to leave traffic for a longer time.

The next part of our series will deal with low-e films and with reducing heat loss in winter.

Online course in Window films, part 8: Thermal stress

Glasses can break for a variety of reasons. Before we take a look at these reasons, it is important to highlight that the vast majority of breakages happen as a result of damages of the glass panel; even a small chipped part below the frame (which is impossible to notice) is enough. Breakages can occur because of the following reasons: tensile stress (the glass breaks as a result of its own weight), mechanical bending stress (e.g. as a result of strong winds), impact stress (e.g. when the window is broken by throwing a piece of stone through it), twisting stress (if the warped frame causes a glass deformity) and thermal stress. Breakage because of thermal stress means that different temperatures and expansions occur in the glass panal, therefore it breaks. This is not an issue with tempered and thermally treated glasses, where all types of window films can be installed, even the most heat absorbant black ones. The next level is that of float glass, where a thickness of 10 mm is critical – you have to take care even with thinner windows, but the majority of films can be installed. Laminated glasses are in danger with a thickness of more than 6 mm, while wire-glass is the worst from this aspect.

Further treatements and the whole structure of the glazing system are also affecting factors. Since the probability of breakage because of thermal stress is directly proportional to energy absorption, dyed glasses and ones treated with a low-e layer are more dangerous than untreated windows. Another aspect affecting thermal stress is the number of glass layers. Nevertheless, factors beside the glass also have to be taken into consideration, such as partial shading of the glass, which also creates temperature differences on the window surface. Interior and exterior shading devices hinder ventilation, and for this reason they are also dangerous. The frame system is also definitive – the more flexible it is, the lower the probability of breakage. The pattern of breakages caused by thermal stress is charactetistic: it runs from edge to edge in a curve, always starting and ending perpendicularly to the edge.

Anyway, if you follow the guidelines of the compatibility tables in SkyFol’s data sheets and install window films accordingly, you do not have to worry about breakages as a result of thermal stress.