Why absorption matters when you install window films!
There is increased attention for the absorption rating of window film, and any window film installer and expert should understand why absorption matters, and why this is becoming a crucial technical parameter to analyse and understand.
Let's start from the beginning
Basically, when the sun shines and sun waves hit the glazing of a building, there are three things happening:
(drawing for regular double pane glass)
The transmission, reflection and absorption percentages of solar energy are always equal to 100%, and for 3 mm single pane glass the values are as follows:
- Transmission: 83% of the solar waves are immediately transmitted to the inside.
- Reflection: 8% is instantly reflected to the outside
- Absorption: 9% is first absorbed by the glass, and emitted to the outside (6%) and inside (3%)
The g-value improvement by installing a window film is the result of changing the transmission, absorption and reflection of the visible light in combination with the total solar energy, as follows: Higher transmission = less solar protection
Higher absorption = higher emittance of solar energy and heat inside the building= less solar protection
Higher reflection = more solar protection
Most of the conventional solar control films e.g. vapour coated, dyed and ceramic films, absorb a larger amount of the infra-red solar heat, and make the glass panel very hot.
Superior spectrally-selective films have high infra-red heat reflection capabilities that keep both the glass and interior of your building at a comfortable temperature.
The role and impact of solar energy absorption
The higher the absorption, the more the glass pane will warm up and emit heat to the inside.
Obviously this is added to the transmitted solar energy when calculating the total amount of solar energy that enters the building.
When installing window film on the inside of double or triple glazing, a high absorption can cause thermal glass breakages because of excessive heat build-up in the glass pane. Therefore, high absorbing window films cannot be installed on double or triple glazing units without risk.
As glazing systems become more complex, with low-e coatings being applied in between triple or double glazing, thermal glass breakage is becoming a significant risk when installing window film on the inside.
This is why more and more manufacturers are promoting the installation of outside window films, and although there is a minor efficiency gain, they are a lot more complex to install and the overall lifetime of an outside window film is always shorter.
Does this mean that we cannot install anymore on the inside of double and triple glazing?
No, it does not. It simply implies that you are obliged to verify the window film's absorption before installing on the inside of more complex glazing systems.
There are also several tools that help you to analyze the film/window compatibility, such as Window Optics or Vitrages Décisions.
An overall rule of thumb is that, when the window film's solar energy absorption does not exceed 28% (measured following EN410 on 4mm glass), it is likely to be safe to install on the inside of complex glazing.
You can also always ask your supplier for assistance, they should be able to advise and help you with the film/glass analysis.