Nice to see a great writeup on window tint being used by european soccer teams that walks through window film conception, history, and how it fits into the overall green initiatives taking place across the world.
Ironically the window film industry has been banging this drum for some time. Their argument is that it makes no sense to allow the heat into the building only to have to cool it down. Particularly as modern solar control window films allow high levels of natural light through whilst keeping the excesses of the sun out.
The cost of heat gain can be counted in many ways. The wasted energy used to disperse warm air is the more obvious cost. The cost to the environment created by unnecessary emissions is another. Add to that the reduction in productivity created when temperatures rise, as evidenced by a research study in the USA by Wyon, albeit some while ago, which found that productivity in an office environment dropped by 40% at 75°F compared to 68°F, the most comfortable temperature to work in. Other research from the USA by Vernon found that accidents are 30% more likely to occur at the higher temperature because people become fatigued. Finally, glare from the sun falling on monitors is also a problem with 75% of people that work on computers suffering from some kind of eye strain.