(Thermal Window Film applied to large front window.)
The concept of using a film to retain heat is not new and a number of different manufacturers have jumped on the bandwagon with their own versions.
In essence, all the brands block UV, reflect heat from outside to some degree and reflect heat from inside, keeping a home warmer. These are the promises they make. Cost wise, it is not cheap, and with many different ways of "Double" Glazing, it may not be the best investment.
The heat retention is the key point and is based on Low-e principles. These reflect radiation back inside the room. In my own house, I have a couple of panes installed in the lounge, with the rest either left clear, or with just "normal" window film. On a cold day, the glass with the thermal film fogs up very quickly, the rest are still clear to see through. The condensation is caused by the glass with thermal film being colder than the other panes. The moment you have condensation, the effectiveness of the heat retention drops away, at a time, when it should work most efficiently. In fact, you are wasting your money. The other potential negative is caused by the condensation over time. As a rule, no window film likes getting wet, cold... heat up in the sun.... dry.... cool down, get wet and so on. It's called thermal cycling. It often results in premature failure of the scratch resistant coating, and needs replacing.
Another fact to remember: Thermal film needs a heat source to reflect, such as radiation from a fire, heatpump or any other heater. A cold, empty room will not be warmer.
In order to keep you warmer, you might have to first invest in a system to reduce the moisture in your home, before considering Thermal Window Film. The benefits will be felt very quickly, as dry air warms up faster and everything will be dry, and your curtains and walls will not get mould any more.
In my opinion, the concept of Thermal film is great, but the environment must be correct in the first place, before considering this.