Why do some cars get hotter after being exposed to strong sunlight for a period of time after being coated with window film?

Ordinary window glass neither blocks light nor insulates heat. Tinted, coated glass, while capable of blocking light, does not insulate heat.

A light-colored film that both insulates and blocks light (called "automotive window film" ) has emerged as a new type of glass covering material and is proving popular.

Window film only provides a certain degree of protection and blocking. Although Window film can block a significant amount of infrared heat, it doesn't mean the car interior won't get very hot. Heat can enter the car through other parts of the car, and visible light itself is a heat source.

The factors that increase the movement of air molecules inside a car can be divided into two categories: active and passive.

Active factors refer to factors that directly transfer energy to air molecules inside the car. Passive factors refer to factors that, through radiation, increase the movement of molecules in the air and interior decorations, causing them to release more energy, thereby increasing the temperature inside the car.

window film can only suppress and eliminate the passive factors that cause the temperature inside the car to rise, but it cannot prevent the active factors that cause the temperature inside the car to rise.

The heat sources that affect the temperature inside the car include the following:

(a) Heat generated by sunlight absorbed by the car's roof and structural materials;

(b) Solar radiation absorbed by air molecules inside the car;

(c) Heat radiated by interior components after absorbing solar radiation;

(d) Darker exterior surfaces absorb more heat than lighter ones.

Without window film, the heat is not blocked at all and the car may be even hotter.