Performance metrics & physics
Thermal Conductivity Glass
Glass thermal conductivity (about 1.0 W/m·K) means heat passes through windscreens primarily by radiation, not conduction — which is why window film targeting radiation has such large impact.
Glass is a moderate conductor of heat. A typical 5 mm laminated windscreen conducts heat at about 200 W/m² per degree Kelvin temperature difference across it.
However, the dominant heat input to a parked car cabin is radiation, not conduction. Direct solar input through the glass area is 10–20× larger than conductive transfer.
This is why window film makes such a large difference. It addresses the dominant mechanism (radiation) rather than the minor one (conduction).