In the absence of a coating, the absorbed heat would be removed by convective currents and by the emission of infrared radiation (wavelength longer than visible light). The presence of crystals or plastics prevents the transport of the accumulated heat towards the outside by convection and obstructs the exit of a part of the infrared radiation. The net effect is the accumulation of heat and the increase in the temperature of the room. See, in solar greenhouse (technique), a more detailed discussion on a solar greenhouse.
The glasses have very little resistance to the passage of heat by transmission (in fact, for simple glazing, the thermal transmission coefficient is considered zero and only the sum of the surface resistances is taken into account), so that, against what Some believe, having two different temperatures on each side, there are notable transmission losses (the glass has a transmittance of U = 6.4 W / m² · K, even higher if it is in an inclined position with respect to the vertical). The result is that the higher the temperature, the lower the heat retention effect, that is, as the temperature increases, the losses will increase and the performance of the system will decrease.