There is a very big difference between reflective paint and the paint that has an insulating factor such as ceramic microspheres.
Air, or even better a vacuum, is one of the best insulators, next only to inert gases such as Argon. That is why double and triple glass windows work, not because the glass has any thermal value, but because it has air trapped between the pains of glass and air is a very poor thermal conductor. The same applies to any insulation, it is not the material, such as foam or fiberglass it is the amount of air pockets that are trapped within the insulation. That is why insulation has little benefit if you crush it or shove it into a space.
The ceramics microspheres are small hollow balls, with a vacuum in them, hence the vacuum acts as a thermal barrier, keeping heat out or in depending on which direction is hotter.
Reflective paint, has the ability to reflect some UV rays which slows down the heating of objects, however, once the object (paint) is heated it transmits the heat as a thermal conductor. It is similar to a well insulated home, it takes longer to heat it from outside hot air and sun, but once it is heated up it takes longer to cool it down.
As reflective paint is exposed to the elements it picks up dirt and grime. The reflective effect of the paint only works if it is getting directly hit with the UV rays. As soon as there is a layer of dirt or grime the UV rays heat those items and the reflective paint has no value.
The advantage of the ceramic microspheres is that you can paint over it with other paints, it can get covered in dirt and grime and it still performs because the insulation is in the transfer of heat through the ceramic microspheres, the surface has no effect.
If you want a perfectly smooth finish, and are using a paint with microspheres, the manufacturers recommend painting over the insulating paint with an enamel.
Additional information on insulating paint.
- Drafts From Recessed Light Fixtures
- Drafty Door & Baseboards
- Reflective & Insulating Paint?
- Attic Insulation