Energy savings by reflection – GreenBuildingAdvisor

For walls, the heat transfer by radiation is going to be determined by the temperature difference and the albedo of the surface. I’m not sure how different the albedo of a white painted wall is going to be from a reflective surface, so let’s set that aside and look at temperature. Is the surface of a wall going to be at room temperature? No, because there is a film of air covering it that contributes some R-value, we usually assume R-0.5. The temperature drop over that film is going to be proportional to the R-value of the film compared to the total R-value of the wall, and proportional to the temperature difference between indoors and outdoors.

As an example, let’s assume it’s 20F outdoors, 70F indoors, and the wall has a R-value of R-20. There’s a 50F drop from indoors to outdoors, the drop over the film is going to be 0.5/20=2.5% of that, or 1.25F. So the surface of the wall is at 68.75F. When the rest of the room is at 70F there’s going to be pretty negligible radiant flow. The surface of your skin is at 90F, so there will be more, but there won’t be appreciably more at 68.75F than at 70F, which is what you’re feeling from the rest of the room.

If the wall is poorly insulated, say R-5, then the drop would be 5F, which makes radiation more of a factor.

Windows are a different case, because light can go straight through glass, and radiation is light. But the heat is going to be in the infra-red spectrum. Modern windows will have an IR blocking coating.