Radiant heat: invisible but deadly

On a warm afternoon with full sun a darkish thin surface (such as a metal roof or a tile roof) heats up to 50 degrees C or more, with the heat being almost immediately transferred undiminished to the underside, as you can see from the temp reading of the underside of this rubberized canvas tent which is supposed to provide cooling shade, but really provides heating shade, which will radiate on the order of (52-32)*5=100 watts per square meter to your head, plus perhaps as much again convected to the air below. Not as bad as the 700 watt sun, you might say, but not comfortable either – and suffocating if the underside of the roof is not radically ventilated. So next time you are sitting under one of these tents at a Thai wedding or funeral, be warned -- it's the heat, not your emotions, that is causing you to feel like you are going to faint.


Here is an excerpt from the manual of the Australian company Bluescope Steel who make (and promote) steel roofing in Thailand. This indicates a reduction in heat transmission through thin roofs (steel, cement tile, etc) if they have foil insulation, properly installed (air space above and below foil, and foil with shiny side down.) For a dark roof, the thermal transmission is said to drop from 165 to 22 watts, almost 90 percent. But I can say from my own experience that there are many qualifications to this supposed effectiveness of thin foil, more specifically that bubble-backed foil that comes in rolls, to be discussed later with a real example.