Is a hot tin roof cooler?

Interestingly enough, the answer might be yes if we are talking about the time that matters most to many folks, that being at night when the outside temperature is slowly falling from the low thirties (too hot!) to the upper twenties (reasonably cool for sleeping). A "tin" (galvanized iron) roof is fairly light, perhaps a kilogram per square meter and has a very low "R" value. So hot as it might be even at sundown, it quickly cools to the ambient air temperature, even two or three degrees below if the humidity is not too high and the effective sky temperature is low, say 5 degrees or so. So you may even get some free radiative air conditioning from such a roof at night.

Lets look at Brother Jack's roof:

This roof weighs about 10 kg/sm and has an "R" value of about 5, and each square meter retains perhaps 10,000 joules of heat per degree. If you want it to cool as fast as the outside air, say 1 degree per hour, you've got to make it discharge 10,000 joules per hour per sm, or 3 watts/sm. If the "R" value is 5, this will require a temperature gradient across the roof from inside to outside of about 3 degrees. In other words the interior temperature will be 3 degrees higher than the outside air temperature--which is what we saw earlier on our April 3 measurements (vertical difference between red and blue lines). Therefore even when the outside air dips to a nice 29 degrees, the inside of the room will still be a sweaty 32 -- too warm for sleeping. And if in your desperation to keep cool in the daytime you had installed some bulk insulation instead of the foil insulation, the "R" value might be double, and the required temperature to push 3 watts out through the roof even more, maybe 5 degrees, an even worse situation! Meanwhile the poor farmer next door will be happily snoozing with his tin roof at or below outside air temperature! His red line will closely track the blue line, maybe be even lower.

So the joke is on you, Mister high-tech fahlang--you tricked nature in the afternoon, but she got back at you when you were trying to sleep!