Local wisdom, upstairs/downstairs

There's a lot to be learned from the natives.

Here I am going on about how terrible hot tin roofs are, and then I discover, from reading my own blog, that if it's a good sleep you're wanting, you might be best off under a hot tin roof.

Here's the idea: even though the night sky in the tropics is fairly "warm", that is, its temperature as measured by a radiant themometer is not close to freezing as it would be in the desert (so don't try to build a heavy masonry building in Thailand and expect it to be cool like a Mexican church), but rather closer to 12 deg C, owing to the high moisture content of the air. But the sky still does does manage to pull off some heat by radiation, so that the temperature of, say, a roof surface will be suppressed by perhaps 2 degrees. With the outside night air in the hot season remaining at a sweaty 30 degrees, the roof temp might fall to say 28 degrees. In the case of a roof with no insulation, say a metal roof or cement tile roof, this cooling would carry through to the underside of the roof--where it might be a big help to sleepers below, happy to be able to radiate from their hot 35 degree brows to a 28 deg surface above, maybe pick up some convective air cooling as well.

Result: the Thai family that avoids the upper floor during the day, but sleeps there at night, would do best without any roof insulation!

So what does this family do in the day?

Everyone knows that they spend their time under the house. Because it's cooler there.

How much cooler?

Here are the measurements above for three hottish days in early march 2007. And we can see that Jack and Dah's house is about 4 degrees cooler downstairs during the day.

Note that the upstairs remains a little warmer throughout the night as well. The moderate insulation of the roof (R=4) doesn not encourage the sky radiant temperature suppression we discussed above.