Cool night sky

I got a giant case of ptomaine from eating a giant hot dog at the Robin Hood and while bedridden began to read a copy of Proust that I found at Jack's place, he and Dah were upcountry. A few days in the world of Combray put me in a contrarian frame of mind and I began to imagine that much of what I believed about roof insulation could be wrong, that rusty tin roofs might be the best of all. Thai country folk lived under them. They just stayed under the house, not in the house, in the daytime when the uninsulated roof created oven-like heat in the room. But at night maybe the outward radiation would cool the simple metal roof below air temperature.

An email from technology innovator Steve Baer reminded me of work by the solar master Harold Hay who invented and promoted a roof pond concept in California that relied on night radiative cooling to store "coolness" for the day use.

I built a "upcountry" room with a metal roof and zero ventilation and let it sit on my roof for a couple of days. Of course not a real room, but one of about a cubic foot size with a sheet of mat black aluminum foil for a roof and the room itself a foam picnic box that I bought for a buck at the Chinese shop down the street. Into which I put a brick for thermal mass and a hobo to measure temp and dew point.

You can see the temp history of the country room in red and the temps of Jack's bedroom and the outside temp (last two almost the same) over several days. The country room is impossible in the daytime but gets really cool at night. At least toward the end of the test period when the dew point got fairly low and the night sky temp fell to as low as -5 degrees C. So you might be very happy with a simple tin roof at night -- though not at day. And, this being a dryish night, note that the suppresssed temp is still way above the dew point, so there will be no problematical condensation. The question being, how about during the hot season, when the dew points are a lot higher, maybe 18 C instead of 10 C? Would you get any radiative night cooling then? If so, would water start to drip from the ceiling?

By the way, I say test boxes--there were two of them. One had foil with shiny side down (the blue line) the other had the bottom surface of the foil spray painted mat black (red line). The shiny foil reduces the peak heat of the day but also slightly reduces the cooling effect of night radiation.