So last time we saw that the underside of tile roofs (or metal, same thing) get very hot. And I said that foil would help, shiny side down.

You don't believe it? I did an experiment. First I built an upcountry house made out of parts of old wood houses in the village, folks didn't want them anymore because they were low class.

Since I figured on hanging around the house in the day I didn't want that radiant heat blasting down so I went to Home Pro and bought rolls of foil insulation. I bought the plain foil, but my wife switched the order because she thought that thin bubble stuff "maybe better". She chatted with the sales guy about R values which neither understood and which are advertised in the most misleading way. Never mind, it didn't cost much more, even if it's not much more effective either.

I had to fight with the contractor to get the stuff shiny side down but here it is.

It works. Everybody agrees, the house is quite pleasant, no hot radiation from above and enough natural ventilation that the hot air never builds up. So it's as nice as being under a tree, which is the gold standard. Very hot behind the foil, over 40 in the afternoon with the sun on the roof.

Maybe you don't think it looks so nice. But there's a bonus: when you turn on one bare bulb at night, the room lights up with a pleasant glow, reflection from the ceiling.

Here's your exam question: at night the temp behind the insulation is about 15C when the air temp is about 18C. How can that be, where is the cold coming from?

Swelters

where is the cold coming from?

From your aircon