The fact that he measures the back of the plywood controls for quite a bit I think. At 3m27s into the vid you see the plywood that has the 'paint' on the _other

The fact that he measures the back of the plywood controls for quite a bit I think. At 3m27s into the vid you see the plywood that has the 'paint' on the _other_ side is even cooler than the square that had been in the shade all along. The insulator is just to prevent the plywood side that is measured to be heated by the ambient temp.