Saturday, August 29th was a really hot day here. I hooked up a set of sensors to the inside and outside of the south garage wall, and also measured the air temperature near the floor.

The wall gets direct sun in the late afternoon, and also gets a direct reflection off light from a patio. It is the hottest place in our house.

As you can see, the temperature of the outside wall shot up to 55C (130F) during the late afternoon, but the inside of the wall didn't go up in temperature in even remotely the same way. This proves that a wall with a radiant barrier does not conduct heat well as the temperature goes up.

In this case, this wall has two radiant barriers. One is under the stucco on the outside and one is behind the drywall on the inside.