I put in a radiant barrier in my attic and I verified attic temp reduction of 20F at the insulation. Best $100 I ever spent.

True that it only reduces upstairs living space about 5 deg but that's misleading. It basically eliminates evening heat soak. Before house actually got hotter from about 5pm onward. The other advantage is that before if I turned on AC at 3pm, it would basically never make any headway. Now I turn it on at 3pm and 30 mins its almost at the thermostat setting because my evaporator and ducting is in attic.

When temps are above 85f my neighbors turn on AC at 1pm and they run pretty much constant whereas I almost never use AC anymore unless there is a string of 100 deg days which is rare.

​

I considered attic fan (I have a big gable vent) and came to same conclusion that they don't really work. Ridge vent is better but also has issues.