I think it would work better on the outsides of the containers to block some heat before it reaches the container. Otherwise you may be trapping heat IN the container.

Some of my containers I have blocked the sun with 1/2" plywood about 4 inches away from the containers to block the sun from warming them too much (had soil temps of 85 degrees in march a few days - back to 65 with the plywood.)

The heat you need to worry about isn't conduction through the container walls as a result of ambient heat transfer, it's the heat build-up from passive solar gain. This occurs when light strikes the container and is turned to heat. If you insulate the outside of the container with clear bubble wrap, the light will strike the container walls, turn to heat, and be trapped there by the insulation, so it will conduct through container walls and raise temperatures HIGHER than if it wasn't used at all. It's much more effective to reduce the amount of light striking the container by shading your containers, using the pot-in-pot system with a cache pot, or using a white (or light colored) container to reflect light before it is turned to heat.

Using it on the inside of the container won't solve the problem because it will only serve to slow heat gain via its insulating value, but once the soil temp becomes too warm, it keeps it from radiating from the container walls and being lost to the surrounding air.

Al

Good points, as usual. I did buy some white spray paint for plastic, figuring it couldn't hurt to make it white instead of a dark color.
Was just a thought, but shade via plywood or whatever would surely work better.

Here is a link that might be useful: sb158's Valley Garden