Originally Posted by
tjspiel
I think the only thing to worry about is if at any point during the pedal stroke the sandal is compressing the insulation layer. That's pretty much a given at the bottom of your foot so you'd want to get some degree of insulation from the sole of the sandal itself.
Less overall compression than a shoe and cycling sandal soles are typically thicker than road shoes and many have a bit of a squishy rubber layer directly under the foot.
The lack of compression in the toe area is why I think sandals and just wool socks (not even a rubber outer sock needed if dryish) are warmer than shoes and the same wool sock. There is more of an air trapping layer in the toe area.