I was reading another thread and this question came to mind. That is the use of a (probably) clay pot to hold an orchid that is planted in a plastic pot. Being a bit sensitive to moisture retention of potting material I take my plants out of the plastic pots and put the plant directly into the clay. One less barrier for moisture to get past.
The vent holes help moisture escape in both cases, but I don't know... seems to me the pot used as a holding fixture for the plastic one would inhibit or have the potential to inhibit moisture evaporation from the potting medium.
A large roomy clay pot with holes and an air space between the it and the plastic pot would not be a concern. but often I see a snug fit at the top and
I wonder what the effect of this double potting is, or if it ever comes into play.
There is also the (for me) beauty of seeing roots work their way out through the vent holes and wrap themselves around the pot.
