i was having the same problem with my substitutus.. i was using 2 pots of water under the ytong.. Now i just use one which is the other side of the heat mat and i get no problems at all.
Thanks!
The condensation has gone, i left it for a couple of days and it seemed to right itsself.
They still havent moved in to the main ytong, i have made the main bit warmer and dark. I'm shining light on the place where they are to encourage them to move to the dark area.
Anarky