Best bet is an AC unit. *You put some duct between the AC unit and the terrarium and hook the output of an ultrasonic humidifier into the duct before it enters the tank. *Have them wired so the ultrasonic humidifier turns on when the AC is operating. *This will rehumidify the air before it enters the grow chamber.
During the cold outdoor months you can modify the system to just pull cold outside air through the duct and bypass the AC unit.
Swords at www.jurassic-gardens.com uses this type of system. *He might have some info on how to set it all up.