Usually the person inside the chamber feels warm as it is being pressurized, comfortable, when it is under pressure and feels a bit cold when the pressure begins to drop. Before the therapy the operator can give you blankets made of pure cotton, so you do not get cold.
Most good quality hyperbaric chambers are fitted with air cooling systems which can be adjusted to ensure that the atmosphere inside remains comfortable throughout your therapy session.
It is recommended to have the chamber in a well ventilated or air conditioned room.