A proven effective "low-end" solution to CO2, humidity, and odor in a rodent holding room is to direct ventilated cage exhaust that is exhausting into the room to the room exhaust port using a simple PVC pipe. No extra fans involved -- just a passive exhaust conduit. Called "Directed Passive Ventilation."

Learn more about ventilated cages.

- From Tradeline's Animal Research Facilities 2005 Conference