Imagine it this way, when you open the door you allow all the warmed air inside to escape; however, if you have Some water bottles already heated to the correct temperature the air temp will rise and stabilize much faster at your desired temps.
As Lisa said the substrate in each incubation container will help maintain that individual boxes humidity, if you are having problems keeping the desired levels you can always add an open container with water which will evaporate and help with humidity. If you have close to airtight boxes this won't likely be necessary though.
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