Well if its sealed then you must do like everyone said above and Create Air exchange your plants need Co2 during Lights on period.. New Co2 must be enterduced into the room and the easyest way is by creating "negative" pressure with one big exhaust fan which will give you "passive intake" ....... This means you will need An intake hole at bottom and and Exhaust hole at top and one Inline fan at top connected to the Exhaust hole .....
Or you can leave it sealed and supply supplemental Co2 eather by Co2 burner(natural gas, propane) or exchangable compressed Co2 bottles...... Eather way you dont want your plants sitting staggnet in a shed....Not only that but you should have a vew Osilating Fans in the room to keep air circulating around the leafs...This will help with mold issues too....
Also i would exchange the air more then a couple times a day.... Large plants can quickly use up available Co2 in the air in minutes and should be exchanged with fresh c02 supplies most likely more then a few times a day......
