If you mean the larger vent holes on the top of the unit, yes, keep those plugged. But the smaller ones, (located on top and bottom of an LG) keep those open! You must have fresh air entering the unit. This is irregardless of whether you have a fan or not. All that fan is doing is keeping what air is inside the incubator moving. (i.e. circulation) It does not draw in fresh air. If you plug all of the small air hole vents, then you run the very real risk of suffocating your chicks. Especially if you have a large number of eggs hatching all at once. The smaller air vent holes will not normally affect your humidity nor temperature levels to any appreciable degree, so just leave them open. Good luck, and let us know how your hatch goes!