My experience with a bath fan in a window as a vent ( I have built 6 darkrooms prior to the one I am in now ) is that, no, out of the box stock big box store vent fans are not light tight. I used one with a chrome steel interior louver. I dissassembled it, and spray painted the inside of the fan housing, and back side of the interior louver flat black. I also spray painted the inside of about 18" of flexible exhaust duct that attached to the outlet, and then bent it 90 degrees. That makes it light tight.