Do You require a Ceiling at a Water Heater Closet?
The International Residential Code requires ventilation ductwork inside a closet which houses a gas water heater, along with setup of that ductwork asks a ceiling. The code doesn’t mention a ceiling requirement for electric water heaters, but typically the insulation it provides is a benefit.
Gas Water Heaters
Section 2407 of this IRC is the one which addresses water heater installations, and for petrol units, it requires the environment inside a cabinet to be hermetically sealed from the rest of the house. Air for combustion must come via a duct which goes through the ground to within a brief distance of the floor, and air air must be vented through another duct that opens just below the ceiling. After these ducts are installed, the ceiling must be shut off.
Electric Water Heaters
Electric water heaters do not require air for combustion, and they do not give any exhaust off, so that they do not require the same ductwork as gas water heaters. If you have an electric water heater in a closet, you could conceivably leave the ceiling accessible, but that would allow cold air to circulate inside the closet and boost the energy intake of the heater. In this instance, it’s energy efficiency — not venting concerns — which demands a ceiling.