After experiencing a nasty draft in my living room today I tracked the problem to a damaged exterior fireplace clean-out door. It looks like I can replace the door, but we only use the fireplace with natural gas, and thus have no use for the clean-out. Could I brick the exterior door as well as the interior door in the bottom of the fireplace so the clean-out is permanently sealed? Even when functional it doesn't appear that the clean-out doors are very energy efficient. Does this pose any code issues, and would there be a need to fill the clean-out void with a non-flammable insulation material?