The type of construction used for substations is generally governed by requirements, e.g. fire and air-raid precautions, which may conflict with the maintenance of the atmospheric conditions necessary for keeping the equipment in good order. These conditions are not necessarily the same as those needed for human comfort, and the application of heat alone has often been found to be ineffective and costly.The authors were faced with this difficulty and, after much investigation, evolved a method of overcoming it.Although containing a general consideration of the problem, the paper is not intended as an exhaustive treatise on the subject but merely as a record of the authors' experiences in overcoming ventilation problems in particular cases.