15.—(1) Subject to paragraph (2), any sidescuttles or windows in a sleeping room, day room, mess room, recreation room or hospital ward without which the standard of lighting required for that room by regulation 13(1) or (2) would not be attained, shall, unless the room is served by an air conditioning system complying with regulation 14, be of the opening type; if the room is served by such an air conditioning system then only 50 per cent of such sidescuttles or windows need be of the opening type.
(2) No sidescuttle or window shall be of the opening type if it is required by other regulations under the Merchant Shipping Act 1995 to be of the non-opening type.
(3) Where the application of paragraph (2) precludes the use of natural ventilation, then where natural ventilation or trunked mechanical ventilation is required by regulation 14, there shall instead be required a trunked mechanical ventilation system or an air conditioning system respectively.
(4) In every ship of 3,000 tons or over every sidescuttle shall be at least 300 millimetres in diameter.