The need for a 50 mm gap above the insulation causes problems for modern building design. The U value needs to be as low as possible to pass the SAP test and any space not filled with insulation is wasted. A permeable membrane can be used, sometimes called a breathable membrane. This allows warm, moist air to pass out through the roof but prevents water coming in. The large ventilated air gap is no longer necessary. This is called an unventilated roof. There is still a small air gap caused by the battens which separate the tiles from the rafters below and this air gap is considered adequate for condensation.
If building an unventilated roof get some drawings from the manufacturer of your insulation. It is important to make sure that the construction is completely right and in accordance with the manufacturer’s handbook. The insulation itself needs to be waterproof, or to have a vapour barrier underneath. Celotex is naturally very waterproof. If thermal plasterboard is used under the rafters it may have to be the kind that has a built in vapour barrier – the manufacturer’s datasheet will tell you what to use. Some mineral wool foam insulation is designed to go between rafters but isn’t intended for an unventilated roof. It absorbs water very easily.
You tend to use an unventilated roof if you have a room in the roof insulated to a very high specification. A room in the roof can have really low ceilings and increasing insulation by 25 mm is extremely noticeable to the occupants who will increasingly complain about banging their heads. It is also possible to add an extra layer of insulation above the rafters. This can help to increase the head space in a loft room.
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