Lining the inside of an attic or under-roof space with plasterboard needs more attention to insulation than a standard home partition, because that surface is in direct contact with the outside through the roof.
Why insulation is more critical here
A sloping ceiling under tile or slate suffers far greater temperature swings than an interior wall: extreme heat in summer under the sunlit roof and intense cold in winter. Without enough insulation the room is practically uninhabitable in the extreme seasons, a problem that is expensive to fix afterwards.
Vapour barrier: the layer you can't forget
As well as thermal insulation, it is essential to fit a vapour barrier on the warm (inner) face of the build-up. It stops moisture from the indoor air condensing inside the insulation or against the roof, a problem that over time can rot the timber structure if there is one.
Recommended insulation thickness
For a habitable under-roof space, a noticeably greater insulation thickness than in standard interior partitions is recommended, usually between 80 and 120 mm depending on the climate zone, compared with the 45–70 mm typical of an interior partition.
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