15 Aug 2023
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United Kingdom
So I got a single skin outhouse with a dpm below concrete and I’ve tanked it for good mesure.

Plan was to 2x1 batten out walls at 400mm apart with a dpm strip behind & not touch the floor with them.
Then 50mm PIR celotex over the top to create a small cavity (taping joints) then foil back plasterboard over the top. Leaving an inch above the floor to create ventilation/use brick vents all the way round. Do I need a vapour barrier somewhere here?

For the flat roof with OSB below, I was planning to do a cold roof insulation, so 50mm air gap then 50mm PIR celotex wedged between joists, then a celotex slab on top, foil backed moisture resistance plasterboard on top.

Anything you guys would change here?
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