Having flat cold roof in the extension done and just wanted to check if installing 27.5mm Knauf XPS Laminate Plus Tapered Edge insulated plasterboards from Wicks directly underneeth the joist (there is 100mm in between the joist) is acceptable?
What about Vapour Control Layer? I don't think there will be one with just insulated plasterboard or this type but than again not sure if it's needed here.... Builder claims it will be fine.
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