Hi all, 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. Please help. Thanks
I would never use the preinsulated or prefoiled plasterboard. I always use: normal insulation (e.g. Celotex/Kinspan/Quintherm), fitted between the rafters/joists, as well as below (in front of) the joists (about 30mm) a good separate vapour barrier (https://www.toolstation.com/ybs-general-purpose-thermawrap-insulation/p90532) that is as continuous as possible, with joints taped closed (https://www.toolstation.com/ybs-thermawrap-foil-tape/p16904) a normal 12.5mm nontapered plasterboard tidied off with a good couple of coats of skim plaster finished off with good quality paint I've yet to experience a condensation problem with that set-up.
What about installing down lights, speakers?..... I would need to cut into the Vapour barrier.... the lights are probably like 70-100mm deep. Then there is no continuity at all.
If you've got to cut holes, then loss of continuity can't be avoided. Try and keep the holes as tight a fit as possible ... preferably taping up any gaps. The more vapour you can prevent getting into the roof space the better. If a little does make its way in there, then good ventilation above there should dry it out.
yes, there is good ventilation above (between the joists above the insulation boards) and the builder says nothing else is needed. That's why I'm asking.... He says everything will be OK the way he wants to do it... I was just suspicious and wanted to double check.