Vapour control layer or not?

Roystone · 22 Aug 2021

We are in the middle of a large redevelopment - the property has a mansard roof (I didn't know what this was until recently). Long story short, the first floor rooms are kind of in the roof (see example pictures).

The builders have put 100mm Kingspan between the rafters, and 25mm PIR on top of that - they intend to fix the plasterboard directly onto the 25mm PIR (as an alternative to fitting insulated plasterboard). The PIR will be taped at seams.

They question is - is a vapour control layer advisable, given that the mansard is technically a loft conversion. If a VCL is advisable, should it be between 100mm Kingspan between the rafters and the 25mm PIR to the underside - or should it be over the combined 125mm (so the plasterboard goes directly onto the PIR? The builders say they don't usually put VCL in.

Kind of related - we have installed 2 wood velux windows in these rooms - and I've noticed that the velux vapour barrier is missing (it seems like an optional piece of kit from velux) - is it necessary?

catlad · 22 Aug 2021

Optional depends how long you want things to last!
They are using foil backed plasterboard right?

Roystone · 22 Aug 2021

@catlad I want it to last a long time - we plan to be here at least 15 years. The spec said 80mm insulation between the rafters and 62.5mm foil back PIR.

The contractors have said they always do 100mm between the rafters and 25mm on the underside followed by standard plasterboard fastened to the 25mm PIR.

catlad · 22 Aug 2021

Never use standard plasterboard on external timber framed buildings.

Roystone · 22 Aug 2021

catlad said:
Never use standard plasterboard on external timber framed buildings.

@catlad is that even if fastening 25mm PIR to the plasterboard (was told by contractors - that this would create a foil backed plasterboard as the spec of foil backed is PIR bonded to standard plasterboard).

^woody^ · 22 Aug 2021

PIR is foil faced, and if the joints are taped, then that performs the same job as a separate vapour check layer or foil backed plasterboard

Roystone · 22 Aug 2021

^woody^ said:
PIR is foil faced, and if the joints are taped, then that performs the same job as a separate vapour check layer or foil backed plasterboard

@woody thanks, been doing a bit of research on this. The 100mm PIR between the rafters / joist has the foil surfaced ripped off all over the place, loads of 2mm to 4mm gaps, not fastened in place with any expanding foam, and no tape to join cuts of PIR . The second 25mm sheet of PIR has been taped at joins and tares - is that suitable (I'm thinking of rectifying myself before it goes further).

^woody^ · 22 Aug 2021

Anything used as the vapour check layer needs to be complete/continuous.

If you are going to mess about covering numerous gaps and tears, then you may as well cover the whole ceiling with polythene, or get the builders to use the specified plasterboard if that is what they quoted for and are charging you for.

Roystone · 22 Aug 2021

@woody are you saying (from inside out), plasterboard, 25mm PIR, polythene on to joists, 100mm PIR between joists (we then have double OSB, the GRP finish).

^woody^ · 22 Aug 2021

No. Any vapour check layer needs to go on the room side just behind the plasterboard. The whole point of it is to stop moisture from the room getting into the ceiling structure

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Vapour control layer or not?

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