Vapour Control Barrier.

[Danwex]

Hello, I’m new to the group so please be gentle.
Had a builder in recently to do a bedroom extension above existing garage. All done well, however when building control gave visited he has said the insulation layer between garage roof and new bedroom floor is not thick enough. He has said this can rectified by placing 15mm pir board over plasterboard ceiling in garage and taping the joints to provide a vapour control barrier.
My question is suppose is if you use the 15mm pir board as suggested, surely the foil side would go against the plasterboard ceiling so then the extra layer of plasterboard on the 15mm pir can later be skimmed. How do you tape the joints if foil side up as I presume plaster won’t stick to foil if facing downwards? I have seen folks of green plastic that state they are for vcb?
Any advice on how best to proceed greatly appreciated. Thanks guys

 

[Danwex]

This is more info receive if it makes more sense. Thanks.

Please see attached calculation – with 175mm of mineral fibre between the joists they will need an additional 15mm PIR board under joists and additional boards to meet fire regs. One thing I did forget to mention is they must tape the joints on the 15mm PIR foil board with foil tape to create the vapour control layer as I imagine they wont have installed a VCL under the joists?



Let me know if you need anything extra

 

[Dereekoo]

This is more info receive if it makes more sense. Thanks.

Please see attached calculation – with 175mm of mineral fibre between the joists they will need an additional 15mm PIR board under joists and additional boards to meet fire regs. One thing I did forget to mention is they must tape the joints on the 15mm PIR foil board with foil tape to create the vapour control layer as I imagine they wont have installed a VCL under the joists?



Let me know if you need anything extra

Sounds as if he is not expecting you to skim the garage therefore you basically have a foil outer face to act as a VCL on the ceiling of the garage and have ended up with double layer of plasterboard for fire control? Out of interest what U value is required for the floor, my calc using mineral fibre with thermal conductivity value of 0.035 gives a U vale of 0.19 without any PIR. Not quite sure why he wants a VCL on what basically is a floor above a non heated room as you don't have them on the ground floor of a house above the damp crawl space beneath, you could ask the question but that might smeg him of.

 

[Danwex]

I was wondering if he means we need to take down the plasterboard, put in the pir board with taped seams and then put the plasterboard back up over top?

 

[Dereekoo]

Yes from how the calcs are stacked PIR goes under floor joists and plasterboard and fireboard beneath. His heat loss spreadsheet is a bit more detailed than mine (as would be expected from BC and their resources) so I would have to concede to his U value. Still can't see the need for a dedicated VCL but since its part and parcel of the PIR just tape it and keep him happy.