Need help with airtightness detailing for room above garage

[Thorfun]

afternoon all. today I started to look at the detailing for the airtightness for the room above our attached garage (I will simply refer to that as the 'studio' for ease of typing) and it's proving difficult to see the wood from the trees. if I knew now when I started this project I would've got the architect to completely change what he designed for the space as it's really hard to maintain airtightness in that space. some details....

 

the garage is, obviously, outside the airtight layer, but the stairs going up to the studio start in the house but then have a half-landing that is part way in to the garage. here's a couple of drawings to illustrate.

 

 

 

 

in the drawings the green line is the current airtight membrane but the stairs pierce that layer where the landing goes in to the garage (the red circle in the first image).

 

so, I was thinking of putting some AVCL underneath the joists of the garage ceiling and on the garage side of the stairs and landing to join the existing green layer. that way any heat that travels through the stairs will get trapped underneath them and not leak in to the garage.

 

is this ok to put the barrier on the underside of the joists with the insulation on the garage side? in the rest of the house I'm using Intello Plus but I was thinking of using some Protect VC Foil Ultra that I have lying around to save buying more Intello Plus. I presume it's ok to mix and match AVCL? and is there a 'right way round' for the Protect VC Ultra? i.e. does it matter which way the silver side is facing with regards to vapour transfer? I guess that if the silver side is facing upwards away from the insulation and in to the joist void then any heat in there will be reflected back up through the floor to the room?

 

and, one final question, if I put the membrane in the yellow location is it a problem to put some Rockwool between the joists (thinking 100mm) on that side of the VCL to reduce noise transfer?

 

sorry for all the questions. it's probably a bit of a silly question but I want to get this done so I can, hopefully, get an airtightness test completed in a month or so and then I can start boarding!

 

as always, happy to take some in-situ photos if anything doesn't make sense.

 

[Thorfun]

further reading on the Protect VC Ultra in this document it shows the membrane fitted either way round but with the foil surface facing the airspace. so foil facing the airspace in between the joists is ok.

 

it also says:

 

Quote

6 When used as a vapour control layer, the membrane must be fixed on the warm side of the insulation, covering all the internal area, including joists, rafters, rails, studs, noggings, window reveals, lintels and sills. All joints should coincide with battens, studs or noggings, be lapped by at least 100mm and sealed in accordance with point 2 above.

 

so, that to me sounds like fitting the membrane to the posi-joists and then insulating on the garage side is actually the correct thing to do!

 

unless I'm completely misreading the information I think my 'yellow-line' plan is correct. anyone got any reason to disagree with that?

[Sparrowhawk]

1 hour ago, Thorfun said:

so, that to me sounds like fitting the membrane to the posi-joists and then insulating on the garage side is actually the correct thing to do!

 

unless I'm completely misreading the information I think my 'yellow-line' plan is correct. anyone got any reason to disagree with that?

That's my reading too unless you want to put any insulation between the joists?

[Thorfun]

5 minutes ago, Sparrowhawk said:

That's my reading too unless you want to put any insulation between the joists?

Was thinking of some Rockwool for sound insulation. Again, can’t see that being an issue as you get walls with mineral wool between the studs then VCL then PIR. Also, will be more insulation garage side (at least 100mm PIR) so membrane will still be on ‘warm side’.