Discontinuous airtightness layer in refurbishment

[OwenF]

I am in the situation where I am converting a garage with a pitched cold roof above. However I’ve also removed the ‘external’ wall that separated the garage and kitchen.

 

I’m taking the opportunity to improve insulation with IWI in the garage and want to also improve airtightness. I’m happy with the floor-wall, wall-ceiling & wall-window details for VCL/airtightness layer with the garage itself. However I’m not sure what to do when I get to the original house.

 

e.g. Crude line drawn on photo below showing continuous airtightness layer coming up external garage wall on left, under insulation in garage ceiling, under steels (to be insulated).

then I arrive at kitchen ceiling with bedroom above.

What happens when you have to terminate an airtightness layer in a refurb where not all rooms are being tackled?

 

all I can think to do is apply liquid membrane between joists in GF/FF void to underside of FF chipboard & around joist entries into blockwork.

Then tape/seal the membrane from the garage onto the liquid one.

My air leakage path becomes up into the bedroom which sounds not great given kitchen has lots of moisture.

 

Or carry the membrane right across kitchen ceiling and terminate midway into the house. Also seems wrong.