Airtightness question - best option

[Moggaman]

Hi. I am about to put wideslab precast flooring onto my block walls. The detail shows the wideslab being wrapped by a  Siga Majpell type material. The wideslab will ultimately have a 150mm suspended ceiling housing the MVHR and services. The majpell will ultimately come down the block to 20mm below the ceiling.. the sand and  cement render will then lap the majpell.. do I need to tape the majpell to the block first before plastering or just keep it in place and lap the plaster over it.

the other option is to bed the slabs in mortar creating a seal horizontally. Then airtight paint the block below the slab to 20mm below the finished ceiling level and plaster over this. Where the slabs run parallel to the inner leaf , I can either paint that before slab goes in or fill the gap with a wet mortar seal to create a seal on the block . Appreciate any comments.

 

[Russell griffiths]

Your membrane option is best, ask the plasterer, but you could include a strip of stainless mesh to trap the mesh to the block work and allow the plaster something to grab to. 

Also why not plaster up higher so top edge will be above false ceiling, then you can put your mesh up high and any fear of cracking will be above ceiling height. 

[Moggaman]

Thanks . I presume that means the walls are plastered before the false ceiling goes on?

[Russell griffiths]

Certainly first coat, then fit ceiling then skim it all.