Is scrim+skim over plasterboard airtight?

[WWilts]

Builder believes it is.

 

We think that in addition to scrim+skim, airtightness tape necessary for plasterboard junctions.

 

What will suffice? Tight budget. Not aiming for Passiv Haus, just what is achievable on a budget.

Edited November 18, 2021 by WWilts

[epsilonGreedy]

Not according to @ProDavewho describes this setup as a "plasterboard tent".

 

Is your question in the context of a masonry brick & block build?

[WWilts]

34 minutes ago, epsilonGreedy said:

Is your question in the context of a masonry brick & block build?

Yes

[Declan52]

It is until it moves the during drying out phase and then cracks. 

[Mr Punter]

It would need to be sealed to the floor and ceiling, which in turn would need to be airtight and the perimeter of all openings, with all fixings and penetrations sealed, so not really.

[WWilts]

2 minutes ago, Mr Punter said:

It would need to be sealed to the floor and ceiling, which in turn would need to be airtight and the perimeter of all openings, with all fixings and penetrations sealed

Yes.

It is the junctions between plasterboard units that is of concern, in addition to the above. To use airtightness tape or not. Scrim & skim presumably can go over airtightness tape.

[epsilonGreedy]

1 hour ago, WWilts said:

Yes.

It is the junctions between plasterboard units that is of concern, in addition to the above.

 

 

I suspect you should be more concerned about heat loss via convection and airflow in the air gap behind the plasterboard formed around the dabs of adhesive. Many years ago @ProDaveobserved the drafts he encounters at wall plug and switch openings when he replaces a fitting.

 

Also what blocks are you using? Some thermo blocks air quite porous to air and water. 

[ProDave]

Decent air tightness is not expensive, it just requires everyone working on the build to understand what they are doing.

 

The issue with dot and dab if not done properly is the void between the PB and the wall can become the famous "plasterboard tent" if the top and bottom are not fully sealed to prevent them venting to a cold loft or under floor space.  THAT is where you should be talking to your builder.

 

As this is a conventional block build, I would suggest you plaster straight onto the blocks. Very little to go wrong with that.  The main reason dot and dab plasterboard is used is to prevent the need for a plaster skim and it is just taped and filled at the joints.

[WWilts]

21 minutes ago, ProDave said:

The main reason dot and dab plasterboard is used is to prevent the need for a plaster skim

Have I understood correctly?

 

option 1

No plasterboard, plaster on to blocks. Good airtightness

 

option 2

Plasterboard on adhesive strips (sealed especially top and bottom). Airtight only if taped at plasterboard junctions, OR possibly if scrim+skim applied over plasterboard.

Other consideration is decoration. Can plasterboard with adhesive strips be painted without a skim on top? Has budget implications.

[Andehh]

When plastering onto the block, how do you you manage cables & sockets? 

[WWilts]

Passiv Haus airtightness guide recommends
a) airtightness paint to seal such areas
or
b) parge coat