Sand Cement over Chasings Sufficient for Airtightness

[Gaf]

Wasn't sure to post here or in ventilation - my main concern with airtightness is the heat retention as we have MVHR looking after the ventilation (as least that's my understanding!).

 

Builder said that after the chasings are serviced (e.g. electrician does his thing), he uses a sand cement mix to fill in the chasings for airtightness. He doesn't use airtightness paint (e.g. blowerproof).

 

I'd seen posts here stating that sand-cement internal skimming is a perfectly good airtightness layer, so would there be any drawbacks to the builder's proposed method for the chasings?

[Russell griffiths]

What is your airtight strategy, wet plaster, dot n dab boards, airtight membrane over the walls. 
as long as what he does ties in with the rest of your airtight strategy and doesn’t hinder it then that’s fine.