Air tightness for dense concrete block walls and brick walls

What’s everyone’s preferred method for making blocks airtight ? I’ve got some aerosanna fibre paint and tapes for the tricky bits like joists and steels. But the main faces on my extension walls are all dense concrete blocks. The old part of the house is brickwork which I’ve repointed. It would cost a lot to paint the whole lot.  
 

reading threads some recommend gypsum  soundcoat type parge coat or cement parge coat. Others say it’s all about the plasterboard -  I was planning on battening out and making a service void then plasterboard.