Airtightness Paint

I used Partel and blowerproof, there is also Belgacoat google all them and you will find suppliers.

Blocks are porous to air and water and are not airtight, we paint the box chasing and wall chasings and make the conduit airtight if going into attic for example with Acrabond or any airtight membrane adhesive and then airtight tape.

So both are susceptible to air leakage.

No, the plaster will make the wall airtight, problem is the electrician will install the conduit before the plaster goes on. 😁

The purpose of painting the chase is because the plaster, which is your airtight layer does not get around the back of the conduit. Every house I have tested has air leakage through the earth pin on all sockets where the chasing has not been sealed. Proof if there ever was that it is required.