Airtightness Barrier - what's best?

[tuftythesquirrel]

Please could anyone help with the following, re. airtightness barrier. We are carrying out a restoration of a 1950s church hall, rendered cavity, brick and block construction. We have a Kingspan Quadcore composite steel roof, fitted with additional seals. The floor build up is 300mm concrete, 200mm PIR and 70mm Cemfloor screed, painted at the floor junction with Pro Clima Aerosana Visconn to continue the radon barrier under the concrete. The task is to make the walls airtight. There is a 200mm EWI system on the outside of the building. It is twin skin cavity, but having been filled with concrete due to the poor state of the block work. I was going to continue the Pro Clima Aerosana Visconn from the roof panels down to the floor. I’ve  used the airtight paint but it seemed a little frail for our site conditions. e.g. we used it on some of the wall sections, where the joists run parallel to the wall and would be subsequently difficult to access to plaster. But having had a few knocks with bits of flooring  etc. the paint has peeled off the wall. There are lots of maybe’s…. The EWI maybe reasonably airtight at the external render surface. The concrete in the cavity maybe airtight where it is continuous, but it is impossible to know. The cement render on the internal brick/blockwork maybe airtight. 
So my question is, would a plaster parge coat be a better bet and if so what plaster? The plaster will lap airtight tapes or paint at the window/door junctions. Please note, this is only to provide the airtight barrier. A decorative board will create a services cavity to run pipes, ducts and cables etc. British Gypsum obviously make a full range but they don’t have any real world figures regarding airtightness. I spoke to their technical support and they recommended their Soundcoat Plus. However, they can’t say that it is 10 times better than their other products, since no one has conducted any tests. The Passive House Guide To Airtightness (my Bible!), suggests 6mm of wet plaster is airtight.
I wanted to use the Thistle Universal OneCoat plaster to get a good build up and reduce the labour costs. Has anyone had a similar situation and achieved good results with this solution. Any comments welcome. Cheers.