Sealing single skin wall

Hi,

I converting an out building to become living space. I have started to apply flexible bitumen based waterproofer to the inside of the single skin walls.
I was then going to timber frame all way round room and fill frame with IB. Then plasterboard etc
Is painting the walls with flexible damp proof a good or bad idea?
Or will this cause damp rather than prevent it.

It had contrete roof and floor which will also be timber framed and insulated.

Any advice would be much appreicated

Thanks
James