A relative is considering converting half of their double garage to a playroom/study for their kids. It was built in 2006 but doesn't appear to have a radon barrier in the foundations. A black membrane is present presumably damp proof course. He is wondering about retrofitting measures for radon given that the kids will be using it a lot. Is the best approach to fit a sump or paint a sealant on the concrete floors and walls. Anyone been down this route ? Any idea of costs ?