retro fitting a radon barrier in existing house

Hi

anyone retro fit a radon barrier in an existing house. I know some people will say this will not work and I kinda agree this is the case but I need to put down a dpc so I might as well use a radon barrier and seal it as best i can. Just wondering what tape was used, how you got on etc

cheers

Just an update if anyone is every considering this, There is a crowd necoflex who supply a tape calld easy flashing which will stick to the wall and stick to the membrane. Tape is expensive though 40 euro a roll for 10 meters