Dave Liquorice wrote:

I guess most comes up from the ground, but what about walls made of
that nice cornish granite?

The reason suspended floors with a 'sump' arrangement work is because
radon is very heavy, and will collect at the lowest point - even if it
is coming from the walls.


Ventilation is the way to deal with it
though.

Agreed - far more practical to install in an existing house.


Can Radon get through plastic DPM?

Only to a small degree.


--
Grunff