My plot is in an area where there is some radon. My Segal-like construction will have a 400 mm void under the floor structure which originally was going to be completely open but more recently I've thought to put some boards round the side just to stop wind blown rubbish and moderate sized animals, etc, accumulating under the house [¹].
From a practical point of view I think the combination of this very-well-ventilated solum undercroft and the roughly Passivhaus levels of airtightness above will mean that there will be no radon issues. The BCO is not averse to this argument.
However, that doesn't match any of the drawings in the BRE document the BCO has to hand which have pictures of cavity walls and the like. So what I'm wondering is if there is any documentation or case history I can point to which says that this sort of construction will be OK. Nick? Anybody?
CAT must have come across this so I might try them later. Findhorn who have a number of similar houses isn't shown as a radon area on the maps.
[¹] Edit to add: another reason for the boards is to substantially reduce wind blown rain hitting directly on the bottom of the floor. Any residual drops which get through will dry out very quickly (particularly in the sorts of wind when this could happen).