There is an area near to us that has a radon problem. None of the older houses have a radon barrier built in and measurements have been taken that show a radon level 20% over the max permissible level. The houses have solid concrete floors, typically 100mm straight on the sub-soil without any insulation.

Would it be possible to retro-fit a radon barrier - and how?

Are the floors structural, do the walls have separate foundations or stand on the slab?
We took up a concrete floor, dug out and relaid over DPC and insulation, which transformed the room.
Not sure what the wall floor junction details would be for a radon barrier but must be doable.

http://www.ukradon.org/cms/assets/gfx/content/resource_2722cs33267e5dc5.pdf

Always struck me as slightly implausible, or more accurately, genuinely hard to understand, that all the radon in a big house would decide to migrate to the bucket-size hole you've dug (in the case of one I did the sump was in the end room of a 14m-long house), but that's the advice offered.

Posted By: Nick ParsonsAlways struck me as slightly implausible, or more accurately, genuinely hard to understand, that all the radon in a big house would decide to migrate to the bucket-size hole you've dug

Reading the article the radon doesn't migrate to the sump, rather the sump is provided with suction (either a fan or stack effect) to forcibly remove the radon. The sump does not drain the house but rather the soil under the house. The sump it would appear doesn't connect to the house.
Quote
For most houses, a single sump and fan will have an influence over an area of 250 m2 (a radius of approximately 9 m around the sump). If the subsoil is particularly permeable a sump may be effective as much as 15 m from the sump. However, obstructions below the floor slab may reduce effectiveness.
What is a sump?
A sump is simply an underfloor space or cavity into which a pipe is inserted and used to extract radon. The cavity prevents obstruction of the pipe with loose fill from the underfloor space
Active sump systems
An open space of about the size of a bucket (volume approximately 10 L) is excavated beneath the ground floor, fitted with pipework and an electric fan.
Passive sumps
Passive sumps rely on the natural stack effect and wind action to extract radon from the ground,

MVHR and PIV would also probably help

Laying a membrane under a new floor my work but the wall/floor junction may be an issue as suggested above especially if there is no DPC to join up to.

The linked article says that for suspended floors the ground beneath the floor needs to capped with concrete OR a polythene sheet covered in gravel or concrete. That suggests to me that if the floors are not cracked or the cracks can be sealed, then the existing slabs are likely to be adequate caps and a sump could be installed under the slab. Probably need to seal around the perimeter of the slab as well as sealing any cracks.

Unfortunately the idea that heavy molecules 'sink' out of the air, is a bit of an urban myth. The same idea was put about in the global warming debate that the CO2 would just sink out of the atmosphere as CO2 is heavier than air, this doesnt happen (fortunately!)

It works if the heavy gas is very concentrated, or cool, so the bulk gas is much less buoyant than air. But not if the heavy molecules are dispersed at ppm levels as then diffusion takes over from buoyancy.

As PiH said, the sump is just a hole in the subfloor to stop the extractor pipe getting blocked up with gravel. The extractor creates a small negative air pressure under the house to suck clean air under and dilute and extract the radon.