Chimney to increase subfloor venting? Liquid radon sealant application?

[YodhrinForge]

Okay brief background - property is circa 1900 semi-detaches stone cottage with early-mid 90's extension/attic conversion, slab in the latter suspended timber in the former. Building is getting an EnerPHit-style eco retrofit with all the proper vapour open materials wood fibre yadda yadda. Original plan was for new insulated slab floors with underfloor heating throughout but the project is going through a bit of the old "value engineering" at the moment and my current instinct is to cut down to just the kitchen extension and treat the existing suspended timber floors with a special system that uses a vapour open but windproof membrane to seal the floor structure(allowing them to still dry out to the solum cavity), then wood fibre batts between the joists, then a vapour barrier on top before application of the subfloor. My one reservation about this is it's a former shale oil mining area and so there's a possibility - albeit a small one - of elevated levels of soil gasses and radon. The inhabited space will have MVHR so I'm probably just being paranoid, but I'd rather do too much now when everything's being taken apart anyway than find out come next winter there's an issue and have to dig everything out again. The solid floor plan would have involved a ventilated sump and radon-rated DPM and while I *might* be able to use one of those as the vapour barrier(yet to hear back from architect about it) I still think I'd like to increase the ventilation of the subfloor space as right now there are only three grates(imagine division between the two properties as interlocking L shapes, mine has the narrow front and wide rear so one front vent two at the back).

 

The actual question then: There are two chimney flues originating in the downstairs front & back rooms which I wanted to close up anyway, so I wondered, could extending the flue down into the subfloor space with some ducting and sticking anti-backdraft caps on the chimneys provide extra passive ventilation via stack effect creating a stronger draw on the space? And if that could work, is there a process and what sort of ballpark price could I expect to pay to have the interior of the chimneys coated with something like a Visqueen liquid gas barrier(no point removing any potential radon from the subfloor just to let it diffuse through the completely vapour-open wall buildup into the interior anyway)? I've seen companies that will do a concrete-ceramic spray coating for chimneys with active fireplaces in them, but I've no idea if that sort of equipment would work and if any would be willing to run something else through them.

[ProDave]

Why not drop a flue liner down the chimneys and extend the flue liner only down to the sub floor space.  That should be air tight.

[saveasteading]

Radon maps  are published so don't guess.

"Soil gases"... what are these?

6 hours ago, YodhrinForge said:

to have the interior of the chimneys coated with something like a Visqueen liquid

If you went that ventilation way then it doesn't need to be high tech. Either leave the chimneys as they are or put liners in.  It all feels as if you think you need "special systems" whereas simple is often better.

 

What free space do you have under the floor? 

[Mike]

8 hours ago, YodhrinForge said:

it's a former shale oil mining area and so there's a possibility - albeit a small one - of elevated levels of soil gasses and radon.

There are Radon measuring services available, if you want to be sure. But if not, then assuming that there is a problem is wise. There are published recommendations for Radon barriers & sumps and I'd stick to one of those to avoid potential problems with home-rolled solutions when you eventually come to sell it.