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.

[YodhrinForge]

On 18/08/2025 at 09:23, ProDave said:

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.

 

On 18/08/2025 at 12:16, saveasteading said:

Radon maps  are published so don't guess.

"Soil gases"... what are these?

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? 

 

On 18/08/2025 at 13:55, Mike said:

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.

 

Re flue liners; the issue there is *air* tight is not necessarily *radon* tight. Radon is capable of diffusing through a lot of materials that can stop air molecules no bother, it's why you're supposed to use a specially rated DPM in known radon areas. It would also - assuming the standard metal option - be putting a huge condensation generator in the middle of a stone wall outside the insulation envelope which doesn't seem great for the health of the wall long-term, as you'd have an air cavity around the pipe where moisture in the wall assembly could evaporate and then a metal tube drawing cool air from the solum space - completely coating the interior of the flue would firstly seal the flue from the wall assembly so water vapour couldn't pass into it from the wall itself, would make condensate less likely in the first place as nothing in there would be cold enough to let significant moisture form out of the air before it was expelled through the chimney, and any that did form could drain back down into the solum rather than being trapped in the air cavity. The Visqueen(or other brands) stuff is designed for exactly this sort of issue it's just normally applied to seal floors or walls rather than the inside of unused flues.

 

At the three access points the measure depth of the cavity between the bottom of the joists and the earthen solum is ~270mm.

 

Re soil gasses; other than maybe radon in a former mining area there's also potentially methane, though that's more common on former waste dumping sites that have been built over, and also just water vapour - you have to be really careful with moisture when renovating these old stone buildings because they were designed to be draughty as hell and have active or residually-hot fireplaces pushing air out through the chimneys pretty much 24/7, so even using the proper vapour-open materials as I will be you have to watch the solum space with so few vents compared to modern structures, soil moisture can elevate the humidity of the space and then you get water condensing around joist-ends and other inconvenient places.

 

Re radon maps & measuring - the maps only report testing that's already been done and most people don't even know radon is a thing they should be testing for. The map data is also not particularly refined - your neighbour a dozen doors down could submit an all-clear result and if that's the only result your whole area could be marked as lowest possible risk, but *your* house could still be sitting on top of a plume of the stuff. You also really have to wait for winter to get proper results, and on top of that results before & after a renovation can be totally different. For my money testing is more something you do to confirm that there isn't an issue *after* you've already done all the mitigations you can within your budget and project scope. And the issue with the recommended methods Mike are they all pretty much assume either old and draughty or modern and airtight, whereas I'll be making an old property airtight(or as close as I can manage) but using vapour-open(and so radon-open) materials to control moisture buildup, so they won't really work in this context because if it stops radon it'll also stop water vapour. Everything about these kinds of renovations is non-standard really.