Old cottage on granite

[StevePB]

Hi folks - I'm renovating a largely stone cottage here in Co.Down, and battling what looks like fairly typical damp and deterioration issues. I've taken the basic steps of removing all the internal render back to bare stone and raking out to lime mortar, and digging down outside to just below the level of the inner floor, which is currently earth, having removed the old concrete slab.
The house is build directly onto granite, which is more evident in some places than others. There are a few places outside where outcrops are at or above the level of the inside floor, and inside we have had to chip off some of the higher places to get the floor level (the old slab was maybe 1" thick and simply followed the undulations).
Headroom inside is at a premium, so I can't raise the floors significantly, and I can't dig down through what is a particularly hard form of Mourne Granite. We're in a mild radon area too.
My quandry is whether to install a plastic dpc under the new slab, which I had intended to be concrete. In theory we need a radon barrier, which is basically a thick dpc, but I've read a variety of tales of woe regarding them.
In all other respects, we are using breathable materials - lime-hemp plaster on the walls, sheep's wool insulation, and lime pointing and plaster on the exterior, which I have yet to strip.
The 2 internal walls (its a typical 3 room cottage) are old brick, and have some fairly wet patches which I can attribute to the locations of previously leaking gutters, high ground, etc.
So - will the radon barrier / dpc leave us with terrible damp issues? The old concrete slab was definitely not the driest, but as I said, I've improved external levels since then so that 'might' improve.
Here's a taste of the kind of damp that we had when first encountering the place -

 

[Zebra]

Sounds like a quandary indeed! Others more knowledgeable will be along to help soon I'm sure, but it would seem to me that if you've found the causes of the damp and addressed them, ie. external ground levels being higher than internal and faulty gutters, then the house might dry out yet, when we get a huge heatwave later this summer
You won't know until you've addressed the causes you can see and given the house time to dry out whether you still have a problem. In any case, an impermeable dpc won't cause "huge damp issues", it might only exacerbate an existing damp issue. If the subsoil under the slab isn't damp, then a dpc won't make that damp go up your walls instead of up through the floor.
How wet was the floor under the slab when you took it out? Try perhaps laying a rubber car mat on the floor for a while, and see how much damp is trapped underneath it when you take it up.

 

[JoceAndChris]

Your house looks intriguing, and welcome to the forum. I'd be really wary about pouring something impermeable down there, have you considered limecrete?

But I write as someone unaware of the implications of radon, I'm afraid. Will flagstones laid on earth not do? We've got that here - it works a treat, but where the previous owner has put down plastic and concrete we've got problems.

 

[Flyfisher]

My understanding is that radon is naturally occuring in various areas of the country, typically in granite-rich areas. It accounts for the majority of naturally occuring background radiation and is not considered to be generally dangerous (perhaps because there's nothing we can do about it? :wink: ) unless it is allowed to accumulate.

I've read that radon accumulation within affected homes is significantly affected by the level of ventilation, which seems fairly obvious really, so the good news is that measures that work to reduce radon accumulation are likely to be effective against damp.

Having said that, the only other thing I know about radon is that I know almost nothing about the practical issues and that specialist advice should probably be sought. As ever, there seems to be a lot of information on the web but a good place to start might be the UK Health Protection Agency, which has extensive information here: http://www.ukradon.org/article.php?key=reduceradon

 

[Penners]

We lived in a radon area when we were in Oxfordshire. The Environment Agency provided us (on request and for a modest payment) with a special radon detection gizmo that we had to leave in the house for three months. It then went back to them for testing. See: http://www.ukradon.org/article.php?key=measureradon

In our case the results showed that we were well below the "action level", but it was reassuring to have done the test and to know.

 

[Penners]

Sorry - just realised that I've duplicated FF's link!

 

[StevePB]

Thanks folks. I'm pretty certain that building control would insist that we installed some form of radon barrier - the alternative is a sump and ventilation, but the granite pretty much rules that out.
I have investigated limecrete, but all the examples I've seen involve some sort of non-capilliary sub layer such as leca, and again, we don't really have the depth for that.
Our floors vary in dampness. Rather counter-intuituvely, the one that has the most rock up against the outside has the driest floor. A the other end of the house is a lovely 1950's extension that has been built slightly uphill, on the south side of the building, right up against 2 windows (one of which was hiding behind dry lining). The builder left an 18" void between the two buildings, which they dumped rubble into, and which funnels moisture down the back wall of the old house. Hence the room behind it is properly damp (and is the one with the black fungal growth in my original post). Removing all the internal render has made a huge difference, but until we remove the extension (which contains the only working loo, at present), we won't see how dry it can get.
So - the earth floors were surprisingly dry when we removed the concrete, apart from in the places where damp is currently pretty much unavoidable.
Any limecrete-laid-directly-on-rock afficionados out there? My only worry here is cost. We're doing this place on a derisory budget, in time for a baby arriving start of September. No pressure...

 

[AMc]

If you have to have the radon barrier then there is no point in putting limecrete over the top for breathability (which I'm sure you knew). It still has a better carbon footprint than ordinary portland cement based screed but it will be more expensive and complicated than calling your local readmix provider.

IF you can be confident that the sources of the damp are all external - and given the shallow depth of subsoil before the bedrock that seems probable to my non expert eye - then I would be tempted to go with a DPM and regular concrete screed.
If you can't then you're going to have to look at breathable solutions and possibly some form of forced ventilation. I don't know if Mechanical Ventilation and Heat Recovery is something you are considering and if that meets the needs for radon proofing?

As you are "renewing the thermal element" of the floor you should technically be insulating it to meet Part L1B of the building regs. As you have restricted height this may be waived but I would speak to BC before you make any hard and fast decisions on what you're going to do. In fact if your construction is typical of the area then a call to BC might help too, I found my local officers were quite helpful with questions I had before applying for formal consents.

 

[CliffordPope]

You seem to be saying that parts of the exposed "floor" are solid granite, as you had to chip it away to get a semblance of level? How thick is the soil in the other parts - inches? feet? Would excavating all the soil down to the granite offer any possibilities, perhaps for a void to collect and ventilate any argon? Is a partly solid, partly suspended floor sensible?

 

[robgil]

Where abouts in Down do you live?

 

[worms]

The wise man built his house upon...?

But that was before the National Radiological Protection Board came along!

 

[StevePB]

The earth floor levels vary considerably, but not consistently, so there are numerous peaks in the rock, the tops of which we've chipped out to get a level (ish). In the end, I've put a radon barrier in, with a straightforward screed, as there wasn't any benefit to using limecrete in this instance.
Even then, I'm seriously short on headroom, and by the time the floor surface goes on, it'll be tight. We used screed with a fibre in it, and there were doubts about using that without a slab below it, but its worked well.
The drying continues, and I've now lowered the outside levels sufficiently, I think. The interior of the walls are visibly drying downwards.
The radon barrier is a compromise, but I think in the overall scheme of things, one that we couldn't avoid. Given the nature of the building, and the rock issue, floor insulation wasn't an issue. Additional insulation in the roof will probably make up the difference.

 

[StevePB]

Oh and @robgil - I am near Rathfriland, in the Mournes.