We have a 16c house with a stone vaulted cellar. We are currently working to minimise water ingress, but we are limited by practicality, and the fact it's grade 2 listed. The floor is random stone flags, then soil. We have to temporarily lift the flags to dry the floor out as much as we can, but when it goes back down we'd like to drop the floor level slightly, possibly install a radon barrier, and improve the drainage for when water does get in. I feel we need to accept the probability that the cellar will flood at one pont in the future. We have an ASHP that has plenty of spare capacity, so we're thinking about under floor heating, but we won't have room for any significant amount of insulation. We also feel the floor should be breathable (despite the theoretical radon risk). Could we look at the earth floor as some form of thermal store, or will we be wasting half our heating bill heating up the countryside with no benefit to the occupants?
Anyone with any experience or recommendations?

If the cellar floods, then that presumably means high water table, underground streamways etc.

All of that is very bad for storing heat in the ground (the water would just wash the heat away).
A bit of damp soil would be another matter but that does not sound like your case.

Why lower the floor ?
Presumably it is not going to be used for storage...

If you decide to lower the floor, take the usual precautions at the wall foundations; I'd just go for a rock drainage course then a limecrete "ratslab" on top, but with a sump to take a cellar pump. You could reinforce the slab with bamboo trellis. Despite what you said in an other post about structurals, it is likely that the existing floor is acting in compression, against wall loads; digging it out would decrease that compressive component; vertical loads could come in to play... Water inflow and flooding would complicate all of that.

If you wanted to heat the cellar, best to use house air, and treat it like a conditioned crawlspace, or make a solar thermal air-soil heat sink. This is what I'm doing in my crawlspace.

gg

I did read an interesting paper from down under about wet and damp substrata under buildings and the conclusion was that it made no difference unless there was a flowing aquifer and this was extremely unlikely in upper layers.

If you have good drainage away from the house, then you do have the option of installing a French drain around the outside. That should keep the cellar dry.

Provided you've got the permission, one assumes...

(Not to mention the cash...)...

(and the need to keep the shovel pressure well away from your wall...).

But you would still have to tank the back of your wall, as nothing would force percolation water to head down to the drain, it would likely just take the easy route into the wall...

Could you run such a french drain to daylight ? otherwise it might not be very effective.

Another solution might be to ram a steel pipe under your cellar, into the earth behind your wall, then hydrojet the pipe out, and hope it turns into a drain...

In which case you might consider hiring the services of a water-diviner first.

FYI, having running water under one's house is, for some folks, a definte NIET...

cf video.

https://www.youtube.com/watch?v=jrKc_vkyDQA

gg

Posted By: gyrogearBut you would still have to tank the back of your wall, as nothing would force percolation water to head down to the drain, it would likely just take the easy route into the wall..

Err, I'd have thought the loose-packed gravel that forms the French drain would be a much easier route for water to take than a solid wall?

Posted By: gyrogearYes, but if the drain invert lies at minus three meters, then the wetness in that 3 meters of earth between the drain and the wall will still go for the wall !

I've no idea what you mean by this. What earth? There's no earth next to the wall; it's gravel there.

See e.g. https://www.sudswales.com/types/permeable-conveyance-systems/filter-or-french-drains/

Posted By: jemhaywardEvents, events...
Yesterday we cleared some clay off the floor and found what may be a drain. It had some water in it, and it didn't seem to be going anywhere, so a partially blocked drain.
I went down there this morning to see if the water level in the hold had changed, but the lights tripped the RCD.
The water level had changed, it's now about 25cm deep in water all over the cellar floor.
So, the job has now changed. We need to fix the drainage, and I think we're going to have to accept that water is going to get in, but hopefully it will flow out as fast as it's getting in, but that makes UFH a complete no-no, and so insulating the floor is probably a pointless waste of time and money as well.
Even dropping the floor may not be feasible now.." alt="" src="http:///newforum/extensions/Vanillacons/smilies/standard/cry.gif" >

Cannot see a problem just put in a foamed glass floor which will act both as french drain and insulation. sounds like you already have the drain for the french drain to drain into it just needs unblocking. Then your 75mm screed containing your ufh followed by 25mm screed to reset your flags. Obviously you would need to apply for Listed Building Consent before doing anything.

Posted By: tonywet and damp substrata under buildings and the conclusion was that it made no difference unless there was a flowing aquifer and this was extremely unlikely in upper layers

But rise and fall of ground water level is entirely possible, maybe even 'likely' in the top 3m layer - and that's just as much of a no-no as horizontally running water flow - will efficiently carry away stored heat.

In fact, as any stored-heat scheme will be relying on the next 2 or 3m down, we should be talking of the top 5-6m layer. The deeper you go, the greater the risk of moving water (permanently still water/saturated ground is not a problem - in fact greatly improves the thermal capacity of the soil).

With rise and fall of water table, no amount or depth of french drain will help - the water table will just flood the trench, with nowhere for the drain to take it away to.

This is a 16C house no way would they construct a cellar without a drainage system to keep the cellar dry. Living in a similar age house I am dismayed at the damage done to the original construction by the use of modern cements, plasters and paints.

Posted By: fostertom

Posted By: Peter_in_HungaryFrench drains should have an out flow - otherwise you are constructing a moat.

Which should be a bit lower than the bottom of the french drain. If groundwater rises temporarily above that level, it's got nowhere to go - unless by some miracle (downslope?) the ground water level dips

It depends what the outflow drains into, doesn't it? Normally it would be a watercourse, or as you say, an area downslope of the French drain. A French drain is basically just a land drain around the house.

Posted By: fostertomthe drain's outflow should still find air voids in the ground to discharge into

The outflow is a regular drain pipe leading to a surface discharge somewhere. Air voids in the ground are irrelevant.

It sounds like what you're saying is that when there is a flood and everything is under water, then a French drain doesn't work. Well, duh!

I'm pretty confident we have a drain, and it's blocked, but it's route 500yrs ago would have been across the road, and down the hill to the stream. However, the road is now tarmac, and is at least 3m higher than it was in the 1500s. I suspect the drain doesn't exist much beyond the walls which are only 6ft from the road anyway.

My current thinking is: At the front, a sump in the garden just outside the wall, below the level of the cellar floor, with a pump pushing water down the hill towards the storm drain just over the road. Hopefully even in rainy weather the outflow would be modest as we'd not be pumping out a flood, just fighting the rising water table.

At the back, a french drain with some form of impermeable membrane against the rear wall and under the gravel, to form a drain, run that drain onto our land, and create another sump, with another pump, and pump the water into a drain that exists in the rear courtyard taking rainwater away. When we re-instate the stone flags above the french drain, put a membrane under them, and point them in so rainwater from our thatch at least doesn't get into the french drain in the first place.

I suspect there is room under the flags in the cellar itself for water to flow below, as there seems to be a double layer of flags, so we'd have to accept that there will occasionally be water flowing below our feet (but at least not over them!) - abandon the idea of insulating the floor, so as not to upset the listed building people, and just install a radiator in there to warm the room, and run a dehumidifier to protect furniture and fittings, as we do in our (slightly damp) barn.

<blockquote>If you put flags on top of the french drain you will defeat some of the point of a french drain</blockquote>

We probably have no choice on that one, as it isn't our garden, so we'd have to put it back as it was, but the garden does slope towards our house so I suspect the main issue is't water from our roof, but water from their garden.

Posted By: jemhaywardWe probably have no choice on that one, as it isn't our garden, so we'd have to put it back as it was, but the garden does slope towards our house so I suspect the main issue is't water from our roof, but water from their garden.

Not necessarily, a lot of heavy rain falling on a slope will run off, depending upon soil type. If the cellar floods quickly after (during) heavy rain then I would suspect run off causing the problem. If the cellar floods some time (next day?) after rain then I would suspect peculation through the soil. Either way the catchment area of the roof dumping next to the wall can't help anything. Flooding caused by run off is easier to fix! BTW did you find the cause or fix the water entering through the window and did it make much difference?

If you are on good terms with your neighbour you may be able to put in a good french drain for the cost of asking politely.