Do we dig up the concrete floors?

[FightingTheDamp]

We are renovating a very damp cottage in the Lake District. The sources of damp are many and varied, and we’re trying to deal with all of them. The big ones are a new roof, stripping off failing cement render & re-rendering in lime. The thing I’m not sure about is the floors. Some of the ground floor has (uninsulated) poured cement floors. Predictably, you can see that this has forced some damp up into the neighbouring walls (even the internal ones).

In an ideal world, we would dig this all up, lay vapour permeable insulation, lime screed with underfloor heating, and then tiles with lime mortar as grout over the top. This would be warm and vapour permeable. BUT the foundations appear to be very shallow. So I’m nervous about digging down and destabilising the wall. As with most cottages, the ceilings are low, so we can’t steal depth upwards either.

Perhaps once we have the rest of the damp issues addressed, and the house is well heated, we might get away with non permeable floors? Or do you think we absolutely need to get rid of that concrete, in which case, how can we have a permeable floor that’s not freezing that doesn’t need much depth?

 

[Itchinfilth]

I dug the concrete out as had the same issue. I'm up north and pretty high up so the weathers not the dryest and warm. Luckily when we dug down we found the majority of the 400 year old flags underneath layed on earth. We used the minimum intervention method and have kept the flags layed direct on earth and simply repointed in lime. We accept the fact that there is permeability and damp spots come on the flags but they disappear equally quickly. The walls are now perfectly dry. We have only Stone floors layed to earth on the ground floor and don't find it cold.
Do you know what's under the concrete yet?

 

[aockenden]

We have the same in our stone cottage. Retrofitted poured-in uninsulated cement slab with plastic DPC underneath which is absolutely pushing significant amounts of damp out from underneath into the base of one wall. We had hoped to avoid digging out the floors originally but after 3 winters on this cold, clammy floor we have decided it needs to go. We also have relatively shallow foundations (or more accurately, big foundation stone laid onto compacted clay), but I’m not worried as long as when we’re digging it out we leave a 45 degree slope down and away (into the interior of the rooms) from the bottom of the foundation stones until we hit the total depth needed, and then flatten out.

If we dig it carefully by hand when the ground is not saturated and then immediately refill the depth with compacted recycled foam glass and then lime screed it, I think we’ll be fine.

 

[Glenn586]

That interesting and sounds like a very sensible approach @aockenden. I would imagine you a forfeiting a little bit of u-value to retain the structural integrity.

 

[G_amsonad]

We are renovating a very damp cottage in the Lake District. The sources of damp are many and varied, and we’re trying to deal with all of them. The big ones are a new roof, stripping off failing cement render & re-rendering in lime. The thing I’m not sure about is the floors. Some of the ground floor has (uninsulated) poured cement floors. Predictably, you can see that this has forced some damp up into the neighbouring walls (even the internal ones).

In an ideal world, we would dig this all up, lay vapour permeable insulation, lime screed with underfloor heating, and then tiles with lime mortar as grout over the top. This would be warm and vapour permeable. BUT the foundations appear to be very shallow. So I’m nervous about digging down and destabilising the wall. As with most cottages, the ceilings are low, so we can’t steal depth upwards either.

Perhaps once we have the rest of the damp issues addressed, and the house is well heated, we might get away with non permeable floors? Or do you think we absolutely need to get rid of that concrete, in which case, how can we have a permeable floor that’s not freezing that doesn’t need much depth?

“Perhaps once we have the rest of the damp issues addressed, and the house is well heated, we might get away with non permeable floors?”

I’m in a similar position and this is exactly the question I find myself asking. Interested to see what people think.

Tempted to leave ours but obviously the cement being damp can’t be good in terms of the houses efficiency.

 

[aockenden]

That interesting and sounds like a very sensible approach @aockenden. I would imagine you a forfeiting a little bit of u-value to retain the structural integrity.

You mean with the 45 degree slope down from the foundation stones to the final depth? Yes the edges of the room would be less insulated than the middle as those edges would have less foam glass depth. Worth it imho though when weighed against the risk of triggering a destabilization of the stone wall.

 

[DaveG]

Is the concrete itself damp or just the walls? If the concrete is dry, this suggests polythene sheet was put underneath to at least avoid a damp floor.
I’m in the process of digging one of my concrete floors out. One of 3 rooms that were all originally suspended timber floors and now have concrete poured floor and gypsum/concrete lower half walls. As you describe, wet walls so much so in my case, timbers on internal walls have turned to dust.
I’m going all out with this first and smallest room but based on learnings I’m going to consider whether this all out approach is needed everywhere else or there are simpler less invasive methods.
For instance I’ve since been tackling outside ground levels which anre too high and it got me thinking, if I focussed on that for the other rooms, would the concrete floors be such a big deal? Of course each case is different depending on water table levels etc.

All I would say is many people jump straight in tackling on multiple battlefronts (myself included) and while it will likely be the most sure fire way to resolve the damp it’s also incredibly expensive and tiresome.
It sounds like if you have other more pressing areas of ingress like roof, focus on those first and see if other areas start to resolve themselves.
Another thing you could try would be to just take the edges of the concrete slab up and make those limecrete and breathable. It could resolve things at a fraction of the cost and hassle. It’s a bit of a trial and error approach I grant you.
If the room is not inhabited like mine, take up the edges, ensure outside ground levels and gutters are in good shape and leave it over winter to see if that’s all you need to do? If it doesn’t improve you know you’ve got to go more invasive.

 

[noj]

@DaveG with something similar I just removed the cement and bitumen on the inside of the plinth walls, replastered with lime/hemp and lowered the external ground level. The wall and sole plate took about six months to fully dry but have been perfect ever since despite the poured concrete floor over a sheet of plastic.
It's been like that for 12 years now and the limewashed plinth woul be a great indicator of damp...

 

[Kearn]

In an ideal world, we would dig this all up, lay vapour permeable insulation, lime screed with underfloor heating, and then tiles with lime mortar as grout over the top. This would be warm and vapour permeable.

Not entirely true…
A limecrete floor system isn’t about being moisture permeable, it’s about moisture management and insulation.

The RFG layer acts as a capillary break, only the lower section of it will absorb, then release moisture. Unless there are extreme conditions, your slab isn’t dealing with any moisture from beneath. Therefore what goes on top of limecrete can be non porous. Limestone or similar porous stone slabs need sealing, otherwise they’ll stain and become filthy.
Which leads me to the fact that the slab doesn’t have to be limectete, it can be a concrete slab. Which is precisely how I’m approaching my floors. It saves a ton of effort and cash.

The important element is the RFG as a capillary break and insulation layer. If depths are limited, you can get away with 65-70mm of slab, save the rest of the depth for RFG. At least 200mm compacted.

BUT the foundations appear to be very shallow. So I’m nervous about digging down and destabilising the wall.

Same for all old buildings, it just requires care when excavating. As others have said, dig at a 45deg angle away from walls by hand and take it slow. You never know what you’ll find…!

Georgian Hot Tub Time Machine??! what is this...

right, I am at this stage resigned for this to remain a mystery, but thought it would be an interesting one to throw out there for thoughts now that I've finally dug out my laptop and can upload photos... Context: - house is late 1500s/early 1600s - timber framed, built with some wealth...

www.periodproperty.co.uk

As others have said, make sure you really understand where any unwanted moisture is coming from before going nuclear on the floors, as it’s very disruptive and hard work!

 

[FightingTheDamp]

Oh wow - you lot have a wealth of knowledge on this, thank you!

The ground level thing is a good point - this is something we are addressing around that part of the house, which might help a lot.

45 degree angle makes total sense, and this may ultimately be what we do if the other bits of work don’t seem to address the issue.

Also interested in the idea of a limecrete border - sounds like much less mess and cost than digging it *all* up.

I suppose our next steps will be a bit more investigation to see exactly what we’re dealing with under this concrete and will go from there… I will report back in due course!

In the meantime, thanks to you all again - very helpful!

 

[Kearn]

It's been like that for 12 years now and the limewashed plinth woul be a great indicator of damp...

as would the hemp in the lime - by all accounts when used on wet substrate, it can rot, discolour and stain through and be visible on the finish.

 

[Niall]

Oh wow - you lot have a wealth of knowledge on this, thank you!

The ground level thing is a good point - this is something we are addressing around that part of the house, which might help a lot.

45 degree angle makes total sense, and this may ultimately be what we do if the other bits of work don’t seem to address the issue.

Also interested in the idea of a limecrete border - sounds like much less mess and cost than digging it *all up

This site has a slabless shallow limecreate UFH solution with visible 45deg in the diagram.

Limecrete - The Limecrete Company

A limecrete floor consists of a compacted layer of Recycled Foamed Glass insulation on top of prepared sub-base and geotextile membrane. This is followed by another layer of geotextile and limecrete produced using lightweight insulating aggregate rather than conventional sand.

limecrete.co.uk

 

[CliffordPope]

I take it the 45 degrees is just an average figure for rough guidance, depending on the actual nature of the subsoil?
Sand I can imagine being unsafe at any slope greater than 5 degrees? Ours is sort of silty clay in a solid compacted pudding with a lot of flat irregular rocks stirred into it, anything from a few inches to several feet across. It's jolly hard to excavate, but an exposed vertical wall several feet high cut through a bank stands indefinite exposure to weather without any propping.

 

[aockenden]

I take it the 45 degrees is just an average figure for rough guidance, depending on the actual nature of the subsoil?
Sand I can imagine being unsafe at any slope greater than 5 degrees? Ours is sort of silty clay in a solid compacted pudding with a lot of flat irregular rocks stirred into it, anything from a few inches to several feet across. It's jolly hard to excavate, but an exposed vertical wall several feet high cut through a bank stands indefinite exposure to weather without any propping.

Common sense is always in order!

 

[Frenchpierre]

I have a similar situation with a large cellar in a 200 yr old stone building. A well sits directly outside the cellar and the whole space is extremely damp. Local builders do not believe in using lime and are coercing us into putting a concrete slab, citing structural issues if we dig past the shallow foundations. They are saying that limecrete needs a minimum of 50cm depth - the diagram above does not have measurements. Does anyone know if there is a minimum depth required for limecrete? Also, in France it is traditional to do a "Herisson" in place of lecadur or RFG (which are extremely hard to find here) - the "Herisson" is basically river-rock stones placed on an earthen floor, compacted down with volcanic gravel (pouzzalane) and then limecrete. Anyone ever come across this before?

 

[almostatmywitsend]

I have a similar situation with a large cellar in a 200 yr old stone building. A well sits directly outside the cellar and the whole space is extremely damp. Local builders do not believe in using lime and are coercing us into putting a concrete slab, citing structural issues if we dig past the shallow foundations. They are saying that limecrete needs a minimum of 50cm depth - the diagram above does not have measurements. Does anyone know if there is a minimum depth required for limecrete? Also, in France it is traditional to do a "Herisson" in place of lecadur or RFG (which are extremely hard to find here) - the "Herisson" is basically river-rock stones placed on an earthen floor, compacted down with volcanic gravel (pouzzalane) and then limecrete. Anyone ever come across this before?

Hi, we have similar problems here in the UK. You might find this of interest, a family renovation of a chateau in France. This video covers installation of a Herisson...

 

[mh12]

Not entirely true…
A limecrete floor system isn’t about being moisture permeable, it’s about moisture management and insulation.

The RFG layer acts as a capillary break, only the lower section of it will absorb, then release moisture. Unless there are extreme conditions, your slab isn’t dealing with any moisture from beneath. Therefore what goes on top of limecrete can be non porous. Limestone or similar porous stone slabs need sealing, otherwise they’ll stain and become filthy.
Which leads me to the fact that the slab doesn’t have to be limectete, it can be a concrete slab. Which is precisely how I’m approaching my floors. It saves a ton of effort and cash.

The important element is the RFG as a capillary break and insulation layer. If depths are limited, you can get away with 65-70mm of slab, save the rest of the depth for RFG. At least 200mm compacted.


Same for all old buildings, it just requires care when excavating. As others have said, dig at a 45deg angle away from walls by hand and take it slow. You never know what you’ll find…!

Georgian Hot Tub Time Machine??! what is this...

right, I am at this stage resigned for this to remain a mystery, but thought it would be an interesting one to throw out there for thoughts now that I've finally dug out my laptop and can upload photos... Context: - house is late 1500s/early 1600s - timber framed, built with some wealth...

www.periodproperty.co.uk

As others have said, make sure you really understand where any unwanted moisture is coming from before going nuclear on the floors, as it’s very disruptive and hard work!

Kearn. How is the floor project going? All well I hope.
Can you please share the depths and the makeup of your new floors buildup please?
Was it Geo membrane followed by 200mm RPG followed but 70mm concrete slab?
Did you have to have to dig approx 300mm from existing floor level?

Thank you

 

[Kearn]

Kearn. How is the floor project going? All well I hope.
Can you please share the depths and the makeup of your new floors buildup please?
Was it Geo membrane followed by 200mm RPG followed but 70mm concrete slab?
Did you have to have to dig approx 300mm from existing floor level?

Thank you

Almost finished the second one…well, still need to tile next week.

This floor was 150-200mm compacted RFG, all wrapped in membrane (can be standard garden geomembrane, or omitted).
I did add a cork perimeter here as the stone plinths are wet.
Plastic grid with UFH pipes cable tied.
Then 100mm NHL 3.5/5 slab, 1:2.5 with 4mm grit and with synthetic fibres. I had a mix of St Astier to get rid off, hence both, either will do.
I’m still 40mm from finish floor level, so there will be about 18mm NHL bedding mix to lay the 22mm slabs.
It’s now tucked up under a bubble wrap duvet at night to resist freezing as the room is unheated and doesn’t have all its walls…!

I didn’t dig this one, it was already taken down to bedrock when the suspended floor was added, hence the edges are straight down not 45deg at the plinth walls and fireplace. Not ideal as there were no footings underground.
Equally, in wet periods, the limestone bedrock doesn’t drain quickly, so there was often standing water under the floorboards, rotting joists and mould aplenty.

For that reason, my starting depth varied, hence the variation in RFG. Ideally I would’ve gone a bit deeper on RFG and reduced the slab thickness. Especially given the horror day it was mixing that much limecrete, but materials dictated.
Minimum 150mm normally for RFG and at least 60-70mm slab with wet UFH.

In hindsight, I’d go for a concrete slab pumped in, I mixed literally all day non stop from 7am-6pm with two 200L Barons continually running. Shifted about 8 tonnes by shovel and barrow for 33m2.

It’s a 4 man job really, 1 mixing, 1 prepping next mixes into buckets, 1 barrowing, 1 screeding - mixing is the limiting factor.

There were a few thermal considerations, but mostly I had a lot of NHL needing to be buried!

 

[mh12]

Almost finished the second one…well, still need to tile next week.

--This floor was 150-200mm compacted RFG, all wrapped in membrane (can be standard garden geomembrane, or omitted).
I did add a cork perimeter here as the stone plinths are wet.
Plastic grid with UFH pipes cable tied.

--I didn’t dig this one, it was already taken down to bedrock when the suspended floor was added, hence the edges are straight down not 45deg at the plinth walls and fireplace. Not ideal as there were no footings underground.
Equally, in wet periods, the limestone bedrock doesn’t drain quickly, so there was often standing water under the floorboards, rotting joists and mould aplenty.


--In hindsight, I’d go for a concrete slab pumped in, I mixed literally all day non stop from 7am-6pm with two 200L Barons continually running. Shifted about 8 tonnes by shovel and barrow for 33m2.
View attachment 16051View attachment 16052

Kearn, many,many thanks for this. You've saved the day. I'd like to follow a similar approach minus the limecrete finish simply because of lack of experience me and the worker(s) who will be doing the work. Going to take active role in the bits that I feel I can do (to save some money) and leave the rest to more experienced person(s).

Can I clarify please the make up of your floor especially the bits about Geomembranes insertions.
Geomembrane on soil followed by varying levels of RPG followed by Geomembrane followed by plastic mesh for UFH piping clips followed followed by varying levels NHL 3.5/5 mix to reach finishes floor level?

Does the Geomembrane have to be breathable? Is it essentially a DPM?

Looking at the first picture in your response there appears to be some sort of white textile under RGP- is this the Geomembrane?
2nd picture- again some white textile above the laid floor- I'm assuming this is bubble wrap covered with Hessian membrane.

I think I would prefer to have the base as your except to finish with concrete floor or screed to floor level.

So I'm thinking the following order, various permutations:
1) RPG (200-250mm depth), then Geomembrane with plastic grid mesh and UFH pipes clipped into Geomembrane, then concrete slab (100mm)to finished floor level
2) RPG (200-250mm depth), Geomembrane, concrete slab (100mm), UFH pipes clipped into concrete slab, screed (60-70mm) to finish to floor level.
3) RPG (200-250mm depth), Geomembrane, concrete slab (50 to 100mm?), Geomembrane, Solid Woodfibre or Corkboard (30mm) UFH pipes clipped UFH pipes clipped into WF/Corkboards, wet screed (60-70mm) to finish to floor level.

The reason for options 2 and 3 is UFH pipes will be closer to finished floor level and quicker to heat the rooms. With option 3 the addition of corkboard will help to reflect some of the heat back upwards? rather than heat travelling down to Glapor RPG and then being reflected upwards into room

Just for my clarification (this may seem a silly question so forgive my ignorance) is the possible to have a hybrid see below?

4) RPG (150-200mm), Geomembrane, Concrete slab (70-100mm), PIR (50-100mm) in the centre of rooms but not touch the walls so away from walls approx 15 cm away from any wall perimeter and the 15cm gap between wall perimeter and PIR made good with WF/Cork board to similar thickness level, UFH pipes clipped to insulation finished with wet Screed (60-70mm) to floor level?

In ALL options corkboard upstands will be placed next to wall perimeters.
Essentially what I'm trying to ascertain is any form of PIR which is suitable distance away from perimeter walls a definite NO, NO.

Please critique the above, I welcome whole heartedly constructive criticisms.

 

[Kearn]

Kearn, many,many thanks for this. You've saved the day. I'd like to follow a similar approach minus the limecrete finish simply because of lack of experience me and the worker(s) who will be doing the work. Going to take active role in the bits that I feel I can do (to save some money) and leave the rest to more experienced person(s).

Can I clarify please the make up of your floor especially the bits about Geomembranes insertions.
Geomembrane on soil followed by varying levels of RPG followed by Geomembrane followed by plastic mesh for UFH piping clips followed followed by varying levels NHL 3.5/5 mix to reach finishes floor level?

No varying levels of RFG, just my floor depth wasn’t uniform - ranged from 150-200 compacted due to the slope. No varying levels of NHL, one single slab, I just happened to have 3.5 and 5 so mixed them up. You can use either. To don’t use the clip rails, just cables tie to the mesh. My NHL slab is shy of final floor level, in this instance it happened to be 40mm, I’m using 22mm slabs, so there will be an incremental NHL bedding layer of around 18mm…

Does the Geomembrane have to be breathable? Is it essentially a DPM?

It is not a DPM. It’s a communal garden membrane for weed suppression. I’ve left it out before, think it prevents unwelcome plant growth really. Totally breathable.

Looking at the first picture in your response there appears to be some sort of white textile under RGP- is this the Geomembrane?

Yes - you lay it on the ground, lapped and up the walls, then wrap it over the top of the compacted RFG. It’s a burrito.

2nd picture- again some white textile above the laid floor- I'm assuming this is bubble wrap covered with Hessian membrane.

No. Geotextile membrane wrapped over the top of RFG.

I think I would prefer to have the base as your except to finish with concrete floor or screed to floor level.

So I'm thinking the following order, various permutations:
1) RPG (200-250mm depth), then Geomembrane with plastic grid mesh and UFH pipes clipped into Geomembrane, then concrete slab (100mm)to finished floor level

That works fine. The membrane wraps around RFG remember. It’s not a layer as such.

2) RPG (200-250mm depth), Geomembrane, concrete slab (100mm), UFH pipes clipped into concrete slab, screed (60-70mm) to finish to floor level.

No real benefit of laying two slabs. The RFG is the insulation layer. You’re now heating the bottom of the slab unnecessarily. It’s a deep slab. Better to add more RFG if you want this depth.

3) RPG (200-250mm depth), Geomembrane, concrete slab (50 to 100mm?), Geomembrane, Solid Woodfibre or Corkboard (30mm) UFH pipes clipped UFH pipes clipped into WF/Corkboards, wet screed (60-70mm) to finish to floor level.

No. Now you’re complicating things…! The RFG underneath is the important bit of insulation. And you’re starting to head towards Oz at this point…

The reason for options 2 and 3 is UFH pipes will be closer to finished floor level and quicker to heat the rooms. With option 3 the addition of corkboard will help to reflect some of the heat back upwards? rather than heat travelling down to Glapor RPG and then being reflected upwards into room

Slab doesn’t need to be more than 70mm. Heating rooms quickly doesn’t happen with UFH, it’s not how it works. You want low and continuous heat. The slab maintains a warmth - it takes time to get to temp and time to dissipate. You don’t switch on UFH and expect a room to heat as you’re heating a large thermal mass, not air like with rads.

Just for my clarification (this may seem a silly question so forgive my ignorance) is the possible to have a hybrid see below?

4) RPG (150-200mm), Geomembrane, Concrete slab (70-100mm), PIR (50-100mm) in the centre of rooms but not touch the walls so away from walls approx 15 cm away from any wall perimeter and the 15cm gap between wall perimeter and PIR made good with WF/Cork board to similar thickness level, UFH pipes clipped to insulation finished with wet Screed (60-70mm) to floor level?

Can’t even understand the goal here I’m afraid! The system is relatively simple, insulation, capillary break and structural sub base at the bottom in RFG - thermal slab above with pipes in that heat it up and keep it warm. Slab doesn’t need to be more than 70-100mm. If you have the depths you’re talking about, then max out on RFG.

In ALL options corkboard upstands will be placed next to wall perimeters.
Essentially what I'm trying to ascertain is any form of PIR which is suitable distance away from perimeter walls a definite NO, NO.

Please critique the above, I welcome whole heartedly constructive criticisms.

I’d always say, pick up the phone to Eden Hotlime, Ty Mawr, Mike Wye, Cornish… they will all happily talk you through the options and build up. They also have diagrams, detailing online. Both will answer the Qs above and anymore…