Best way to insulate solid 9 inch wall with no DPC?

[Matthew309]

Hi all

I have an old 150yo cottage with 9 inch solid wall on load bearing walls, and single skin on gable ends. The bricks are red brick and very porous and soft. The house is in full demo at the moment right back to brick.

I want to improve insulation and sort the damp problem, both penetrating and rising.

Things to consider:

• Water table is 15 Inches from ground level
• Foundations at only 4 bricks deep on clay
• No original dpc ( signs of retrofit but compromised)
• The houses original suspended floor is now a dpm lined concrete floor

With this in mind I can only think of a few options:

1. Build 30mm cavity wall with cls timber and add cavity vents, this solves both rising and penetrating damp and prevents mould. But reduces room size and looses thermal mass of wall, chances of spallng damage long term.
2. Add insulated plasterboard directly onto wall and tanking the base. Utilises thermal mass, more space efficient but high risk of interstitial condensation.
3. Use old breathable materials (something I'm not sure I'm willing to do as that requires removing 80sqm of good concrete flooring.)

There's then other extreme options that may include retrofitting slate DPC, adding internal channels in the concrete, underpinning with concrete creating a barrier, using sump pumps under the concrete floor.

I'd be interested to know people opinions, there no "correct" solution here imo that's doesn't have some form of downside, unless I've missed something?

 

[Cubist]

I think you may need to consider: -

1. Current Water Table at 15 inches - when was this established, how reliable is the measure now, what might the future effects of global warming be?
2. Current foundation is circa 10-12 inches below ground - so just a couple of inches from surface of Water Table.
   1. What is the typical annual variation in Water Table height?
3. The floors are concrete which may be circa 6-8 inches thick with DPM below.

All your walls and the concrete floors will almost certainly be foot-wet at some points during the year and perhaps for protracted periods. Setting aside the penetrating damp, which I am assuming is rain-fall associated, then you you need to add and consider another option. Presently the concrete floors will be 'pushing' damp toward and into the walls and this, in my opinion, is the most likely and significant threat to the house.

I therefore believe you need to give serious consideration to removing the concrete floors and replacing these with a breathable limecrete solution.

 

[Matthew309]

I think you may need to consider: -

1. Current Water Table at 15 inches - when was this established, how reliable is the measure now, what might the future effects of global warming be?
2. Current foundation is circa 10-12 inches below ground - so just a couple of inches from surface of Water Table.
   1. What is the typical annual variation in Water Table height?
3. The floors are concrete which may be circa 6-8 inches thick with DPM below.

All your walls and the concrete floors will almost certainly be foot-wet at some points during the year and perhaps for protracted periods. Setting aside the penetrating damp, which I am assuming is rain-fall associated, then you you need to add and consider another option. Presently the concrete floors will be 'pushing' damp toward and into the walls and this, in my opinion, is the most likely and significant threat to the house.

I therefore believe you need to give serious consideration to removing the concrete floors and replacing these with a breathable limecrete solution.

With the water table so high, surely the ground would be permanent damp with limecrete, therefore any wood flooring or tiling over the top is going to rot and and thave the same effect of pushing damp to edges...

Regarding water table it drops to around 24 inches in summer

 

[Cubist]

When was the 15inch WT measure established? Was this a product of direct and recent observation or was it drawn from publicly held local records? What is the nature of your local topography and how is the cottage sited in relation to local hillsides and slopes? Sadly you have to work on the basis of the worst case measures as to do otherwise opens you up to a world of pain.

The purpose of a limecrete floor to to create a much larger breathable surface from which rising damp can evaporate into the atmosphere and therefore prevent the creation of particularly wet areas in the enclosing brickwork and other walling materials. However, and as you rightly imply, it does limit the range of choice for final floor coverings to similarly breathable materials such as quarry tiles and stone flags.

 

[Kearn]

With the water table so high, surely the ground would be permanent damp with limecrete, therefore any wood flooring or tiling over the top is going to rot and and thave the same effect of pushing damp to edges...

Regarding water table it drops to around 24 inches in summer

Nope, the RFG layer acts as the capillary break and keeps the moisture off the slab. Providing, it’s not more than 30 or 50% saturated - I forget the threshold, but it’s not invincible!

 

[Matthew309]

When was the 15inch WT measure established? Was this a product of direct and recent observation or was it drawn from publicly held local records? What is the nature of your local topography and how is the cottage sited in relation to local hillsides and slopes? Sadly you have to work on the basis of the worst case measures as to do otherwise opens you up to a world of pain.

The purpose of a limecrete floor to to create a much larger breathable surface from which rising damp can evaporate into the atmosphere and therefore prevent the creation of particularly wet areas in the enclosing brickwork and other walling materials. However, and as you rightly imply, it does limit the range of choice for final floor coverings to similarly breathable materials such as quarry tiles and stone flags.

I have a well out the front of my house and can clearly see it vary as the weather changes.

Topologically, the house is on a sloped down from the road to a low point around 100 meters away, my house is probably 30 meters down that slope.

 

[Lacan07]

No solid floor for me, whether limecrete or not, I would reinstate the suspended floors with plenty of ventilation and maybe install some kind of french drain / pump under the house if things are liable to get a bit fruity after heavy rainfall. Then wood fibre the internal walls, they look pretty flat in the photos so no need for a lime base coat.

My neighbour once told me about a chap who specialised in retro-fitted slate DPCs, he had an adapted wheel barrow with a giant circular saw attached to it!

 

[Matthew309]

No solid floor for me, whether limecrete or not, I would reinstate the suspended floors with plenty of ventilation and maybe install some kind of french drain / pump under the house if things are liable to get a bit fruity after heavy rainfall. Then wood fibre the internal walls, they look pretty flat in the photos so no need for a lime base coat.

My neighbour once told me about a chap who specialised in retro-fitted slate DPCs, he had an adapted wheel barrow with a giant circular saw attached to it!

That's a HUGE amount of work and tbh doing that justifies just knocking house down as it already requires so many jobs. I think retrofitting a slate dpc would be a better option then going through reinstating a suspended floor

 

[a twig]

I have a well out the front of my house and can clearly see it vary as the weather changes.

Topologically, the house is on a sloped down from the road to a low point around 100 meters away, my house is probably 30 meters down that slope.

If that’s the case then wouldn’t exterior drainage / water management to divert water around the house be the better option?

Unless it’s an incredibly shallow slope I don’t understand how you can have such a high water table

 

[Jahouli]

Have you considered woodfibre down to floor level with a thermal strip along the bottom edge to prevent cold bridging from the concrete? I did exactly that on 9 inch thick brick walls and limestone ones. I have a fairly high water table too (as demonstrated by my inspection pit in the garage full of water!), but I'm on firmer footings than you, foundations down to limestone about 2 feet down. Its important to note that even with high groundwater levels I didn't have any issues with damp before I Insulated, even with no dpc. I sealed the surface of my concrete slab for belt and braces and added thick underlay and deep pile wool carpet for comfort and a tiny bit of thermal improvement. Breathable wall coatings may allow moisture pushed up to pass through, supported by adequate heating in the colder months?

 

[Matthew309]

If that’s the case then wouldn’t exterior drainage / water management to divert water around the house be the better option?

Unless it’s an incredibly shallow slope I don’t understand how you can have such a high water table

I thought the same, but filling the perimeter with stone will just cause soil erosion and damage to the shallow foundations. The french drains would need to be placed at least 1 meter out which still helps with damp but not a lot.

I'm not sure, it's a heavy clay soil, water just doesn't drain away.

 

[Matthew309]

Have you considered woodfibre down to floor level with a thermal strip along the bottom edge to prevent cold bridging from the concrete? I did exactly that on 9 inch thick brick walls and limestone ones. I have a fairly high water table too (as demonstrated by my inspection pit in the garage full of water!), but I'm on firmer footings than you, foundations down to limestone about 2 feet down. Its important to note that even with high groundwater levels I didn't have any issues with damp before I Insulated, even with no dpc. I sealed the surface of my concrete slab for belt and braces and added thick underlay and deep pile wool carpet for comfort and a tiny bit of thermal improvement. Breathable wall coatings may allow moisture pushed up to pass through, supported by adequate heating in the colder months?

So you have a concrete slab with a perimeter wall sat directly on limestone with no DPC? How did you not get any damp issues with that?

What do you mean by thermal strip?

 

[a twig]

I thought the same, but filling the perimeter with stone will just cause soil erosion and damage to the shallow foundations. The french drains would need to be placed at least 1 meter out which still helps with damp but not a lot.

I'm not sure, it's a heavy clay soil, water just doesn't drain away.

View attachment 15038

At the risk of asking potentially a stupid question, is that not a cistern rather than a well?

It would explain the drop in summer as there is less rainwater running into it

My creaking memory banks are suggesting that @overlander matt found a cistern in his place? Perhaps he might be able to chip in.

 

[Matthew309]

At the risk of asking potentially a stupid question, is that not a cistern rather than a well?

It would explain the drop in summer as there is less rainwater running into it

My creaking memory banks are suggesting that @overlander matt found a cistern in his place? Perhaps he might be able to chip in.

Well considering when I pumped it dry it filled through seepage within 30 minutes I assumed it was a well. However the rainwater from the gutter does run into it and there also appears the be a pipe to a concrete block adjacent to the house to what I assume is to pump the water out?

It's also about 5 meters deep but only has a diameter of around 0.8-1m

 

[Cubist]

Thanks for the additional information concerning the site on which the house stands but to confirm: -

• The house stands on the slope of a hillside.
• The foundations of the structure stand on/in the clay bed.
• There is a well or cistern close to the house.

As rainwater will always follow the line of least resistance it will flow downhill through the topsoil and over the top of the clay bed. I think your best course would be to install a French Drain situated along the up-slope face of the property with two legs parallel to the downhill sides of the building. The bottom of the drain needs to be sited immediately above or just below the top of the clay bed - but only slightly below the foundation. The purpose of this is to ensure that the bulk of the water run-off from the hill-side is directed away from the buildings structure and floors. This alone may be sufficient to considerably reduce the scale and extent of the rising damp you are finding but it may take a year of so to prove that.

At present the up-slope side of your house is operating rather like a dam and run-off is collecting against the foundation before escaping down its sides. Inevitably some of that water must also be finding its way through the brickwork of the foundation and then under the concrete floors before escaping via to the downhill side. If the well/cistern is situated on the uphill side of the property then you may well find that old French Drains are already in place and that these feed the well/cistern but have maybe become blocked.

You cannot use the depth of water in a well or cistern to be a reliable guide to local groundwater levels, particularly on a hillside, as the topography of the surrounding land and the nature of the soils, clays and rock beds will have a considerable influence on the behaviour of rainwater run-off.

 

[Matthew309]

Thanks for the additional information concerning the site on which the house stands but to confirm: -

• The house stands on the slope of a hillside.
• The foundations of the structure stand on/in the clay bed.
• There is a well or cistern close to the house.

As rainwater will always follow the line of least resistance it will flow downhill through the topsoil and over the top of the clay bed. I think your best course would be to install a French Drain situated along the up-slope face of the property with two legs parallel to the downhill sides of the building. The bottom of the drain needs to be sited immediately above or just below the top of the clay bed - but only slightly below the foundation. The purpose of this is to ensure that the bulk of the water run-off from the hill-side is directed away from the buildings structure and floors. This alone may be sufficient to considerably reduce the scale and extent of the rising damp you are finding but it may take a year of so to prove that.

At present the up-slope side of your house is operating rather like a dam and run-off is collecting against the foundation before escaping down its sides. Inevitably some of that water must also be finding its way through the brickwork of the foundation and then under the concrete floors before escaping via to the downhill side. If the well/cistern is situated on the uphill side of the property then you may well find that old French Drains are already in place and that these feed the well/cistern but have maybe become blocked.

You cannot use the depth of water in a well or cistern to be a reliable guide to local groundwater levels, particularly on a hillside, as the topography of the surrounding land and the nature of the soils, clays and rock beds will have a considerable influence on the behaviour of rainwater run-off.

You make some good points, this would be an enormous task but it might be required. As its all concrete on the upside of the property.

I'm struggling to understand the reasoning behind old french drains leading to a cistern; surely if you feeding them into the uphill cistern, that water would just pass back down the hill and eventually find an equilibrium where half the pipes are just filled with water due to the cistern water level?



Something strange about this well/cistern is that there are two large black pipe either feeding in at about 1m down which I found when pumping it. They go off in different directions but I cannot make out what they are or where they go

 

[Jahouli]

So you have a concrete slab with a perimeter wall sat directly on limestone with no DPC? How did you not get any damp issues with that?

What do you mean by thermal strip?

Luck!? Ground levels around the house are below internal floor slab and mostly gravel. Soil here is also sandy so drains pretty well. Lime pointing externally. So overall a good place to start from.

By thermal strip I mean something like this: Expanding foam tape. Creates a thermal break so the wood fibre isn't resting directly on the concrete slab.

 

[Matthew309]

Thanks for the additional information concerning the site on which the house stands but to confirm: -

• The house stands on the slope of a hillside.
• The foundations of the structure stand on/in the clay bed.
• There is a well or cistern close to the house.

As rainwater will always follow the line of least resistance it will flow downhill through the topsoil and over the top of the clay bed. I think your best course would be to install a French Drain situated along the up-slope face of the property with two legs parallel to the downhill sides of the building. The bottom of the drain needs to be sited immediately above or just below the top of the clay bed - but only slightly below the foundation. The purpose of this is to ensure that the bulk of the water run-off from the hill-side is directed away from the buildings structure and floors. This alone may be sufficient to considerably reduce the scale and extent of the rising damp you are finding but it may take a year of so to prove that.

At present the up-slope side of your house is operating rather like a dam and run-off is collecting against the foundation before escaping down its sides. Inevitably some of that water must also be finding its way through the brickwork of the foundation and then under the concrete floors before escaping via to the downhill side. If the well/cistern is situated on the uphill side of the property then you may well find that old French Drains are already in place and that these feed the well/cistern but have maybe become blocked.

You cannot use the depth of water in a well or cistern to be a reliable guide to local groundwater levels, particularly on a hillside, as the topography of the surrounding land and the nature of the soils, clays and rock beds will have a considerable influence on the behaviour of rainwater run-off.

To be honest, I'm debating just adding a physical DPC and being done with it. So many conflicting opinions and solutions.

 

[Cubist]

You make some good points, this would be an enormous task but it might be required. As its all concrete on the upside of the property.

I'm struggling to understand the reasoning behind old french drains leading to a cistern; surely if you feeding them into the uphill cistern, that water would just pass back down the hill and eventually find an equilibrium where half the pipes are just filled with water due to the cistern water level?



Something strange about this well/cistern is that there are two large black pipe either feeding in at about 1m down which I found when pumping it. They go off in different directions but I cannot make out what they are or where they go

So the cistern is on the upslope side of the house? If so, then the black pipes, which at 1metre underground are presumably situated below the clay bed, may be routing the water to soakaways in the surrounding land. In which case surface rainwater must be entering from upslope and the only available route would be from an old French drain situated above the clay bed and directed toward the cistern.

To be honest, I'm debating just adding a physical DPC and being done with it. So many conflicting opinions and solutions.

Considering the cost, effort, materials and time needed to do that I would suggest that a French drain - sited a few meters away beyond the edge of the concrete outdoors - as per my previous suggestion - would be far less expensive and troublesome.

 

[Matthew309]

So the cistern is on the upslope side of the house? If so, then the black pipes, which at 1metre underground are presumably situated below the clay bed, may be routing the water to soakaways in the surrounding land. In which case surface rainwater must be entering from upslope and the only available route would be from an old French drain situated above the clay bed and directed toward the cistern.


Considering the cost, effort, materials and time needed to do that I would suggest that a French drain - sited a few meters away beyond the edge of the concrete outdoors - as per my previous suggestion - would be far less expensive and troublesome.

The black pipes also run downhill though, one passes somewhat under the house and the other goes off towards a neighbouring house.

What your suggesting makes sense, but I would imagine they would technically be overflow pipes that redirect water towards a soak way or the like?