Lime Render in Exposed Setting

[MarkAldo11]

Hi All,

I’m begging a restoration of an old chapel in the north east of England and the property is very exposed to the elements, with lots of wind and rain from the west.

There is currently a layer of cement render on the exterior of the building, keeping the westerly weather at bay. 
Our architects have proposed to repoint the interior and exterior of the stone in lime, batten the internal walls, fill with sheeps wool and then cover with wood fibre and lime render.
I understand the logic behind there proposal (breathability etc), however the local tradesman take a different view, as they think the property is too exposed to high levels of wind and rain for the this breathable system. Their recommendation is to keep the weather out by lining the internal walls with cement to stop the wind and rain from entering the property. They have seen lime used in the past in these exposed settings and damp and cold spots in the wall have been too frequent.  
There is a definitely a divide on this one, and I’m not sure which way to go. 
Any advice or similar case studies would be a great help. Thanks. 
 

 

 

[nod]

As a business I do use quite a bit of lime plasters each year Usually specified by Architects 

To be clear Whilst lime render is breathable They are poures There are more modern products to insulate and keep the elements outs 

But if you want the lime finish and you have no budget constraints It will do the job and look like it’s been there for a 100 years 

[MarkAldo11]

10 minutes ago, nod said:

As a business I do use quite a bit of lime plasters each year Usually specified by Architects 

To be clear Whilst lime render is breathable They are poures There are more modern products to insulate and keep the elements outs 

But if you want the lime finish and you have no budget constraints It will do the job and look like it’s been there for a 100 years 

Thanks Nod. If we go with the modern method of creating a water tight barrier on the inside and back that up with modern insulation, we will definitely have a warmer house and will be protected from the cold air and rain, but will it have issues breathability? New regs propose vents everywhere, trickles in the windows, so there’s plenty of areas for air to escape rather than breathing through the walls. I’m all for tradition but not to the detriment of cold lime walls and expensive heating bills.

Cheers.

[nod]

14 minutes ago, MarkAldo11 said:

Thanks Nod. If we go with the modern method of creating a water tight barrier on the inside and back that up with modern insulation, we will definitely have a warmer house and will be protected from the cold air and rain, but will it have issues breathability? New regs propose vents everywhere, trickles in the windows, so there’s plenty of areas for air to escape rather than breathing through the walls. I’m all for tradition but not to the detriment of cold lime walls and expensive heating bills.

Cheers.

If you install MVRH you won’t need to have trickle vent 

Our previous build of five years is well insulated But we have windows open most of the time But we’ve never switched the heating on upstairs 

 

You can have the best of both world 

We recently Lime plastered a brand new steel framed building for a football club 

They have the full works Heat pump MVRH etc 

We also installed Parquet flooring throughout Covering UFH 

Entering the building it looks like an old building 

But as I said earlier you will have to fit this into your budget 

If you look at a normal re render and plastering refurb x by five as a guide 

[Redbeard]

@MarkAldo11said: "New regs propose vents everywhere, trickles in the windows, so there’s plenty of areas for air to escape rather than breathing through the walls."

 

Important point is that 'breathability' (water vapour permeability) is not ventilation, and even less to do with air-tightness strategies (someone will find connections I don't have in mind while typing quickly, but this is broadly true). For example you can have a very airtight breathable house, you can have a very airtight non-breathable house, and you can have a non-breathable (or breathable) house which is not very airtight.

[SimonD]

11 hours ago, MarkAldo11 said:

Their recommendation is to keep the weather out by lining the internal walls with cement to stop the wind and rain from entering the property.

 

Time suggests that this approach, especially in older buildings build with stone rarely stands up to the hype and inevitably fails - it is incredibly difficult to detail this to ensure it doesn't fail, hence why so much conservation now proposes using materials that work with moisture. Are you keeping the external cement render, or is this being removed as part of the works? Can you share the full specification of the buildup proposed by the architect and the alternative proposed by the tradesmen?

 

Your other option, which I think works best in exposed areas is to design in a break between the elements and the interior. This could either be a rain screen to the front of the exterior or a ventilation gap between the external wall and a sheeps wool stud wall inside. That way the sheeps wool/woodfibre buildup can buffer excess moisture without the risk of getting saturated if the exposed wall does get wet to the inside.

 

 

[Redbeard]

It is also relevant to note that Thermafleece used to (I don't know if they still do, but I guess so) require that a breathable membrane was used to separate the sheep's wool from the wall itself, for exactly the reason which  @SimonD is suggesting - that the outer (currently cement-based) render *may* fail at some point in the future, in which case the sheep's wool does not want to become a sponge. I second the ventilated (and I mean properly cross-ventilated) void. See for example Stirley Barn, done by the Green Building Company (Green Building Store's building arm).

Edited April 2, 2023 by Redbeard
deleted 'I think'

[MarkAldo11]

The architects are proposing the following internally ; 

Existing Solid stone walls: Ty-Mawr Internal Wall Insulation System -
Sheeps Wool & Woodfibre: existing solid stone wall, breather membrane, 140x50mm C16 regularised timber, treated and dry graded to BS 4978 @ 600mm centres, 140mm Thermafleece insulation fitted between studs, moisture variable vapour check, 60mm T&G wood fibre insulation, lime plaster.

The cement render will be coming off and stone will be repointed in Lime.

[MarkAldo11]

If we expose the stone wall and loose the render, will this then leave too much exposure for the proposed system. This was the concern of the builder.

Thanks all.

[Redbeard]

I am no stone wall expert but I would be trying to find out whether the wall was *originally* rendered externally (granted that it probably would have been in lime then). I have seen a few houses where people have removed the 'horrible render' only to find that the house was originally built with render, mainly because the stone was not of high quality. Take it off, point up the stone, and they had a damp issue because the stone was poor.

 

The system you describe above sounds OK but I would prefer it if it were ventilated, which basically means at least 4 air-bricks per wall in diagonally-opposite corners. If it is not vented then that thickness of insulation is potentially cooling the wall a lo, without anywhere for water vapour to go. Ventilate it and you effectively have the same situation as in a cold roof.

[Iceverge]

Pictures please, inside and out. 

 

How are you fixed for space inside the building? 

 

Equally how about outside? Is the building listed or anything? EWI may be an option. 

 

Do you have any intermediate floors and/or many abutting walls? 

 

 

 

 

[SteamyTea]

21 hours ago, Redbeard said:

Important point is that 'breathability' (water vapour permeability) is not ventilation, and even less to do with air-tightness strategies (someone will find connections I don't have in mind while typing quickly, but this is broadly true). For example you can have a very airtight breathable house, you can have a very airtight non-breathable house, and you can have a non-breathable (or breathable) house which is not very airtight.

I know what you mean, and you are right.

I think that it is a myth that ordinary Portland cement render is waterproof, airtight and has no breathability, compared to lime.

But I cannot find decent numbers to back it up.

I suspect that what happens in a major refurbishment of an old building is that many of the real reasons for damp are cured in the process i.e. gutters fixed, holes filled, roof repaired, soil build up removed from walls, heating and ventilation/humidity levels improved etc.

 

And why specify sheep wool insulation, that is bonkers financially, thermally and environmentally.  I did a calculation somewhere on here that if we took all the flock from sheep, we could insulate about 1200 houses a year, but at a huge environmental cost.  Stick in full fill mineral wool, even if it is not fully filling the cavity.  It has anti-wicking properties.

[larry]

On 03/04/2023 at 08:48, SteamyTea said:

I know what you mean, and you are right.

I think that it is a myth that ordinary Portland cement render is waterproof, airtight and has no breathability, compared to lime.

But I cannot find decent numbers to back it up.

I suspect that what happens in a major refurbishment of an old building is that many of the real reasons for damp are cured in the process i.e. gutters fixed, holes filled, roof repaired, soil build up removed from walls, heating and ventilation/humidity levels improved etc.

 

And why specify sheep wool insulation, that is bonkers financially, thermally and environmentally.  I did a calculation somewhere on here that if we took all the flock from sheep, we could insulate about 1200 houses a year, but at a huge environmental cost.  Stick in full fill mineral wool, even if it is not fully filling the cavity.  It has anti-wicking properties.

It's not just the render. It's the paint, at least that's my theory.

We had a 1920s house, solid brick with sand cement render. Fairly sure it's original. In 2009 Google Street view shows our house brown coloured i.e. Unpainted render. In 2016 previous owner had painted it with homebase standard masonry paint. 2022 and we have penetrating damp. 

 

All render knocked back off and after going around the houses with various quotes and options I've finally found somebody who will do it in lime at a reasonable price. 

 

My theory is the paint basically becomes a layer of cling film. Our problem is the brickwork underneath is horrid (spalled), I guess as water has found its way past the failed render, soaked up into the soft bricks with no way back out (thanks to the paint) and then in winter frozen, popping the front of the brick off with it. 

 

No expertise here but a combination of what I've read and observed and deduction...! 

Inside house now much dryer with render off and only solid brick wall