Internal lime plaster with external cement render debate

[Mrsp]

I’ve read this debate in many threads and understand (I think) what the experts will usually say. However…

I have a cornish cottage built around 1850 which is mainly built in stone. When I bought it around 80% of the external walls were covered in a 2 inch thick sand and cement render, internally was the same
Initially we removed the concrete render internally and on 3 external walls externally. You could literally see these walls dry before your eyes!

The front external part of the cottage was completely covered in the render and our plan was to remove this also. However we discovered on this part of the cottage (we think originally it was 2) only the ground floor part of the wall is stone and the upper part is block - it possibly had a wooden structure at some point that was replaced? No idea when.
So we removed the render from the ground floor and blended the upper render in. Then pointed up the stone with lime mortar. See photo.

In the upstairs bedroom at this part of the house the inside of the block wall had been skimmed with gypsum. I would like to remove this and use lime plaster straight on the block, as the gypsum plaster has blown in a couple of spots.

My question is this… Is this a good idea?

I know some will say there is no point as there is cement render on outside. However my thought is that any moisture that may get in, can either move down the wall to where there is just stone, or evaporate through the internal lime render??

Does that make sense to anyone? Any help very much appreciated

 

[Stevers]

Due to damp proofing and previous repairs we have a blend of cement and lime renders internally. Where my new lime renders have been adjacent to the cement renders, only good things have happened in terms of drying out and tolerance of initial salting in quite challenging conditions - mainly around a basement chimney breast. That render/plaster is an easy to work with NHL 3.5 with sand and stone dust. You have little to lose and much to gain by replacing the internal renders even with a cement render outside. That render might well be cement/lime, and if it's blockwork, there's may well be a cavity anyway.

 

[aockenden]

Agreed with Stevers. The “all or nothing” fallacy is a great enemy of humanity. The more cement you replace with lime, the drier and more breathable the house will become. The less cement you replace with lime, the less dry and breathable the house will be. Don’t overthink it.

 

[Stevers]

Don’t overthink it.

Now if only I could do that consistently on my own jobs!

 

[stuart45]

That render might well be cement/lime, and if it's blockwork, there's may well be a cavity anyway.

It's probably better if there's not a cavity, unless there's a tray at the bottom.
Any moisture that penetrates the outer skin will run down into the solid stone wall.

 

[Cubist]

For me the logic of this debate was and is always that we want encourage the house to 'exhale' moisture laden air, whatever its source, whilst ensuring that we minimise the 'inhalation' of the same. Unfortunately none of the materials I'm aware of offer the means to establish genuinely 'one way' streets. But they often do inhibit the speed of the exit road resulting in, quite literally, 'bottlenecks' for the water we would like to see gone. The question then becomes 'how much damage can such trapped water do to the materials adjacent to and in contact with the bottleneck?'

 

[Mrsp]

Due to damp proofing and previous repairs we have a blend of cement and lime renders internally. Where my new lime renders have been adjacent to the cement renders, only good things have happened in terms of drying out and tolerance of initial salting in quite challenging conditions - mainly around a basement chimney breast. That render/plaster is an easy to work with NHL 3.5 with sand and stone dust. You have little to lose and much to gain by replacing the internal renders even with a cement render outside. That render might well be cement/lime, and if it's blockwork, there's may well be a cavity anyway.

Thanks so much for commenting … sometimes just need to hear things confirmed

 

[Mrsp]

Agreed with Stevers. The “all or nothing” fallacy is a great enemy of humanity. The more cement you replace with lime, the drier and more breathable the house will become. The less cement you replace with lime, the less dry and breathable the house will be. Don’t overthink it.

Thanks… we are getting there slowly with removing… I think where we started 80/20 concrete to lime we are now 30/70

 

[Mrsp]

Now if only I could do that consistently on my own jobs!

I have trouble to not over think

 

[Mrsp]

It's probably better if there's not a cavity, unless there's a tray at the bottom.
Any moisture that penetrates the outer skin will run down into the solid stone wall.

I’m pretty sure there is no cavity