Best way to finish internal wall of a solid stone retaining wall

30 Oct 2019
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United Kingdom
One external wall of a room in my property is constructed from solid stone but also serves as a retaining wall. The room has 2 other external walls which are not retaining walls but are also solid stone. All of the walls have been rendered in what appears to be a cement render, then plastered and painted.

The room is hard to heat, is in a damp area and gets very little direct natural light, especially in winter.

What I'd like to do is strip back the plaster/render, batten the wall out, insulate, board, plaster then paint.

But I don't know what to do about the retaining wall. As solid stone walls are water permeable I presume it is the render that is stopping the water from migrating from the exterior ground level and into the property.

I am not sure what to do, I could try stripping back the render and seeing what's there and seeing if it is damp, but I don't want to make the changes and then only find out its an issue in the depths of winter. I could just leave the retaining wall rendered and just insulate the other two walls. I am open to any other ideas from anyone more qualified!

Any advice on this subject would be greatly appreciated.
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I would avoid drilling into the render, just in case it is a tanked system. My advice would be fix thermal plasterboard dot and dab. Roughen the patches where the dabs will fix with a gritty glass paper and coat with a grit primer - Febond Blue grit or similar. 52.5 or 62.5 board would make a significant difference.
Thanks for the reply. So basically that'd be leaving this wall as a "wet" and non-breathable wall?

I think my issue with that, is whilst that might be the best I can do for the retaining wall, it doesn't seem like the right approach for the other two external walls for that room.

Additionally, the render is really quite thick, and adding an insulation system on top of it is going to start eating into what is already a modestly size room.
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Solid masonry walls don't need to be breathable, and a rendered wall is almost certainly not breathable anyway. As long as the wall is not running with free water, the fact that it is damp is not an issue. Nearly every single solid masonry wall that I have ever tested shows some degree of dampness, and this is often due to condensation from inside the room rather than true penetrating dampness.

D&D thermal laminate is a well proven standard specification for any solid masonry wall, so is good for both types of wall. Yes you would lose 20mm of space by not hacking off render but hacking off may be extremely difficult to achieve and if you find a tanking system behind you will just be re-rendering again - assuming you don't destroy the tanking. For 20mm, which you will never notice anyway, it's not worth it.

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