Retrofitting EWI - solid walls + dwarf cavity - too tricky?

[larry]

Hi, 

I'm after some advice from you knowledgable folks...

 

So you've already helped me with several things related to this, but in essence I've been pulling off the render from our solid wall house to solve penetrating damp issue. It worked - hurrah. We then had a dodgy lintel - advice on here, and we've now got some Helifix in - again, hurrah. Now I've just got to work out what I'm doing with the render. 

 

It's a solid wall house, though with a dwarf cavity wall approx 800mm high. The solid wall sits on top. The old thick cement render, a good inch or two thick, then covered the solid wall and brought it basically level with the cavity wall. I guess the cavity is about 2" thick as that's about the depth of the protrusion. 

 

My starting point was to get somebody to re-render in lime, potentially expensive but bretahable and flexible. 

An alternative was to rerender in a breathable monocouche. Cheaper. 

But in the other posts people have suggested I'd be missing a trick not to put in EWI. 

The only problem with this is that I think it would be a potential pig of a job. It would need to be on three wall faces, one is  a big gable end with no roof overhang. And there are two toilet stacks close to the property. And of course the dwarf cavity wall, which has a brick face, which we really like the look of. I know these are all solvable problems but extending the roof line, moving a cast iron toilet stack, and adding a load of brick slips begins to sound expensive.  Is that a reasonable assumption or are these just issues par for the course?

 

Moreover, I've already started IWI - the top room shown has wood fibre on walls and am pleased with result. Had planned to do similar downstairs. I'm doing this all DIY so no labour costs. It just takes forever.

 

Whilst I think I can just about get away with plastering an (upstairs!) internall wall with lime plaster I don't think my skillset stretches to rendering... though I could probably fix boards to wall, that sort of thing.  

 

My only other thought before giving up on the EWI idea is whether it was possible to add some kind of thin EWI board, say 30mm or so, above the cavity dwarf wall, with only a thin coat render on top, and leave the dwarf cavity wall as is. That would mean the thickness of the board + render would be similar to the thickness of the existing cement render, which means all those adjustements are avoided. I'd still then do IWI in the downstairs room. I could probably fit the boards myself which I guess would save on labour costs. But then do I make the rendering job more or less complex? Is this a bonkers suggestion?  If so, are there any EWI boards that can be fixed straight to brick, are breathable (solid wall, right?), and can be rendered on top with a fairly thin render (also breathable)?

 

I know 30mm is puny but it would I think make a difference particularly in addressing the gap between the two floors 

 

I'd considered some insultaing lime render instead but the insulating values look very low and not sure it would be adding much? Or no?

 

I am talking  the options over with a few people who I hope will give me a quote , but I feel like this is a job where there are so many angles (and different products on the market) so please could I ask the BH brain for collective wisdom - thank you!

 

Am I even asking the right questions?

 

[larry]

Well, I've tried to answer at least part of my own question - attached a very rough comparison of what I could see where the main insulating (mostly lime) renders on the market at the moment. 

Seems they mostly claim to bein the region of 0.12 w/mK but a couple do jump out at somewhat lower (Diathonite for one) but then they both need further finishes on top as well 

 

 

 

 

 

Insulating Renders Comparison - Copy.xlsx

Edited January 25, 2023 by larry
attach right version of file