Thick stone wall, external cement render: how thick can my IWI be?

[Quebrantahuesos]

Hello everyone,
It seems to me that there's no point having the dew point near the outside of my 700mm stone wall if it's bunged up with cement render. So surely the thicker the (breathable) internal insulation the better? The worry is that it will turn to mush (I'm hoping to use wood fibre). I'll have MVHR and possibly a variable ('intelligent') vapour control layer.
What's your experience with breathable internal wall insulation? How much have you put on without getting soaking wet batts/board?
As an extra, some people say plasterboard is fine as a breathable finishing. Any experience? If it saves me the time and expense of lime plaster on wood fibre boards it sounds excruciatingly attractive to me.

 

[Lacan07]

I have no experience of stone walls but I would imagine the thermal qualities of a 70cm thick wall is pretty good straight off the bat. You wouldn't need to go mad with the insulation. For something like this I'd be inclined to plaster directly on to the wall with lime & hemp insulation mixture. No faffing about with batons and boards and skims.

The first thing to determine however is whether the wall is suffering damp due to the cement render, or a concrete floor perhaps. There's no point wasting money on liming a wall that can't breathe, unless you plan to deal with the causes of the damp at a later stage.

Standard plastered board is indeed as breathable as lime (so I've been told) but unfortunately that attribute is pretty useless as it won't withstand actually getting wet. Lime plaster can withstand the wetting/drying cycles, gypsum just turns to mush and collapses.

I'm afraid there may not be a simple fix for you, but I sure wish my walls were 70cm thick.

 

[Cubist]

Hi there and welcome,
I was about to say pretty much what Lacan has just said (beat me to the punch) and its barely worth adding that your biggest problem is the cement render as this will seriously inhibit the buildings ability to breathe at all. Ultimately it is that that will cause and exacerbate future and perhaps significant structural problems.

 

[a twig]

In your situation I would use wood wool rather than wood fibre, fix it straight to the wall and give it a quick skim with lime plaster.

I used Savolit Plus 25mm boards on similar wall thicknesses in my old place to good effect.

It’s quick and easy to do, and will leave you more time and resources to attack the cement render.

 

[Kearn]

Frankly, with that much thermal mass of stone, the internal element is almost just aesthetic. Make sure the stone is dry, and it will insulated instead of conduct… personally I’d go help lime straight onto stone - too much undulation to be fiddling with wood wool/fibre plus the extra cost. you can apply it thicker than just lime in a coat, effectively like a daub

 

[a twig]

I’d disagree I’m afraid. I’ve banged on about it enough in other threads that my biggest regret was not working it out sooner otherwise I’d have done every room like it.

Cheaper, faster, doesn’t require expensive lime specialists and made a noticeable difference to the warmth in the room - between 1 and 1.5 degrees according to my smart heating system at the time comparing before and after.