Ideally you need a lot more than 50mm of insulation 300mm or 200mm, where would the breather membrane go? Behind the slates? But they are draughty and let water out by design

Yes conversion to dwelling but listed so slates have to stay(I would want to keep them anyway)

Will 50mm of cellotex be hardly any benefit?

+1 to Jonti. Worth a discussion with your BCO/Conservation Officer. Also, although it has been done before with Celotex I would be tempted, even if using a plastic insulant, to use graphite EPS. Also since, as I understand, Celotex does not do a non-foil board (and you don't need foil for a VCL as it's the wrong place for a VCL), have you thought of phenolic? This is available non-foiled for EWI.

Also, while 50mm is too little, as others state, if you use exg battens it's 50mm insulation with full-depth timber interruptions every however many mm. If you *had* to stick with 50 could you perhaps do 25 ins against the wall, battens, then 25 between, then slate.

Agree with Tony re membrane. It wouldn't do any *harm* but you almost certainly don't need it.

Edit to add Conservation Officer

Thanks for the replies,very useful.
The slates have got to come off anyway so it will be a good time to add insulation.I like the idea of doing a two layer unfoiled design(Nick Parsons) and will try and push for more than 50mm.
At present I cant see that the other stone (slate possibly) walls can easily be internally insulated.

Oo - so would the EWI behind slates wrap around that return, so standing out from the side wall stonework? Might be hard for Listed Buildings to swallow; on the other hand it would go a long way to creating the overlap between EWI and IWI that Peter mentions.

There's a quite modest limit to how insulative IWI can safely be, before it leaves the wall outboard of it so cold all winter through, like it's never before been, that so much winter condensation builds up within it, that it never quite evaporates in summer, leading to gradual accumulation of water in the masonry. EWI has no such problem, thicker the better, as that keeps the wall warmer than it's ever been!

If wanting to IWI up beyond that danger level, that's where Peter's 'insulated box inside the walls' comes in, which must be coupled, strangely, with really free ventilation to outside air of the airspace between wall and box, so the wall becomes effectively a freestanding garden wall, at which it should have no problem. However I doubt you'll be trying to insulate by IWI up to that danger level.

Posted By: chrisinbrightonDoesn`t external wall insulation need to be breathable.Im assuming warm dampness will work its way through the thick stone walls.

Not necessarily. As long as the EWI is thick enough that the temperature on its inner face is above the dewpoint for whatever humidity you have inside the house then there'll be no condensation on the inner surface so no problem. The problems can come with IWI, because it's cold outer face is against the stone (or other) wall surface, so condensation is much more likely.

...and EPS or phenolic are not breathable I understand ?

EPS is breathable to some extent or at least not impermeable to moisture. µ is 20-40 for EPS 70. Phenolic is a lot more vapour resistant, as is XPS

Thanks for replies,all my questions have been answered..ill let you know how it goes.

Posted By: chrisinbrightonOne thing im not sure about is the 700mm thick granite stone wall is very uneven so will there be a `large` air gap(mabe averarage 15mm) between rigid insulation and stone...will this air gap blow away the warmth from the building?

IMO yes.If there is any air flow in the gap between the insulation and the wall then heat will be removed which will affect the effectiveness of the EWI.

What I do when installing EWI is to use standard EPS 80 grade batts at whatever thickness meets your target U value which I attach to the wall with a band of adhesive on all 4 sides (at the edge) and then 5 dollops in the centre. This I press firmly on to the wall until the adhesive squeezes out at the visible sides and to match the height of the adjacent batts, any adhesive squeezed beyond the visible edges I scrape off to mane the next batt easier to butt up. In this way there is no path for air to wick away heat.

I don't bother to try to get the walls to be straight and plumb but rather I follow any contours the wall may have. No one expects a 200 year old stone/rubble walled building to have straight and square walls

If you want something a bit more breathable than standard EPS Baumit do an EPS with perforations that allow better vapour diffusion than standard EPS (there may be others equally as good)
See https://baumit.co.uk/products/external-wall-insulation/opensystem-breathable-ewi/baumit-opentherm
They also do and internal render designed for such walls.
I have no connection with Baumit, just (so far) a happy user.

If you are going to put slates hung on the EWI then under the horizontal battens either vertical battens or spacers will be needed to hold the horizontal battens off the EWI to allow and water to drain away

I would put the EPS fully across the wall (rather than between studs) and then fix battens through the EPS to the wall.

Hi Nick, yes your `lattice` idea of insulation back in March 2022 seems very good and im thinking of using kootherm papered. People have said that i need an air gap behind the slates though ?. As an alternative to adhesive could i just add some fluffy rockwool between the back of the kooltherm and stone wall?
Also the vertical studs need to stay i think as they are holding alot of weight.

Why not just run a bead of adhesive foam around the perimeter of each piece of rigid insulation?
Should stop any air movement and hold it in place