Renovation of a mid 19th c Cotswolds cottage

[KLD]

Dave,

Did I understand it correctly that the insulated wall in your wall will be (from outside to inside):
cladding
rigid insulation
50mm open cavity
old stone-built wall
plaster etc

To make that system work, you'd have to ensure the cavity is part of the "warm" side of the structure. In particular, it must not be vented to the outside! But then, you wanted it to be there in the first place to let the old stone wall "breathe".
Well, my take is: it doesn't make sense.

Instead I'd stick the insulation boards directly to the stone wall. If you are worried about damp build-up in the walls, you could use a vapour-open insulation material, e.g. a woodfibre based board. Google for Steico, Pavatex, Gutex.
These boards can then be rendered. Natural Building Technology (NBT) near Oxford have a good website as well, with some example wall sections and insulation calculations.

Klaus

[davec]

Aye correct, that was roughly the proposal although we hadn't got as far as talking about actual materials (yet to finish buying the place).

Intention of the gap is primarily to allow the walls to dry out without soaking the cladding insulation; it's 2ft rubble-filled and 100% wet in places which, I'm told, will take at least 2 years to dry out after installing a damp course. Once dry, the stone will be a useful thermal mass... up till then a source of condensation. I was sort of assuming that we'd have to adjust the exterior ventilation over time.

I've been struggling to find good sources of information / guidance on cladding old buildings so thanks for those search tags.

DaveC

[SimonHa]

...Intention of the gap is primarily to allow the walls to dry out without soaking the cladding insulation; it's 2ft rubble-filled and 100% wet in places which, I'm told, will take at least 2 years to dry out after installing a damp course. Once dry, the stone will be a useful thermal mass...

How are you planning to install a damp course? It sounds very tricky with old stone/rubble walls to me.

[davec]

How are you planning to install a damp course? It sounds very tricky with old stone/rubble walls to me.

Electro-Osmosis has been recommended to me as the only effective system for rubble-filled walls - positive charge reverses capillary action and forces damp downwards. For a whole house, it apparently isn't much more expensive than chemical as the hole-spacing is wider; hence less labour for drilling in.

[KLD]

Dave

Electro Osmosis, is that where you run a strip of copper all around the house, earthing it at one or a few places, and having loops poking into the centre of the wall? I'd be surprised if that really makes any difference to the moisture content in that wall section, to be honest.

Running a french drain along the foot of the wall might be more effective, and probably cheaper?

Klaus

PS. Shall we start a new topic dealing with your project?

[davec]

Dave

Electro Osmosis, is that where you run a strip of copper all around the house, earthing it at one or a few places, and having loops poking into the centre of the wall? I'd be surprised if that really makes any difference to the moisture content in that wall section, to be honest.

Running a french drain along the foot of the wall might be more effective, and probably cheaper?

Yep, that's the thing... titanium wire / electrodes in the walls; a big earth spike in the ground and an 'always-on' power supply. Building is sat on a solid sheet of whinstone bedrock so drains would be difficult.

PS. Shall we start a new topic dealing with your project?

Aye, sorry to have hijacked your topic, Klaus... it's developed from a follow-up question to your comment about sound travel... I'll do some more research and then post a call for wisdom in the Green Construction space.

DaveC