Stone & rubble walls: all-natural solid insulation, or a "system"?

[YodhrinForge]

As well as my roofing questions which the forum has been very helpful with, I'm considering how to approach the insulation of the internal walls of my late 1800's semi-detached stone walled cottage. My initial instinct was to go for a solid mass buildup using clay plaster to level the wall and also to bond wood fibre boards to it, then a reinforcing layer of the same clay plaster with mesh, and a final topcoat of a finer grade of clay plaster. This leaves the wall completely vapour open and relies on the combination of MVHR and the properties of the clay & wood fiber boards to manage humidity and condensation(any windows will be triple glazed insulated-aluclad timber, but tbh there aren't that many windows so most of the walls will be taking insulation). However I've recently been put on to "more modern" techniques by a builder pal, exemplified by BackToEarth's "NatureWall" system that has a much more layered buildup of bonded wood fiber as previous but instead of a reinforcing coat they specify a vapour open airtightness membrane with taped seams, then a battened service cavity with the floppy wood fiber to infil, then what seems to be a clayboard type product as your internal surface to then skimcoat.

 

The notion of running services in a cavity rather than having to chase out paths in the insulation is appealing, but honestly I'm not completely sold on all these membranes and tapes that get pushed these days - am I just being a snobby Luddite or is there something to the modern approach?

[Russell griffiths]

If you can add a service cavity then do it, these speed you can add services is brilliant and will save money on trades chasing and filling in chases. 
 

if you use the correct size batten you can get a back box to line up nicely with the front face of your boards. 
 

the board system you state sounds like it will give you a flatter wall, whereas the clay might look more traditionally lumpy bumpy. 
 

My plasterer has just installed some clay finish plaster that you don’t paint, comes in all different colours. 

[saveasteading]

1 hour ago, YodhrinForge said:

am I just being a snobby Luddite or is there something to the modern approach?

You are being a sensible, pragmatic researcher.

Membranes, insulation, and service voids are great but must be used properly. 

 

I'd venture that the "back to nature" methods are the ones to be cautious about. Nature has no idea of house building.

[Mike]

13 hours ago, YodhrinForge said:

My initial instinct was to go for a solid mass buildup using clay plaster to level the wall and also to bond wood fibre boards to it, then a reinforcing layer of the same clay plaster with mesh, and a final topcoat of a finer grade of clay plaster. This leaves the wall completely vapour open and relies on the combination of MVHR and the properties of the clay & wood fiber boards to manage humidity and condensation

Provided the wall is sound externally that sounds OK, although I'd swap to a lime-based plaster (at least behind the insulation) as the alkalinity provides additional protection against mould.

 

13 hours ago, YodhrinForge said:

 The notion of running services in a cavity rather than having to chase out paths in the insulation is appealing, but honestly I'm not completely sold on all these membranes and tapes that get pushed these days - am I just being a snobby Luddite or is there something to the modern approach?

A brick wall that's been plastered or parged will provide all the airtightness necessary, provided you link it to all adjoining components using proprietary products. If your wall is suitably resistant to rain, if you use vapour permeable paint and if you have MVHR, then there's a very good change that you don't need a vapour control membrane on the inside of the insulation, though a WUFI would still be advisable. BTW, in some circumstances a VCL can elevate the risk of moisture damage, rather than reducing it.

 

A service void is a great idea at ceiling level though, as here you are likely to be replying on an airtightness membrane which mustn't be damaged.

 

 

Here's another recent thread discussing some alternatives (including, later on, the method I most recently used):

 

[YodhrinForge]

I hadn't considered the alkalinity factor Mike that's really interesting thanks. I had wanted the clay due to it being really effective at transporting moisture as a defence against possible short periods of penetrating damp(the cottage is in the central belt of Scotland with exposed stonework on the south and west, the west also being the gable, and it's a conservation area so there's no prospect of changing that with a render system - I had considered applying a masonry creme once I've had the wall checked for cement-based repairs or mortars to be raked out and replaced with lime if found, as I had heard that more modern iterations are actually as vapour open as they claim, but I've found it difficult to find definitive information on that as there are so many complaints and warnings about the old style ones) to help draw any moisture through into the wood fiber where the internal warmth can get it back to vapour quicker, but I think I do remember seeing a hybrid product that had both lime and clay so I'll look further into that. My internal finishes were all going to be pigmented clay plaster topcoats like the ones Russel mentioned or clay based paints so that at least shouldn't prove an issue.

 

There will be a void at ceiling level as I was already planning to lower the ceiling ~30cm to run MVHR ducting and make plumbing up the new bathroom easier, but it shouldn't be an issue regardless as there's an existing attic conversion so once I settle on an appropriate method the insulation will continue up into the roof space with only the joists/rafters to impede it. I had planned to carry the vapour retarding membrane the roof probably will require down onto the walls just below the rafter level, tape them off there, and then use adhesive-backed coir or hemp strips over that so they'll take plaster which should link the roof and wall airtight layers.

 

I'll have a read at your thread cheers.

 

Another issue I'm encountering is how to deal with the interface at the rear of the house since it had a block cavity-walled extension added at the same time as the roof conversion some time pre-2000 and I also intend to replace an existing small pointless conservatory(it's north facing and only has two actual "walls", leaning against the existing stonework on one side and the block extension on the other) with a utility room extension which will be timber frame(specifically engineered timber I-beams because I can build the whole thing myself and save a bunch of money), so any suggestions there would be helpful.

Edited 7 hours ago by YodhrinForge