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My problem is that not all lime is created the same. I know the difference between NHL 2, 3.5 and 5, and I know the difference between those and non-hydraulic lime. We had a survey of our house before we moved in by a historic buildings surveyor, who recommended all repointing etc be done with non-hydraulic lime mortar, and that any gypsum plaster be removed and replaced with a non-hydraulic lime based plaster. If you go anywhere near a 'historic buildings' or 'old homes' type community online you get shot for suggesting anything cementitious is used in an older house. The problem being that tradespeople willing to use these products that take many times longer to set than cement or gypsum based materials are rarer than hens teeth! It also means you're very limited in the types of finishes you can use - limewashes/distempers/clay paints. What I always try to do in situations like this is understand the evidence, the science and the data and try and make a plan which balances convenience, cost and results. The problem with that is that not many people have experience of all of the options over a long term. It's either fine because they haven't seen problems with it yet, or it's terrible and to be avoided because it didn't work out for them.

Here's a couple of pictures showing the height of the chaos and where we're at now which is a lot more civilised. For a few weeks it felt like we were recreating a WW1 trench.

Did you get a specialist installer for any of the diathonite products? Did you find it easy to find someone competent? We're putting living space upstairs and have mostly vaulted ceilings, with the exception of one area which will house the MVHR system. It is above the utility room, so this shouldn't be too much of a problem noise-wise. I think we'll probably have 75% of the house connected to MVHR and the remainder will be hard to duct to, so will have decentralised systems installed. We've always been on oil in this house and hate the process of buying oil twice a year. Our road is fairly narrow, so we've recently had two companies take our money then several weeks later say they can't deliver. Our builder is fairly traditional and keeps talking about tanking stone walls, but I think I'm bringing him around to thinking about different possibilities, like lime based products.

Thanks for the detail here. We've ruled out EWI, at least for now. There are just too many moving parts of this job, but the main ones being the roof and windows. If we wanted to do EWI we would need to factor that into the roof design, and we're enlarging quite a few of the window openings as we go through the refurb, so we already have the issue of when we order windows to ensure they fit, but not actually having physical openings to measure. Factor EWI detailing into this with an uncertain timeline and it just didn't feel like the right thing to do. Maybe if our builder or architect were experienced in this area that would have made a difference but that's not the case. I would still consider diathonite internally, however finding a competent installer seems to be the bane of most 'eco tech' for buildings nowadays. My ideal solution for insulation and airtightness is something that is straightforwards enough for our builder to pick up, and using fairly common building materials, which is why I'm leaning towards a parge coat to the stone, with an insulated battened service void, using mineral wool or maybe wood fibre. I'm really going to lean into airtightness and MVHR design will need to come relatively soon. We will likely switch out from our oil boiler to ASHP at some point, so all radiators will be sized to accommodate low flow temperatures. It's good to hear that you are seeing benefits of the MVHR already. Have you measured your airtightness at all?

Thanks for this, but we're way past that point! We looked at it, but a rebuild at the ~200 sqm would have been in the region of £500-600k. We may as well just be moving house at that point.

Although I don't have a huge amount of practical experience, I've done a lot of research in this area because my house has similar construction for the most part. There are several schools of thought without much overlap, but in general the first question should probably be to ask why the wall is wet in the first place. Are all RWG in good working order? Is there a risk of a broken pipe somewhere nearby? You mention that it's an internal wall which would lend itself more to a problem at ground level or coming from within the wall above, rather than damp coming through the wall as you might expect if it was external. Is it on both sides of the internal wall? Is it limited to just one wall or to many walls? You mention the concrete floor appears dry. Do you have a sense of where the water table is relative to the levels of the house? While tanking will stop water physically moving from in the wall to into the room, if the wall is wet it is likely to remain colder and will be more of a heat sink. If you put PIR over the top it will make the wall colder still, and any moist warm air from in the house that manages to get behind the insulation will condense on the surface inboard of the tanking slurry, and potentially lead to mould growth on the wall. I'm sure tanking has a place when there are no other options, but the ideal solution would be to find a way to make the wall dry (or dryer than it is currently at least).

This seems fair. We're extending and fully refurbishing our 200m2 detached house and hoping to get it done for ~£300-320k

I wish we had such cheap skips around here! £280+vat for 6yard a few weeks ago. Loaded up with old broken concrete blocks, stone, old roof tiles etc. Twice had smaller lorries come and pick it up, drive down the road then come and drop it off complaining of being 1.5T over weight!

I've emailed a couple of businesses selling tiles and pedestals, as well as asking kingspan what they would recommend. Kingspan suggested I talk to a structural engineer, one other business said 18mm osb would be fine and another hasn't got back to me. Ideally I'd lay single ply membrane directly onto PIR, then pedestals directly onto the membrane, but my suspicion is that an 18mm layer of osb under the single ply membrane would suffice.

I'm coming back to this and my worry at the moment is about how compression resistant PIR is. I assume I can't have PIR, with single ply over, with pedestals straight on top? Would I need an intermediate layer to support the load of the tiles plus people/furniture so as not to damage the insulation through compression (like another layer of OSB or ply)? A bit of research has shown me that you should ideally have 300kPa/m2 underneath the pedestals, but PIR appears to be in the 120-150kPa/m2 range. Edit - would adding a thin layer of XPS over the PIR allow me to get the load-spreading required? XPS seems to be in the 200 kPa range and higher.

Thanks for this - I think we're likely to use a combination of centralised and decentralised MVHR, as we can likely get ducting to 2/3 of the house relatively easily, but the final 1/3 will be more tricky. This definitely makes sense to me.

Thanks for this. What attracted me to the method of parge for airtightness and rockwool/service void/plasterboard finish is that it decouples the airtightness from the services (no taping socket backboxes etc). It seems simpler from an execution perspective and I think this would work well with our builder and help me stay on top of the quality of the work. It isn't that I don't think he'll do a good job, but I think that we still have work to do when it comes to selling the importance of airtightness. I'm also wary that the more rigid the insulation board is, the less able it will be to match the uneven shape of the stone walls, and the more likely we are to have voids. If we were to use a parge coat of NHL 3.5, rockwool within a CLS timber 'batten' frame, Intello Membrane then Plasterboard (and I assume a skim of gypsum), does that mean that all sockets/holes need to be taped to the Intello membrane? Our project is a combination of extension, reorganisation and renovation, so anywhere we can create a simple solution will be big positive.

Thanks - this all makes sense. What about window/door reveals? Maybe similar but thinner batts and thinner mineral wool insulation?

This is incredibly helpful - thank you! I have a couple of questions about this method if that's OK. What would you do if the walls are uneven stone not solid brick. I'm imagining a parge over the rough surface, then battens create as flat a surface as possible for plasterboard, and mineral wool batts assumed to take up the uneven surface of the stone? In this method there's no airtighness attempted between the room and the parge coat, so moisture is free to get behind the plasterboard and insulation onto the parge coat which (given it's on the cold side of the mineral wool) will condense. Assuming I'm right in this, is the thinking that it's all vapour open enough to allow drying inwards aided by PIV/MVHR? Thanks in advance.