"There are also ventilation channels from the exterior to the middle of the wall which ventilate the channel and the rubble filled interior of the wall"

Are these about an inch diameter, with a circular grille? Could be the 70s(?) method of combatting rising damp by installing such tubes at regular intervals, just above ground level, the idea being that being porous, maybe ceramic like a water filter, they evaporate water out of the wall and ventilate it away.

What sort of insulation boards are you going for?
We did a solid stone walled barn conversion (450mm thick walls) with 100mm foil faced PIR on the inside
Foam glued and plastic hammer-in fixings
Plasterboard foam glued on top.
No cold bridges.
No fiddly cutting of each board to fit between studs, just the top/height and each end of the row.

U=0.22 according to a calculation that I did at the time.

Hi @reed,

I'll briefly describe my approach, having done a lot of stone building refurbs for clients, bearing in mind that there are slight variations depending on building properties, and client wallets. If any of it's of use to you, then great.

- lift the floors an insulate between the joists and ideally below, using a breathable membrane to hold the recycled glass wool, usually knauf frametherm 32. Must maintain the air flow under the new insulation, regs requiring min.150mm to the solum. If there is a reasonable depth of void below the floor, I put 2 layers of 140mm insulation (one layer which wraps under the joist, the other between). If it's a deep fill like that, I sometimes drop to a 40 or even 44 wool density, to help the client's costs. Looks like you might have some floor depth, judging by the lower of the 2 CIron wall grilles, which I assume is for sub-floor.

- walls stripped back to the stone. If you've got the cash, coat the walls in a lime based render, ideally one with cork insulation bits in it, but that's a chunk of cash. It air tightens the walls, binds in the lime pointing, and makes the inner wall stone face warmer. If you've time, but not the cash, have a go yourself. It can look rubbish finish wise, as no one will ever see it, Just use the straight edge (plus spirit level) to scrape over the plastering to keep it reasonably plumb. My guys spray it on, but that's not for the faint hearted.

- timber frame internally 38x95 timber floor and ceiling runers with studs on 600 centres. The stone walls are usually well of the plumb, so the frame is free standing, not fixed to the stone wall. 90mm knauf frametherm 32 between studs. I don't bother with a VCL, though regs would ask for it, but it concerns me reducing the breathability of the build-up. I sometimes use the foil back plasterboard where the Building Warrant requires it, because I know the foil backed plasterboard isn't much good at being a VCL.

- I try to avoid the rigid insulation boards for various reasons, not least is that there is always waste, it can shrink, needs careful installation (undercutting and foaming all edges to full depth), is not vapour open, and is horribly dusty when cutting. I only use it at the window reveals/heads/cills, as these are often angled with reduce void behind the plasterboard.

- get wool insulation into the ceiling-to-1st floor void around the wall perimeter, say 300mm depth all the way around, as insulation and also to stop drafts blowing around in the floor void. Can be from below if the ceiling is coming down, or lift some floor boads on 1st floor, often at same time as services are being installed.

You'll only ever do this once to a room, so don't skimp. I have a number of clients who pulled back from doing he full monty, and admit to regretting it. Key is stopping drafts, as they strip SOOO much heat, and can cause mould where they "jet" onto the back of plasterboard, esp at window reveals or skirtings.

I've checked a couple of the projects, cold days in winter, by removing a double power socket and ferretting in behind the wool. No damp that I could notice. That's a very small sample number, so I wouldn't suggest that supports my approach 100%, but keeping it vapour open and not too much insulation "appears" to work - Uvalue of around 0.3W/m2K (existing without insulation more like 1.4).

For "new build" inside an existing stone building, I've essentially built my usual timber frame construction, with a 25-50mm cavity to the stone wall interior face, with additional weep holes at the wall base, providing Wall Uvalues around 0.13.

Hi GreenPaddy, Thanks for the very useful comments. Your assumptions on the significant space below the floor are correct. I am considering wool insulation rather than PIR board to maintain breathability and to avoid gaps due to poorly cut board. Would be grateful if you could explain what you mean by putting one layer of insulation under the joists? Unsure how you would do this with wool. It would make sense given that the joists would otherwise be uninsulated and they make up a reasonably significant % of the floor area for transmission of heat.

Many thanks GP,
Indeed I will be lifting the floorboards. Your diagram has downloaded well and it is good know that the method works. I have till now been pondering how to best insulate the lower edge of the joists, as although they are not a large % of the surface area of the floor, my calculations show that, if they remain uninsulated, they will nevertheless represent a relatively high proportion of the heat loss, once the gaps between the joists have been insulated, especially because by underfloor space is very well ventilated. Just need to do it now!

Posted By: fostertomvapour proof membrane

I imagine he meant vapour permeable membrane, but that ambiguity is probably why VPM is not normally used as an acronym, I believe.

Apologies if there was any confusion. In 40 years in construction I've only ever seen VPM referencing "permeable". VCL is "control" or "check". I guess it's a reminder not to assume.

Posted By: Dominic CooneyWe've had this before. I think a lot of confusion lies with the very common use of DPM as "Damp Proof Membrane".
I seem to recall VB being "Vapour Barrier" by someone as well. Gotta love acronyms!

Yes - type DPM into google and there are lots of relevant hits. type VPM in and there are none!