Insulating a stone outbuilding on the cheap — where do I even start?

Done a fair bit of this on my static, so here's what I'd actually do rather than what the YouTube crowd recommends...

Start with the walls first, always. Stone buildings are brutal for cold bridging and damp. Battening out with 50mm timber and stuffing with Rockwool is cheap and reasonably effective — just make sure you leave a small gap behind the battens if the stone sweats at all. Found this out the hard way 😅

For the floor, Celotex or Kingspan offcuts are your mate. Check eBay and Facebook Marketplace — builders constantly offload partial packs. Got 60mm PIR boards for basically nothing last spring.

Roof/ceiling is where you lose most heat though. If you've got any kind of loft void above, chuck loose fill mineral wool up there. Wickes and B&Q both do decent own-brand rolls. Otherwise spray foam is tempting but creates a nightmare later if you ever want to sell or do anything structural.

A few things worth thinking about:

• What's the rough floor area? That changes the approach quite a bit
• Is there a damp problem already, or is the stone relatively dry?
• Are you heating with electric (solar/battery setup?) or something else?

The damp question is honestly the big one with stone buildings. No point insulating properly if moisture's tracking through — you'll just be trapping it.

Anyone else tackled a stone outbuilding here? Curious whether people have gone internal or external insulation route — external is obviously better thermally but the cost difference is massive.

@OffGridDawn solid start but nobody ever mentions the floor robbing all your gains — did my static's walls beautifully with 100mm Kingspan, still freezing until I chucked 50mm celotex under a ply deck and suddenly it actually held heat like a proper building rather than a fancy tent.

@BorderVanLifer floor cold-bridging is criminally underrated — my narrowboat taught me that lesson the hard way when I realised I'd basically built a very expensive cold plate to stand on.

Reclaimed rigid PIR boards from eBay are your best friend here — SkipDip (the salvage lot) occasionally has 100mm sheets for pennies, and two layers offset so the joints don't align beats anything you'll buy new from a builders merchant for twice the price.

Vapour control layer position matters more than people think in a stone building — get it wrong and you're just growing mushrooms in your walls, which is a lifestyle choice I suppose.

@OffGridDawn @BorderVanLifer really interesting thread — can I ask a dumb question though? With stone walls that have genuine thermal mass, is there an argument for not insulating internally and instead letting the stone regulate temperature passively? I'm trying to decide this exact thing for a wee outbuilding I want to convert for battery storage (Fogstar lithium pack needs reasonably stable temps year-round).

Also wondering — does anyone have experience with external wall insulation on rubble-fill stone? Seems like the ideal solution to preserve the thermal mass benefit AND stop the cold bridging, but I've seen costs quoted that are genuinely eye-watering. Are there DIY-friendly external systems worth looking at, or is it realistically a professional job?

@Cleggy not a dumb question at all — but the answer depends entirely on which gap. Historic lime-pointed stonework with small voids? Leave it, it's doing vapour management work. Large structural gaps? That's a different problem entirely and insulation comes after you've sorted that.

The mistake most people make is chasing air sealing in old stone buildings the same way you would a modern timber frame. Stone walls need to breathe. Trap moisture behind rigid foam in a genuinely old building and you'll be dealing with wet rot and spalling within a few years.

Lime-based renders on the inside, then a proper insulated service cavity, is the correct sequence for most traditional stone construction. More expensive upfront, yes. But you only do it once properly or twice badly.