Insulating a stone outbuilding – worth doing the floor or just walls and roof?

Done all three in my shepherd's hut and the floor made a massive difference — probably more than I expected going in.

Stone walls are brutal for cold because of the thermal mass, but once you've sorted the walls and roof you'll notice the floor is where all the residual chill comes from. Standing on an insulated floor versus bare stone/concrete is night and day, especially in a space you're actually living or sleeping in.

What I did:

• 50mm Celotex laid flat
• Tongue-and-groove OSB on top
• Finished with a thin floating floor

Total floor rise was only about 65mm which was manageable given my door clearance.

The counterargument I've heard is that for occasional-use spaces the thermal mass of an uninsulated floor actually helps store heat from a wood burner — there's some logic to it if you're heating the space for hours at a time. But for a cabin or hut you want warm quickly, insulate everything you can reach.

One thing worth mentioning — damp. Stone outbuildings often have rising moisture and if you lay insulation boards straight on a damp slab without a decent DPM you'll create a condensation problem underneath. Learn from others' mistakes on that one 😬

What's your subfloor situation like — original stone flags, concrete slab, earth? That'd change my advice a bit.

@SunnyTinker makes a fair point, though I'd push back slightly on the thermal mass being purely "brutal" — it's a double-edged sword. On my narrowboat I've got steel hull acting similarly: takes ages to warm up, but once it does, it holds heat remarkably well overnight.

For a stone outbuilding specifically, the floor is doing two jobs simultaneously: stopping conducted cold from the ground and eliminating draughts rising through any gaps. Walls and roof address radiation and convection losses, but that ground contact cold is almost visceral — you feel it through your boots before you notice anything else.

If I were prioritising with a limited budget, I'd actually do floor first, then roof (heat rises — obvious, but people forget), then walls last. The stone walls can buffer temperatures once everything else is sealed properly.

@SunnyTinker completely echoes my experience with the shepherd's hut I converted last year. The floor was the revelation nobody warned me about — I lost so much heat through cold feet before I sorted it.

What I did was rigid PIR board (Kingspan Kooltherm K103) under a floating chipboard deck, and the difference was immediate. The floor had been sitting on bare compacted earth, essentially a giant cold sink.

One thing worth factoring in: moisture vapour rising from the ground. I ran a 1200-gauge DPM under the insulation before anything went down. Stone buildings breathe differently to timber frames, and getting that moisture management wrong causes more problems than the cold itself ever did.

@ExmoorNomad raises a fair counterpoint on thermal mass — once that stone is warm, it does hold it beautifully. The issue is getting it there initially.

@SunnyTinker @QuietMaker — both right about the floor, and it's the one people always skip because it feels less obvious than walls.

One thing nobody's mentioned yet: vapour control matters enormously in stone outbuildings. Get the layering wrong and you'll trap moisture in the floor build-up, which quietly destroys your insulation's effectiveness over a couple of winters. Seen it on the boat community too — damp insulation is basically just ballast.

For a stone building specifically, I'd strongly recommend going for a rigid PIR board (Kingspan, Celotex, etc.) under the floor rather than anything breathable. Stone floors need a vapour barrier beneath the insulation, not above it.

Worth also checking whether you've got any ground moisture issues before you close everything up — probe the earth first if it's an old building.

Spent three winters on a narrowboat before I moved the experiment to a stone bothy, so I've got form with "why is it cold from below" syndrome.

The floor is basically a giant cold plate pressing itself against your feet and whispering "give up." People ignore it because you can't see heat escaping — it just silently vanishes down