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

Floor is massively underrated and I'd argue it's actually the most important bit in a stone building. Cold ground will just drain heat out of you all winter regardless of how well you've done the walls.

Did a similar job on my static a few years back before I moved onto doing a timber cabin properly. The difference even a basic insulated floor made was night and day — we're talking 50mm rigid PIR board under a floating chipboard deck, cost very little compared to the warmth gained.

Stone walls are obviously a challenge because you lose internal space with a stud and insulate approach, and going external isn't always an option aesthetically (or if it's listed/protected in any way). Roof is usually the easiest win per-pound though.

My rough priority order would be:

1. Floor — biggest bang for your buck thermally
2. Roof/ceiling — heat rises, simple as
3. Walls — still worth doing but more complex with stone

One thing people forget with bothies specifically is moisture management. Stone breathes, PIR doesn't, so you can create condensation issues if you just slap modern insulation against old stone without thinking it through. Lime plaster, vapour-open membranes, that sort of thing — worth reading up before you commit.

Have you measured the floor area vs wall area? Might help people give you more specific suggestions. Also, is it an earth floor currently or concrete slab?

Curious what others have done — especially anyone who's tackled traditional Scottish or Welsh stone construction where the walls can be 600mm+ thick.

@ExSquaddie solid point. Done a tiny house on a trailer and the floor was the first thing I obsessed over — lose ground contact heat and your heating system works twice as hard for nothing.

Stone buildings are worse because the thermal mass of the floor slab just sits there absorbing whatever you put in. You're essentially heating a rock.

My approach:

• Rigid PIR board under any screed (100mm minimum)
• Break the thermal bridge at the perimeter where floor meets wall
• Vapour control layer above insulation

That perimeter bridge is the bit most people skip and then wonder why their corners stay cold.

Roof still matters obviously — heat rises — but a cold floor is constant drain. Cold roof is more of a peak loss issue.

@ExSquaddie makes sense physically but I'd push back a bit — depends massively on the ground beneath. Stone bothy on solid bedrock behaves totally differently to one sat on damp soil or clay.

My static has a suspended timber floor over rough ground and honestly the walls were leaking more heat than the floor ever did once I got 100mm rigid PIR under the deck.

Not saying ignore it — don't — but "most important" is a stretch without knowing what you're dealing with underneath. Bedrock bothies can get away with less floor insulation than something sat on wet Scottish clay.

Do a thermal camera survey first if you can borrow one. Shows you exactly where you're haemorrhaging heat before you start ripping things apart.

@ExSquaddie makes a compelling case and I'd back it from a slightly different angle — thermal mass works against you in winter if the floor is uninsulated. That cold stone slab just becomes a heat sink, dragging your ambient temp down overnight even after a decent fire.

In my van and motorhome builds I use 50mm Kingspan between battens under any living surface, and the difference is genuinely remarkable. For a bothy I'd go 75–100mm rigid PIR under a floating timber deck — relatively cheap, no vapour issues if detailed properly.

@ZFS_OffGrid raises a fair point about solid rock — less conductive than waterlogged clay — but even solid rock sits at around 8–10°C year-round. That's still a massive heat drain when you're trying to maintain 18°C inside.

Do walls and roof first if budget is tight, but floor is a close second, not an afterthought.

Been through this with my narrowboat — different problem, same physics. The floor is where you lose heat you don't even notice losing. You're not getting a dramatic cold draught like a gap under a door, just a slow relentless bleed.

One thing nobody's mentioned: moisture. Stone bothy floor will almost certainly have damp rising through it unless you've got a proper DPM sorted first. Insulation sat on damp stone is just expensive waste — PIR boards will eventually degrade, rigid foam loses R-value when wet.

So yeah, insulate the floor, but sort the damp barrier first or you're wasting your time and money. What's the ground condition like @ExSquaddie — solid rock, clay, compacted earth? Makes a difference to what you can actually lay down without the whole thing lifting or rotting out within a few winters.

@ExSquaddie @MarinePhil this is directly relevant to my static caravan setup — I've got ground-mounted solar and a Victron system running year-round, and the floor cold was genuinely the last thing I cracked.

Quick practical question though: with a stone bothy, are you dealing with a flagstone floor directly on earth, or is there any kind of void beneath? That changes the insulation strategy completely doesn't it — PIR rigid board behaves very differently over a suspended floor versus a solid slab on damp ground.

Also wondering whether anyone's used a vapour barrier between layers rather than just at the bottom, given the moisture wicking potential through old stone foundations. My caravan had a condensation nightmare until I sorted that properly.

What thicknesses are people achieving before it starts causing issues with door clearances? That's my other concern in a space with original stone door frames.