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

Did a similar project on my garden office a couple of years back — not a bothy but a solid stone outbuilding from the 1800s, so same principles apply really.

Honestly, don't skip the floor. It's tempting because it feels like the least dramatic heat loss culprit, but in a stone building sitting on bare earth or old flags, you're losing a surprising amount upward through conduction. I used 100mm PIR board under a floating chipboard deck and the difference was immediately noticeable compared to when I'd only done the walls.

The thing with old stone buildings is the thermal mass works against you in winter — the walls soak up heat for ages before the space warms up. So if you're insulating for intermittent use (which most bothies are), you actually want to decouple yourself from that mass as much as possible. Internal insulation with a decent air gap, then your finish layer.

Roof is obviously priority number one though. No arguments there — warm air rises, you know the drill.

A few things worth considering specific to stone:

• Vapour control matters a lot here. Stone needs to breathe on the outside face or you'll get interstitial condensation issues
• Lime plaster over the internal insulation rather than standard gypsum if you can
• Underfloor doesn't need to be fancy — rigid foam + chipboard is perfectly adequate

Running a small Victron system with a couple of panels on my outbuilding now, and the insulation is what makes a 200W panel actually viable for keeping the space usable in winter.

What's your floor situation currently — earth, flags, concrete? That'll change the approach quite a bit.

Skipped the floor insulation in my static caravan thinking "heat rises anyway" — narrator: the heat did not, in fact, rise.

@HeatherChild that cold floor phenomenon is well documented — radiant asymmetry basically means your ankles are freezing whilst your head's roasting, which is genuinely miserable for extended stays.

On the practical side for stone bothies specifically: ground moisture is your real enemy before you even think about thermal resistance. I'd be looking at a 20mm Kingspan Kooltherm or Celotex GA4000 over a decent DPM before your floor deck goes down. The lambda values on those are around 0.022 W/mK so you're not sacrificing much floor height for meaningful performance.

From my van build experience, ignoring cold bridging at the perimeter where floor meets wall is where most people lose the gains they've just installed. Frame that junction properly with either foam tape or a continuous insulation overlap and you'll notice the difference immediately in actual occupancy temperatures rather than just on paper.

@AndyRobinson is onto something critical here that I'd reinforce from my cabin build — floor heat loss in a stone structure isn't just conduction, it's also significant ground moisture migration wicking thermal energy away constantly.

What I'd add practically: vapour barrier sequencing matters enormously on a stone floor. Get it wrong and you're trapping moisture between the slab and insulation, which degrades PIR performance substantially over time.

My approach on the cabin was:

• DPM directly on the slab
• 100mm PIR (Celotex GA4000 series)
• 18mm OSB floating deck, taped joints

The U-value improvement was measurable — I went from a thermal imaging survey showing the floor as the dominant loss point to it being broadly equivalent to the walls.

@WayneJames the radiant asymmetry point is spot on — that ankle-to-head temperature differential is genuinely miserable to live with long-term, not just uncomfortable.

@AndyRobinson @DucatoSolar seconding all of this strongly. The other thing worth mentioning with stone bothies specifically is ground moisture — even if you somehow convinced yourself the heat loss was acceptable, an uninsulated floor sitting directly on old rubble or compacted earth is going to wick damp upwards constantly. You'll be fighting condensation on your walls and that lovely lime mortar pointing will suffer for it too. I used 100mm PIR board under a floating chipboard deck in my outbuilding and the difference was genuinely transformative — not just warmth underfoot but the whole thermal mass of the building started actually working for me rather than against me. Budget permitting, a decent vapour barrier beneath the insulation is non-negotiable with these old structures.