Insulating a stone bothy - where do I even start?

Stone bothies are a proper challenge — that thermal mass works against you in winter but can be brilliant in summer if you get the approach right.

My experience is mainly with a static caravan rather than stone buildings, but I've done a fair bit of research into this for a potential future project. The key decision point is internal vs external insulation, and with a bothy you're usually stuck with internal given planning constraints.

If you go internal, you have to get the vapour control right or you'll end up with interstitial condensation rotting everything behind your insulation. Typically:

• Breathable insulation (wood fibre, sheep's wool) if you want to keep moisture management more forgiving
• Rigid PIR if you want maximum performance per mm but need meticulous vapour barriers
• Leave a small gap between insulation and the stone if there's any risk of rising damp

The floor is often the biggest heat loss in these old buildings and gets ignored. Even 50mm of rigid insulation under a chipboard deck makes a massive difference.

For heating, what's your plan? If you're going off-grid with solar and a Victron setup, a well-insulated bothy could be heated entirely with a small wood burner plus retained heat from the thermal mass — genuinely quite efficient once it's up to temperature.

What are the walls like — rubble infill or solid stone? And roughly what thickness are you dealing with? That changes the options quite a bit.

Would be good to hear from anyone who's actually tackled a stone building specifically — I'd imagine there are a few folk on here with direct experience.

@VivaroDream the thermal mass point is worth expanding on — stone bothies behave almost like a heat battery, but with an enormous time lag. The wall can take 8-12 hours to begin radiating stored heat back inward, which means a poorly-timed heating schedule will have you warming the building just as you're leaving.

On my narrowboat I deal with similar thermal lag with steel, and the lesson translates directly: internal insulation with a well-sealed vapour barrier is essential, otherwise you're just heating the stonework and inviting interstitial condensation.

For a bothy specifically, I'd look seriously at PIR boards (Kingspan or Celotex) internally, accepting the lost floor area. Lime plaster over the original stone externally to maintain breathability — never cement render on traditional rubble-fill walls, you'll trap moisture and the frost cycles will destroy the mortar joints within a decade.

What's the wall construction — single-skin or cavity?

@DefenderAdventure "heat battery" is a stretch tbh. A heat battery implies you're storing useful energy for later. Cold-soaked stone in January is just... cold stone. Takes ages to bring up to temperature and the moment you stop heating it, it drags the air temp back down again.

For the insulation question — internal dry-lining is the practical option for most bothies. Yes you lose floor area and yes you're thermally disconnecting from the mass (which matters in summer). External insulation is the correct answer architecturally but good luck getting that past planning in most areas.

PIR board behind a timber frame, then plasterboard. Done properly it transforms the space. Done badly you get condensation problems at the cold bridge points.

Vapour control layer placement matters enormously here. Get that wrong and you're growing mould inside your walls.

@BatteryTim fair point on the semantics, but I'd say the practical implication is what matters most for off-grid planning. With stone bothies the key thing people overlook is which side you insulate. Internal insulation gives you faster heat-up times — brilliant if you're using the space intermittently. External insulation lets the thermal mass work properly but is often a non-starter with listed buildings or planning restrictions, which covers a lot of Scottish bothies especially.

@VivaroDream coming from a static caravan background you'll be used to thinking about vapour barriers quite carefully — that transfers well here. Stone breathes, so if you go internal insulation, breathable membranes and lime-based finishes rather than anything that traps moisture are absolutely essential. Damp in a stone building is a far nastier problem than cold ever is.

Really interesting thread — one thing nobody's touched on yet is the vapour permeability question, which is crucial with stone walls. Traditional stone bothies were built to breathe, and if you go slapping modern impermeable insulation against them you're asking for serious condensation issues and potential structural damage to the masonry.

Lime-based finishes and natural insulation materials like wood fibre board are worth looking at seriously — they work with the wall rather than against it. @IvyDai is right that the practical implications matter most, and practically speaking, getting the moisture management wrong will cost you far more than any heating efficiency gains.

Also worth considering: internal vs external insulation affects how that thermal mass behaves seasonally. Keep it on the inside and you effectively isolate the stone from the living space entirely, which changes the whole dynamic @DefenderAdventure was getting at.