Insulating a stone outbuilding on the cheap — worth it or just hassle?

Been wrestling with this exact question for my tiny house project — stone outbuilding, solid walls, absolutely brutal to heat in winter.

Did a fair bit of research and landed on internal insulation as the only realistic budget option. External render systems with insulation boards look great but the costs spiral fast. I went with 50mm rigid PIR boards (Celotex) fixed to timber battens, then OSB over the top. Not glamorous but the difference was noticeable almost immediately.

Few things I'd flag though:

• Interstitial condensation is a real concern with stone walls — you need to think carefully about where your vapour barrier sits or you'll rot your battens within a few years
• You lose floor area, which in a small outbuilding actually matters quite a bit
• Lime pointing on the stone ideally wants to breathe, so some people argue PIR board isn't ideal against old stonework

Has anyone gone the hemp or wood fibre route instead? I keep reading it's more sympathetic to old stone buildings but the cost per m² seems significantly higher. Wondering if it's actually worth the premium or whether the PIR approach is fine as long as the detailing is right.

Also curious whether anyone has tackled the floor — mine is currently flagstone directly on earth and I'm losing enormous amounts of heat through it. Raising the floor level feels like a big commitment.

What's everyone's experience been — is it genuinely worth the effort or do you end up chasing your tail with condensation issues?

PanelChris | 847 posts

@MV_Marine solid walls are a nightmare, aren't they — all that thermal mass working against you in winter rather than for you.

One thing worth considering alongside your internal insulation: once you've sorted the envelope, your heating load drops dramatically, which completely changes the solar/battery sizing equation. I've seen people crack on with a big heating system before insulating and then wonder why their off-grid setup is constantly struggling.

Get the building tight first, then size your generation accordingly. You might find a modest 12V heat mat or small infrared panel does the job rather than something beefier.

Also — with stone outbuildings specifically, watch the dew point carefully with internal insulation. A membrane done properly saves a lot of grief with condensation later on. Worth spending the extra tenner on getting that right.

@MV_Marine stone walls + internal insulation = you've just paid to heat a very expensive cold stone shell, but hey, at least your interior stud wall is cosy 🪨

Seriously though — aerogel insulation boards are criminally underrated for this exact scenario, thin as a biscuit but genuinely impressive U-values, saves you losing half your floor space to 100mm of PIR.

Running my garden office off Victron kit and the insulation job was honestly more impactful than upgrading panels — thermal performance first, solar second, that's the gospel.

@MV_Marine done almost exactly this with a stone barn conversion — internal insulation was the only realistic option without planning drama.

One thing nobody mentions: vapour control layer is absolutely critical with stone walls. Get that wrong and you're growing mushrooms by February 😅

I used 100mm PIR boards between battens, taped all the joints carefully. Made a massive difference to my little Webasto keeping up overnight.

The thermal mass argument @Chopper54 hints at is real, but honestly once it's up to temp it holds well too — swings both ways.

Budget tip — check local builders' merchants for offcuts and returns, picked up half my PIR for next to nothing that way.

@MV_Marine done something similar on the boat actually — internal insulation is basically all you've got when you can't touch the exterior.

One thing nobody's mentioned: vapour barrier placement matters massively with stone. Get it wrong and you're basically creating a condensation sandwich inside your wall. Had a nightmare with that on an older narrowboat before I knew what I was doing.

Also worth looking at Celotex or Kingspan boards if you're going rigid foam — better thermal performance per mm than most alternatives, so you lose less internal space. Every cm counts in a small outbuilding.

Not sure I'd go too cheap on this bit tbh. Skimping on insulation is one of those decisions you really regret come January. 🥶