Insulating a stone outhouse conversion – worth the hassle or start from scratch?

Right, I'll wade in here because I've been down this rabbit hole twice — once on the narrowboat and once converting an old brick outbuilding into a workshop/sleep space.

Stone is an absolute nightmare compared to brick, and brick was bad enough. The thermal mass sounds romantic until you've spent three days trying to get a damp, freezing structure up to temperature in February. The wall thickness gives you nothing for free — you still need to address the cold bridge, the floor, and whatever cursed roof situation you've inherited.

That said, I didn't start from scratch and I don't regret it. What I do regret is skimping on the insulation spec first time round. Went with 50mm PIR boards thinking I was being sensible. Should've doubled it. Now I've got a space that's okay rather than genuinely comfortable.

A few things worth considering before you decide:

• Breathability matters massively with stone — trap moisture and you've bought yourself a rot and mould nightmare
• Internal insulation loses you floor space but is far cheaper and simpler than external
• If the structure is listed or in a conservation area, that'll shape your options whether you like it or not

My honest take? If the stone is in decent nick structurally, the conversion is worth it — character-wise and cost-wise. But don't bodge the insulation to save a few hundred quid. You'll be living with that decision every winter.

Anyone else gone down the breathable lime plaster route rather than PIR? Curious whether it actually performs in a proper cold snap or whether it's just aesthetics dressing up poor performance.

@AngliaCamper stone walls are the thing that nearly broke me when I was setting up my garden office. Old limestone construction, walls breathing like a living thing — any attempt at rigid insulation just shifted the damp problem somewhere more sinister.

What eventually worked: a proper vapour-open system. Woodfibre board against the stone, battened out, then a service cavity. Let the wall do what it wants to do. Fighting moisture in old masonry is a losing battle; managing it is the only sane approach.

The thermal mass argument cuts both ways too — yes it stores heat, but in a poorly insulated space it's storing cold just as happily.

For a garden office specifically, the start-from-scratch argument is genuinely compelling. A well-built timber frame with modern insulation outperforms a converted stone building almost every time on energy efficiency metrics.

@AngliaCamper internal insulation is the usual answer but you lose floor space and you will get condensation behind it if the vapour barrier isn't done properly. Seen it ruin OSB in under a year.

Spray foam's tempting but mortgage lenders hate it if you ever sell, and removal is a nightmare.

Personally I'd go:

• Internal stud wall, 100mm rockwool
• Proper VCL taped at every join
• Leave a small gap between insulation and stone if you can

Stone buildings breathe — seal them wrong and you're just trapping damp inside instead of outside.

What's the wall thickness? Old stone can be 600mm+ which actually gives you decent thermal mass if you're heating it consistently. Different beast entirely to a thin brick shed.

@ZFS_OffGrid thermal mass is doing all that condensation's admin work for free and nobody appreciates it until the damp patch appears on the inside of the PIR board six months later 🙃

@OhmsLaw7 thermal mass is brilliant until it's October and your stone walls have been storing "cold" all summer like a budget Victron battery that only works in reverse — ran a dehumidifier in my static for three months before I twigged the walls themselves were the problem, not the humidity.

Vapour-open insulation is the move for old stone — Pavaflex or similar — let the moisture breathe rather than trapping it behind a membrane and crying about it in February.