Insulating a stone outbuilding - worth doing the floor or just walls and roof?

Done both scenarios across different builds and the floor makes a massive difference — probably more than people expect.

Stone walls have terrible thermal mass behaviour in a UK climate. They take ages to warm up and then just radiate cold back at you. But heat genuinely does rise, so a lot of folk assume the floor is low priority. In practice, a cold concrete slab will absolutely kill your heating efficiency because you're losing ground-source cold upward constantly, especially in winter.

For my garden office conversion I did:

• 100mm Celotex under a new screed on the floor
• 50mm Celotex batted out on the walls (stone was too uneven to bond direct)
• 100mm between rafters in the roof

The floor insulation alone transformed the feel of the space. It went from "damp cave" to actually habitable within a couple of days of heating.

Practical caveat though — if you're on a tight budget or the stone floor is listed/original flagging you want to keep, walls + roof will still get you 70% of the way there. A decent wool rug over flags does surprisingly well as a stopgap.

Also worth thinking about your heating source before you decide. If you're running a small Victron setup with an electric panel heater, every bit of insulation pays back directly in battery capacity. Ground floor losses are brutal for small systems.

What's the current floor situation — concrete slab, original flags, bare earth? That changes the answer quite a bit. And are you planning to heat it year-round or just shoulder seasons?

@VoltWill my Victron kit stops alarming about battery temps once I finally caved and insulated the floor — turns out concrete slabs are basically just fancy cold plates pretending to be floors.

Really glad this thread came up — it's one of those topics that doesn't get enough attention.

On the narrowboat I learned the hard way that cold radiating up from below is relentless. Floor insulation isn't glamorous but the payback in comfort (and battery health, as @MultiPlusFan touched on) is significant.

For a stone outbuilding specifically, I'd prioritise this order:

1. Floor — rigid PIR board under a screed or ply deck
2. Roof — heat loss rises, obviously
3. Walls — stone gives you some thermal mass benefit eventually, but draught-seal first

One thing worth checking: is there any ground moisture risk? A decent DPM under the insulation is non-negotiable in a UK climate, especially older stone builds that were never designed to be sealed up.

What's the floor construction currently — earth, concrete slab, timber joists?

Really interesting thread for my situation — I'm fitting out a tiny house and the floor insulation question is one I've been going back and forth on.

Practically speaking, how thick are people going with? I've got maybe 100mm of depth to play with between the subfloor and finished floor level. Is rigid PIR board (like Celotex or Kingspan) the obvious choice, or is there something worth considering for a smaller footprint building where moisture management is also a concern?

@VoltWill curious whether you noticed any difference in how quickly the space responds to heating after floor insulation — tiny houses can be slow to warm up and I'm wondering if that's partly a floor heat-loss issue rather than just the walls.

Genuinely curious about the practical side of this for my setup — I've got a stone outbuilding with a solid concrete slab floor and I'm torn on whether to go over the top with rigid PIR board or try to inject anything underneath.

@MultiPlusFan did you lose much ceiling height going over the slab? That's my main concern — it's already fairly low in there and my Victron MPPT and battery bank are wall-mounted, so rerailing everything if I raise the floor level sounds like a headache.

Also wondering whether the thermal bridging around the perimeter where the slab meets the stone walls basically kills your gains anyway, or whether you can mitigate that with upstand insulation around the edges?