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

Done my garden office conversion in a stone outbuilding last year and honestly the floor was the game-changer nobody talks about.

Walls and roof get all the attention but cold radiates up from an uninsulated slab something fierce in winter. I had 100mm PIR under a floating chipboard deck and the difference was immediately noticeable — the space holds heat so much better when you're not losing it downward.

Thermal bridging is the enemy. You can have brilliant wall insulation but if your floor is a cold sink, your heating system (and battery reserves) are working twice as hard. For an off-grid setup that matters a lot more than a grid-tied building — every kWh you waste heating a poorly insulated space is a kWh not available for the EV charger or evening loads.

A few things worth considering:

• Damp proof membrane under the insulation is non-negotiable with stone buildings — ground moisture will wreck everything eventually
• Check your floor-to-ceiling height before going mad on insulation depth, you can lose 150mm+ once you factor in the deck
• Stone walls need breathable insulation solutions in a lot of cases — rigid foam can cause issues if moisture gets trapped

My setup runs on Victron kit and the thermal performance of the building directly affects how I size the battery bank for winter, so I treated insulation as part of the energy system, not just a building job.

What's the floor situation in your outbuilding — original earth, concrete slab, or flagstones? Makes a big difference to what approach is practical.

@BreezySparky floor insulation is the unsung hero — did mine in the static van AND the garden office and both times thought "why did I do the walls first like an idiot"

100mm PIR board under a floating deck, job done, suddenly the space actually holds heat long enough for the Victron to stop working overtime keeping the electric panel heater fed all night 🥴

@BreezySparky my tiny house sits on a timber frame but the first winter I skimped on floor insulation I basically paid to heat the Welsh countryside beneath me — lesson learned in kilowatts.

@BreezySparky stone floors are basically a direct line to the Earth's core with your heating bill attached to it.

Did my T5 floor with 50mm Celotex before anything else — same logic applies to my garden office build on a solid concrete slab. Rigid PIR board under the whole floor, taped joints, then OSB over the top. Thermal bridging sorted before I even thought about the walls.

The maths is simple: heat rises, yes, but cold conducts, and an uninsulated slab will cheerfully rob you of every watt your system produces regardless of how many layers you've got on the walls.

Anyone skipping floor insulation to save £200 on Kingspan is essentially putting a heated duvet over themselves whilst sitting barefoot on a glacier.

@BreezySparky floor insulation is where I had my biggest "oh, that's why" moment in the van too. Cold mass beneath you doesn't just steal heat — it actively drags your apparent temperature down even when air temp reads fine. Your body radiates toward the cold surface regardless of what the thermometer says.

In the outbuilding context I'd prioritise floor like this:

• Vapour barrier first — stone floors wick moisture relentlessly
• 50mm PIR minimum (Celotex or Kingspan — don't cheap out with polystyrene)
• Chipboard deck on top, properly taped

The U-value maths on an uninsulated concrete slab is genuinely depressing. You can have perfect Rockwool in the walls and still be hemorrhaging heat downward into essentially infinite thermal mass.

@T5Project is right about the "direct line to Earth's core" thing — it's not even hyperbole when you run the numbers.