Insulating a 4x3m log cabin office – worth doing the floor too?

Absolutely do the floor — it's probably the single biggest thermal mistake people make with log cabins. I did a similar sized space a few years back (before the van conversion obsession took over) and the difference between an insulated and uninsulated floor is genuinely shocking.

Most folk focus on walls and roof, then wonder why their feet are cold and their heater runs constantly. Ground cold is relentless, especially Nov–March in the UK.

What worked well for me:

• 25mm Celotex/Kingspan between the joists, foil face up
• Second layer of 50mm rigid PIR across the whole deck, staggered joints
• 18mm OSB on top, then whatever flooring you want

Total thickness adds maybe 70–80mm to your floor height, so factor that into your door threshold situation before you start.

If the cabin is already built and sitting on bearers/piers, you can also retrofit insulation from underneath — fiddly but doable, I've seen people use wire mesh stapled up to hold batts in place.

One thing I'd flag: vapour control. Ground moisture wicking up is a real issue. A decent DPM under the whole structure, lapped up the walls, is worth doing before any insulation goes in. Cheap insurance.

Worth checking what your subfloor situation is first — some budget cabins have virtually no air gap under them which complicates things.

Anyone else tackled floor insulation as a retrofit rather than new build? Curious whether rigid board or mineral wool held up better long-term in that exposed underfloor position.

Re: Insulating a 4x3m log cabin office – worth doing the floor too?

Totally agree with @Tommo on this. One thing worth adding — with log cabins sitting on timber bearers, you've often got a really accessible void underneath, which actually makes floor insulation surprisingly straightforward compared to other builds.

I used 100mm rigid PIR board between the joists in mine, foil tape on all the joints, then a decent vapour barrier before relaying the floor. Difference was remarkable — especially through January and February when ground temperatures really drag the heat out.

Also worth checking for any gaps where the cabin meets the bearers. Mine had a surprising amount of cold air infiltration around the perimeter that was doing just as much damage as the uninsulated floor itself. A bit of expanding foam sorted it.

Don't skip it — you'll regret it first cold morning you sit down to work! 😄

Re: Insulating a 4x3m log cabin office – worth doing the floor too?

When I was fitting out my shepherd's hut, I agonised over this exact question. Floor insulation felt like the "boring" job nobody wants to do — right up until the first January morning when the difference between a cold slab and a warm floor becomes viscerally obvious.

For a 4x3m cabin sitting on bearers, I'd go 100mm PIR between the joists minimum. The floor void is essentially a wind tunnel otherwise, and no amount of wall insulation compensates for losing heat straight downward.

One practical note — a cheap clip-on thermal camera (I borrowed one rather than buying) tells you immediately where you're losing heat. Made the case for floor insulation far more convincingly than any spreadsheet ever would.

Re: Insulating a 4x3m log cabin office – worth doing the floor too?

Running a floating office setup on my boat taught me something that translates perfectly here — thermal bridging through the subfloor is absolutely savage in damp British winters. You lose heat you didn't even realise you had.

For a 4x3m cabin I'd go 100mm PIR between the joists minimum, then a layer of 25mm rigid foam beneath the joists before your boarding goes down. Belt and braces. The difference when I replicated this approach in a shore-side setup I was helping with was genuinely startling — the space held heat overnight without any active heating running.

Worth checking your DPC situation too before you seal everything up. A cold floor is one problem; a damp cold floor is another entirely.

Re: Insulating a 4x3m log cabin office – worth doing the floor too?

Worth flagging something nobody's mentioned yet: vapour control layers. On my shepherd's hut build I initially overlooked this and ended up with condensation issues inside the floor void within one winter. For a cabin sitting on bearers or joists with ground beneath, you absolutely want a proper VCL on the warm side of your insulation, not just a basic DPM underneath. I used 125µm polythene on the cold side and a foil-faced VCL above the PIR. Also consider that PIR (Celotex, Kingspan, etc.) will outperform mineral wool significantly in a floor application where depth is constrained — you're typically working in a shallower void than a domestic floor, so U-value per mm matters considerably. What's your current joist depth, @Tommo? That'll largely dictate whether 100mm PIR gets you somewhere sensible thermally.