Converted an old shepherd's hut last summer – worth insulating the floor first?

Absolutely do the floor first — and I'd argue it's the most critical layer in a shepherd's hut specifically, because you're sitting on metal chassis rails with air flowing freely underneath. That differential between cold steel and warm interior air is brutal in winter.

When I did mine, I went with 100mm Celotex between the joists before any flooring went down. Once the tongue-and-groove boards are screwed over the top, you've lost your access forever essentially, so get it right first time. I used foil tape on every single joint — belt and braces approach, but vapour control matters when you've got a wood burner creating warm moist air right above a cold subfloor.

A few things worth considering:

• Wheel arches are a nightmare for thermal bridging — pack them carefully, don't skip them
• The chassis rails themselves conduct cold directly into your floor joists if they're in contact — some people wrap them in armaflex or similar
• Ventilation underneath is good for preventing damp rot in timber, but that also means your insulation is doing heavy lifting year-round

My setup now runs entirely on a Victron 200Ah Fogstar Drift LiFePO4 bank with a small Webasto diesel heater, and honestly the floor insulation probably saves me 20-30% on heating demand compared to before I retrofitted it (rough estimate, no proper monitoring).

Would genuinely be interested to know what your current floor construction looks like — are you on original timber joists or has someone already done a steel frame conversion? That changes the approach considerably.

@ExPostie82 makes a great point about the chassis. Quick question though — did you find condensation was an issue between the insulation and the metal rails? I did something similar in my motorhome conversion and the interstitial moisture was a nightmare until I sorted a proper vapour barrier.

Also wondering what thickness you went with? I've seen people use 100mm PIR between the joists but in a shepherd's hut with limited floor depth that seems tricky without losing headroom.

For anyone budgeting this out — Celotex or Kingspan off-cuts from building merchants are brilliant value if you're flexible on sizes. I picked up a decent haul for next to nothing once.

What did you use for the subfloor finish on top? OSB or marine ply? The damp underneath those huts can be brutal in a UK winter.

@ExTrucker73 curious to see where that question was going! Condensation between the floor insulation and the steel chassis was definitely something I had to think carefully about on my hut build. I used a vapour barrier on the warm side of the insulation (above it, not below) and made sure there was still airflow underneath the chassis — don't be tempted to seal it completely shut. The moving air underneath actually helps carry away any moisture rather than letting it pool against the metalwork. I went with 100mm PIR between the joists and it's made a massive difference to heat retention. The floor is genuinely where you lose the most heat in these things, especially in winter — cold air drafts straight through if you've skimped there.

@WingAndPrayer @ExTrucker73 — really curious about this condensation question too!

I'm planning something similar with a tiny house on wheels and the floor void is what's keeping me up at night, honestly. On my boat I use closed-cell spray foam against any steel surfaces precisely because it eliminates that air gap where condensation forms — would the same logic apply to a shepherd's hut chassis?

Has anyone used Kingspan Kooltherm or similar rigid PIR boards tight against the steel rails rather than leaving any cavity at all? I've seen conflicting advice about whether you want some ventilation under there or whether sealing it completely is the better call.

Also — does the insulation choice change if the hut is parked on damp ground vs. hard standing? Mine would likely move between both 🤔

@RustySkipper the condensation thing is SO real — had exactly this on my narrowboat conversion before I sorted it properly. The key is treating that steel chassis as a cold bridge and dealing with it before you lay any insulation.

A few things that helped me:

• Foil-faced PIR board (Celotex/Kingspan) directly against the steel — the foil acts as a vapour barrier
• Make sure there are no gaps at the edges where warm damp air can sneak in underneath
• Some folk swear by a thin layer of closed-cell spray foam first to kill the cold bridge entirely

The worst outcome is trapping moisture inside the build where you can't see it rotting things out quietly. Worth spending an extra afternoon getting the vapour control layer right rather than regretting it in year two! 🙈

@PanelKate narrowboat experience is probably the closest analogy here — you're dealing with the same steel frame / warm interior / cold underside problem.

The bit people miss is that "insulating the floor" isn't just about the insulation itself — it's about making the whole assembly vapour-tight on the warm side. If you're using rigid PIR board (Celotex, Kingspan, whatever), any gaps around the edges are where you'll get interstitial condensation wicking into your timber floor deck above.

Done mine with foil-faced PIR, every joint taped with aluminium foil tape, then a ply deck on top. No issues so far but I'll admit I'm watching it sceptically over winter.

The real test is year two onwards when any trapped moisture makes itself known. Would be wary of anyone claiming it's straightforward — it isn't really.