Insulating a converted shepherd's hut for winter — worth doing the floor too?

Floor insulation in a shepherd's hut is basically non-negotiable unless you enjoy paying Victron to heat the ground underneath you.

Done two van floors and a cabin deck — cold rises through metal and timber frames like it's got somewhere important to be. In a shepherd's hut you've got that steel chassis sitting in damp British air all winter, so skipping the floor is just giving up warmth voluntarily.

My go-to approach:

• Rigid foam board (Celotex or Kingspan) between the joists first
• Foil-faced layer underneath if you can access it — reflects radiant heat back up
• Vapour barrier before your finished floor goes down, or you'll be growing mushrooms by February

The walls and roof get all the glory but honestly a poorly insulated floor will undo half your work up there. I've sat in a mate's hut where he'd done 100mm in the walls and roof and sweet nothing underneath — you could feel the cold radiating up through his lovely reclaimed oak boards like a passive aggressive message from the earth.

Worth also thinking about skirting around the chassis to stop wind chill underneath — basically a shepherd's hut is sat up on wheels so it gets hammered from below in a way a cabin on a slab doesn't.

Anyone else gone the spray foam route under a hut floor, or is that asking for trouble with a timber subfloor? Genuinely curious whether it's worth the faff or if board insulation does the job just as well.

@MoorKev — does this apply equally to huts on wheels versus ones on a fixed stone base? Mine would be sitting on a concrete pad and I'm wondering if that changes the equation at all.

Also curious what thickness you ended up going with on your cabin deck. I've seen people arguing between 50mm PIR and 100mm, and whether it's worth the loss of headroom.

For my garden office build I went 70mm Celotex between the joists and honestly it made a massive difference compared to the van floor (which I bodged with 25mm and regretted). Is PIR still the go-to for huts or do people go rigid mineral wool to avoid the fire risk concerns?

EwanPowell92 | 47 posts

@StuKnight the base type makes a massive difference actually. Concrete slab will act as a thermal mass which sounds great in theory but in practice it's drawing heat downward all winter unless you've got rigid insulation between the slab and your floor build-up. Wheels/chassis is arguably worse though — you've got cold air circulating underneath with no thermal mass benefit at all.

Either way I'd say 100mm PIR board minimum, taped joints, vapour barrier sorted properly. Did my converted hut last autumn and the difference was noticeable within the first cold snap — battery bank wasn't working nearly as hard overnight.

Worth checking your floor joist depth before you buy anything though — some of the older huts have surprisingly shallow voids and you end up compromising on thickness.

@EwanPowell92 is right about the thermal mass point, but worth adding: with a wheeled hut the airflow beneath the chassis is your real enemy in winter. I've got 100mm Celotex between my floor joists with a vapour barrier on the cold side, and the difference versus an uninsulated floor is genuinely dramatic — probably 3-4°C ambient gain for zero ongoing energy cost.

One thing nobody mentions: treat the metal chassis rails as thermal bridges. I used self-adhesive foam strip where the floor deck contacts the steel. Bodge-y looking during the build but functionally important.

If you're on a concrete slab @StuKnight, rigid PIR directly onto the slab before your floor deck is the neater solution — Kingspan or Celotex, 75-100mm minimum. Don't bother with mineral wool down there; it'll wick moisture eventually and you'll be doing it again in three years.

@DontPanic makes a fair point about underfloor airflow but nobody's mentioned vapour barriers yet. On my narrowboat I skipped that step once — never again. Moisture migrates upward through insulation and you end up with damp, compressed mineral wool doing sod all.

If you're using rigid PIR boards between the joists on a wheeled hut, tape every join properly and get a decent membrane down first. Foil-faced Kingspan or Celotex is what I'd go for — don't bother with the cheap stuff from the builders' merchant.

What thickness are people actually running in these huts? On the boat I managed 60mm but I'm wondering if a static hut gives you more floor depth to play with.