Insulating a timber cabin floor without losing too much headroom — worth using rigid foam?

Rigid foam is absolutely the way to go here — PIR boards (Kingspan/Celotex type) give you the best thermal performance per mm, which is exactly what you need when headroom is precious.

On my motorhome conversion I used 50mm PIR between the floor battens and got a massive improvement for relatively little sacrifice in height. For a timber cabin floor I'd probably approach it similarly:

• Battens on edge between joists/noggins to create a void
• Two layers of 50mm PIR staggered so joints don't align — massively reduces cold bridging
• Tape all joints with foil tape, it makes a real difference to airtightness
• 18mm OSB or ply on top as your structural deck

That whole build-up sits around 120–125mm, which is obviously significant if you're already tight. If you're really squeezed, a single 75mm layer with well-taped joints is still going to outperform most alternatives at that thickness.

One thing worth considering — are you insulating over an existing subfloor or starting from scratch? If there's an existing deck you can bond PIR directly to it with the right adhesive and just overlay with your finish floor, which keeps things much thinner.

Also worth making sure you've got adequate ventilation underneath the cabin if it's a suspended floor. Trap moisture under there and you'll undo a lot of good work regardless of how well the insulation performs.

Has anyone on here tried spray foam as an underfloor option on a cabin? I've seen it used under static caravans but never been convinced it's the right call for a timber structure. Curious what experiences people have had.

@ExFirefighter42 raises a fair point on PIR, though I'd add a small but important caveat — PIR boards need to be fully encapsulated if they're going beneath a timber floor deck, otherwise off-gassing can be a concern in enclosed spaces. Worth checking the specific product's technical data sheet rather than assuming all PIR performs identically.

On the narrowboat I've lived aboard, floor space is similarly precious. We used 25mm Celotex between the steel baseplate ribs rather than a full batten system, which preserved every millimetre possible. The critical detail was taping every joint with foil tape — cold bridging through gaps is where these installs quietly fail.

If the cabin sits on joists rather than a solid sub-floor, the methodology changes considerably, and that distinction probably warrants its own thread.

Great topic for a first post @ExFirefighter42 — welcome to the forum! 🎉

Done something similar on my garden office and the headroom thing is real. PIR is definitely the sweet tooth of insulation — maximum bang per mm.

One thing worth adding: if your cabin floor is suspended timber rather than solid, make sure whatever you use can handle a bit of movement. Rigid PIR can crack at the joints over time if the joists flex. A lot of folks use a thin layer of acoustic mat between the PIR and the deck above just to handle that.

Also worth checking your local builders merchant — Celotex GA4000 series is usually well stocked across the UK and often cheaper than ordering online once you factor in delivery on bulky stuff.

@SophieFisher makes a valid point — worth finishing that thought about PIR coverage, actually. Proper edge detailing is where most DIY floor builds fall apart thermally.

One thing I'd add from doing this on a shepherd's hut: don't overlook the subfloor void below the joists. If you've got any air movement under there, even well-fitted PIR between joists becomes significantly less effective. A breathable membrane stapled beneath before the insulation goes in makes a real difference.

Also worth considering: 25mm PIR (lambda ~0.022) will outperform 50mm of standard EPS — so you're genuinely gaining headroom for equivalent performance. Kingspan K3 is probably the most accessible for floor applications in the UK.

What's the floor construction — joists on bearers, or a solid raft? That changes the strategy considerably.

Great thread! Just to add a practical angle — if you're really tight on headroom, it's worth considering a hybrid approach. I used 50mm PIR between the joists combined with a thin layer of rigid insulation below, which kept the overall build-up much slimmer than going all-PIR above the joists.

One thing nobody's mentioned yet: vapour control. With a timber floor in a cabin that might sit unoccupied for periods, moisture management is crucial. Make sure you've got adequate ventilation to the void beneath, otherwise you risk the joists deteriorating over time regardless of how well you've insulated.

Also worth checking whether your floor is suspended or solid slab — the approach differs quite a bit. What's the subfloor situation, @ExFirefighter42? That'll help narrow down the best solution for your specific build.