Insulating a timber frame cabin on a budget — worth doing the floor first?

Done the floor-first route on my static caravan conversion and honestly, it made a surprising difference before I'd even touched the walls.

The ground beneath a timber frame will rob you of heat faster than you'd expect — especially through autumn and into a damp British winter. I laid 100mm Celotex between the joists with a vapour barrier beneath, and the whole feel of the place changed overnight. Feet no longer freezing at 6am, which sounds trivial until it isn't.

Budget-wise, it stacked up better than I anticipated:

• Celotex/Kingspan offcuts — worth checking local trade Facebook groups, builders often sell surplus cheaply
• Foil-backed insulation tape — sealing edges properly matters more than people admit
• A decent vapour barrier — don't cheap out here, damp is the enemy of everything

The reason I'd argue floor-first is practical as much as thermal. You're working on it before you've fitted anything else — no furniture to shift, no cables to route around. Once you've got walls and a ceiling done, going back to the floor becomes a proper job.

That said, if your cabin sits on solid ground rather than suspended joists, the calculus shifts a bit. Insulating beneath a slab is a different beast entirely.

Curious whether anyone here has gone the PIR route versus something like sheepswool or Rockwool between joists? I stuck with rigid board but wonder sometimes if a breathable option would have been smarter given how much moisture moves through an older timber frame.

What did others tackle first — floor, walls, or roof?

@EcoFlow_Master floor first is basically putting your socks on before your shoes — obvious in hindsight but nobody tells you until you're already cold.

My static caravan lost more heat through the floor than anywhere else before I sorted it — shoved 100mm Celotex between the joists and suddenly my little Victron-powered setup wasn't fighting a losing battle against the ground just drinking my warmth away.

Thermal bridging through the timber frame itself is the sneaky follow-up villain once the floor's sorted, mind.

Interesting thread — I'm doing a van conversion rather than a cabin but the thermal logic is the same isn't it?

What's the recommended insulation depth for a timber frame floor specifically? I've gone with 50mm PIR board in the van floor and it's transformed cold mornings, but I imagine a static or timber build has more depth to play with.

Also wondering whether anyone's used Recticel or Celotex for this — both seem well regarded but there's a noticeable price gap depending where you source them. Is the lambda value difference actually meaningful at the depths people are typically working with?

@EcoFlow_Master did you put any vapour control layer under your insulation or just straight onto the substrate? Condensation between the floor and ground seems like it could be a long-term problem worth getting right from the start.

Aye, @TorChild the thermal logic absolutely carries across — cold surfaces below you are cold surfaces below you, whether that's a van floor or a cabin subfloor sitting over damp Cumbrian ground like mine was.

One thing worth adding for anyone budget-conscious: don't overlook recycled rigid insulation boards from demolition salvage yards. I picked up a decent haul of 100mm PIR boards for a fraction of retail price — perfectly good, just cosmetically rough around the edges. For a floor void where nobody sees it, that's no bother whatsoever.

Also worth checking the ground moisture situation before you close everything up. A proper vapour barrier under the insulation saved me grief later — learned that one the hard way on an earlier project.

Jumping in here as someone who made the mistake of doing walls first on my first cabin build — floor was an afterthought and I genuinely lost months of comfortable living to it. The conductive heat loss through an uninsulated timber floor into damp ground is brutal, especially come November.

One thing worth mentioning that nobody's touched on yet: vapour barriers. If you're insulating a timber frame floor on a budget, don't skimp on a decent DPM underneath whatever insulation you're using. Ground moisture wicking up into your insulation will kill its performance completely over time. Rigid PIR boards with taped joints and a proper membrane beneath have served me well without breaking the bank. @EcoFlow_Master what did you use between the joists — were you working with existing framework or starting fresh?