Insulating a 4x3m log cabin office - worth doing the floor as well?

Did the floor in my static caravan a couple of years back and honestly it made more difference than anything else I'd done up to that point. Cold rises from the ground something rotten, especially on those damp autumn mornings when the ground hasn't warmed up yet.

For a 4x3 you're looking at a manageable job. The approach I'd take:

• Lift the floorboards if they're accessible from underneath
• Pack 100mm rigid PIR board (Kingspan or Celotex) between the joists
• Tape the edges properly — don't skip this bit
• Lay a vapour barrier before you put the boards back down

The floor is often the forgotten layer. People spend ages on wall insulation and draught-proofing the door, then wonder why their feet are cold and the little diesel heater is running flat out.

One thing worth considering — if you're running a solar setup to power the office (laptop, monitors, maybe a fan heater), reducing the heat loss through the floor directly means your battery bank isn't working as hard. My Victron setup in the cabin holds charge noticeably longer in winter now compared to before I sorted the floor.

The ROI on a 12m² floor isn't massive in monetary terms, but in comfort? Significant.

Has anyone here tried spray foam between joists rather than rigid board? I've seen a few people on here mention it but never been entirely convinced it's worth the faff compared to just cutting PIR to fit.

@StormyMechanic yeah floor insulation is massively underrated tbh. In my van build I put 50mm PIR under the ply floor and the difference in thermal comfort was night and day compared to vans I've done without it.

For a static cabin on legs/joists I'd go Celotex or Kingspan between the joists then a layer of 25mm PIR board across the whole lot before your deck goes down. Stops the cold bridging through the timber.

Only thing I'd flag - vapour control layer on the warm side else you'll get condensation issues over winter. Seen a few cabin floors rot out from people skipping that bit.

@StormyMechanic did my 4x3 garden office floor with 100mm Kingspan between the joists and suddenly my Victron setup stopped working overtime just to heat the place — turns out the ground was basically drinking all my solar gains for free! 🥶

Worth noting with log cabins specifically — moisture barrier under the insulation is non-negotiable or you'll be growing your own mushroom farm by spring. Cheap DPM from any builders merchant sorted mine.

Also if you're on a budget, reclaimed PIR from salvage yards is absolutely worth hunting down — paid about 40% of new price for mine.

@BitsAndBobs that's the bit nobody talks about — battery performance in winter is directly tied to ambient temp, not just the cells themselves. My LiFePO4 bank used to throttle down noticeably from October onwards until I sorted the thermal envelope on my setup.

For a 4x3 on legs I'd go minimum 100mm PIR between joists, then tape every single joint properly. People skimp on the taping and wonder why it does nothing. Also make sure you've got a vapour barrier sorted on the cold side or you'll be pulling rotten timber out in five years.

Floor first, then roof, walls last — that's the order of priority if you're doing it in stages. Anyone telling you otherwise hasn't actually lived with the results.

@BitsAndBobs this is exactly what convinced me to insulate the shepherd's hut floor properly — my Fogstar batteries were showing genuinely alarming capacity drops last January and I kept blaming the cells. Turned out the floor was basically a cold sink dragging the whole interior down overnight.

Did 100mm PIR between the joists, thin ply on top, and the difference going into this winter has been noticeable on the Victron dashboard — resting voltage holds much better.

One thing I'd add for a log cabin specifically: make sure you've got a decent vapour control layer sorted, because ground moisture wicking up through timber joists is a real issue. Lose that battle and your insulation becomes a soggy sponge pretty quickly. @VitoProject is right about ambient temp being the hidden variable — worth solving properly rather than just throwing more panels at it.