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

Absolutely do the floor — it's probably the biggest heat loss area that people overlook. I made that mistake with my static caravan setup years ago, insulated walls and ceiling properly then wondered why it still felt cold underfoot and the heating was working overtime.

For a 4x3m cabin the floor is a relatively small area to tackle, so the effort-to-reward ratio is pretty decent. A lot of log cabins sit on timber bearers with an air gap underneath, which means cold air is circulating right under your feet all winter. Even 50mm of rigid PIR board between the joists makes a noticeable difference — I used Celotex in mine and it transformed things.

A few things worth considering:

• Vapour control — get this right on the floor or you'll have condensation issues later
• Boarding over — 18mm T&G OSB or ply on top of the insulation gives you a solid, warm surface to finish on
• Air gap sealing — worth blocking rodent access at the same time, ask me how I know 🐭

If you're running any off-grid power for the cabin — I've got a small Victron/Fogstar setup in mine powering lighting and a laptop — a well-insulated space also means your heating load is lower, which obviously helps if you're not on mains.

What insulation are you considering for the walls and ceiling? That might affect the best approach for the floor too. Happy to share more specifics about what worked in my setup if useful.

@OldSailor79 completely agree on the floor, and worth mentioning that with a log cabin sitting on bearers or joists, you've often got a lovely accessible void underneath to work with. Rigid PIR board (like Celotex or Kingspan) between the joists is brilliant for this — far better than trying to stuff rockwool up there. Even 50mm makes a noticeable difference, though 100mm if you've got the depth available. Just make sure you're also addressing any draughts around the perimeter where the cabin meets the base — that cold air infiltration underneath is sneaky and often undoes good insulation work. A breathable membrane stapled across the bottom before your boards go in keeps moisture and critters out too.

@OldSailor79 Done the same on my narrowboat — spent a fortune on Celotex for the walls then basically sat on a raft of cold misery for two winters before I admitted the floor was the problem.

@OldSailor79 oh this takes me back — I did exactly the same with my static caravan before I sorted the solar and battery setup. Freezing legs in January despite lovely warm walls!

What finally sorted it for me was a layer of Kingspan TF70 between the joists, then rigid OSB on top to protect it. Absolute game changer. The difference in how long my heating holds the temperature overnight was dramatic — and honestly I think it's reduced how hard my system works to maintain comfort, which I notice on the battery draw monitoring through my Victron.

One thing nobody mentions — vapour barrier placement matters massively with suspended floors. Get it on the warm side or you'll have condensation issues within a couple of winters. Learned that the hard way! 🙈

Slightly off the main topic but this is really relevant to my shepherd's hut build — I've been agonising over whether to prioritise floor insulation or spend the budget on upgrading from my current 200W panel setup.

After reading this thread I think the floor has to come first. My hut sits on a steel chassis with a good 400mm of airflow underneath, which I suspect is absolutely hammering my heating efficiency.

@PanelTina what thickness Celotex did you end up going with between the joists? I'm working with fairly shallow floor joists so wondering if 50mm PIR is a reasonable compromise or whether I'd really notice the difference going up to 75mm.

The van conversion I did previously had 25mm floor insulation and I always regretted not going thicker — lesson learned I suppose.