Insulated Log Cabin Floor: Wood-Fibre Boards and OSB
Our log cabin stands on a load-bearing timber frame on strip foundations, so air flows freely underneath. That is exactly what we want for the timber – it keeps it dry – but the moving air would also chill the floor. So we built a floor that keeps our feet a bit warmer: insulation between the joists and a hard-wearing OSB floor on top.
Quick answer – our floor build-up from bottom to top:
- Battens 4 × 6 cm (approx. 1 1/2 × 2 1/2 in) screwed flush to the underside of the joists
- Bitumen-impregnated wood-fibre boards laid on the battens between the joists
- Electric cables in the void
- 22 mm (7/8 in) tongue and groove OSB screwed to every joist
- Three coats of one-component clear varnish
Step 1: Battens under the joists
We screwed 4 × 6 cm roofing battens to the sides of the joists, flush with their underside. They form ledges for the insulation boards. No mitres are needed – the battens simply butt against each other, without a gap. We fixed them from the side with 6 × 80 mm wood screws.
Pre-drill near the ends of the battens, otherwise they split. The photo also shows how the base frame sits on the strip foundations, with a bitumen damp-proof layer in between, and otherwise stays about 14 cm (5 1/2 in) clear of the ground.
Step 2: Wood-fibre insulation between the joists
To close the bays between the joists we used bitumen-impregnated wood-fibre boards (in our case GUTEX). They are fairly firm and are not gnawed through as quickly by rodents as soft insulation – so far we have had no problems at all.
To cut the boards, we laid two battens across trestles and the board on top. Cut along the marked line with a plunge saw and guide rail, set about 2 mm deeper than the board is thick. That way the boards are cut quickly and accurately.
Tip: If your saw drifts sideways despite the guide rail, the blade may simply be worn out. Check it.
With the insulation in place, we first ran the electric cables for the cabin in the void below the floor – before the floorboards went on. Have electrical work connected and checked by a qualified electrician.
Step 3: OSB floor
Which floor for a log cabin?
Our cabin floor had to be robust and easy to keep clean. Ordinary softwood floorboards were not up to that, so we chose OSB flooring boards. OSB looks a bit like chipboard, but the wood strands are larger and oriented, it contains less glue, and the boards are very strong. Ours are 22 mm (7/8 in) thick with tongue and groove.
As the laying plan shows, there is hardly any waste if you start each new row with the offcut from the end of the previous one. This also avoids cross joints where four corners meet.
Laying and screwing the OSB
- Cut to size with a plunge saw and guide rail – quick, straight and clean if the blade is sharp.
- Slot the boards together and tack each one with just a few screws at first.
- Mark the joists on the boards with a chalk line. It is surprising how often you would otherwise miss the joist underneath.
- Screw down every board to every joist with five 5 × 50 mm chipboard screws.
Tip: Buy good screws – no cheap ones. If the screw heads do not sink flush into the OSB surface, switch to a better screw.
Finishing
Finally we sealed the OSB with a one-component clear varnish, applied with a roller in three coats – observe the drying times between coats. The result is a hard-wearing floor that copes with workshop use and is easy to sweep.
Our floor build-up at a glance
| Layer | Material | Fixing |
|---|---|---|
| Base frame | 8 × 16 cm bearers on strip foundations, about 14 cm above ground | see log cabin base |
| Ledges | 4 × 6 cm battens, flush with the underside | 6 × 80 mm screws, pre-drilled at the ends |
| Insulation | Bitumen-impregnated wood-fibre boards | laid on the battens |
| Floor | 22 mm T&G OSB flooring boards | 5 × 50 mm chipboard screws, 5 per board per joist |
| Finish | One-component clear varnish | 3 coats with a roller |
Frequently asked questions
How do you insulate a log cabin floor?
We screwed battens flush to the underside of the floor joists, laid bitumen-impregnated wood-fibre boards on these battens between the joists, ran the electric cables in the void and then screwed 22 mm tongue and groove OSB boards on top.
What is the best flooring for a log cabin?
For a workshop or hobby room, we found 22 mm tongue and groove OSB boards the best choice: robust, easy to keep clean and quick to lay. Ordinary softwood floorboards were not hard-wearing enough for our use. We sealed the OSB with three coats of a one-component clear varnish.
How do you screw down OSB flooring?
We tacked the boards first, marked the joists with a chalk line and then fixed every board to every joist with five 5 × 50 mm chipboard screws. Use good-quality screws: if they do not sink flush into the OSB, change to better ones.
Do mice get into wood-fibre insulation?
Bitumen-impregnated wood-fibre boards are relatively firm and are not gnawed through as quickly as soft insulation. In our cabin we have had no problems with rodents so far.
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