Start With the Wall, Not the Label

Most cabin walls in the UK follow the same basic recipe: a timber stud frame at either 400mm or 600mm centres, sheathing board on the outside, a drained and ventilated cavity, then cladding. The insulation goes in the gaps between the studs, and how well those gaps are filled matters as much as the material you choose.

A cabin wall is rarely a sealed laboratory. It moves with the seasons, it gets driven rain on one side and warm, damp air on the other, and it is often heated intermittently. That last point is the one that catches people out. A shepherd hut used at weekends, or an off-grid shelter that sits cold for weeks, will see moisture condense inside the wall in ways a permanently heated house never will. So the question is not simply which material has the lowest thermal conductivity, but which one suits the way your building will actually be used.

Mineral Wool: The Dependable Default

Glass and rock mineral wool remains the sensible starting point for most timber cabins. It is inexpensive, non-combustible, and available in slabs and rolls in thicknesses that match common stud depths, typically 50mm, 75mm, 100mm and 140mm.

• Thermal performance: around 0.035 to 0.040 W/mK, so a 100mm stud bay gives you roughly 2.5 to 2.8 m²K/W.
• Breathability: vapour-open and forgiving. It lets moisture migrate rather than trapping it, provided your outer layers are not sealed shut.
• Fitting: the easiest of the three to work with. Slabs are compressed slightly and sprung into the stud bay, and they hold themselves in place while you work.
• Watch out for: over-compression at the edges. A slab crammed into a bay loses performance at exactly the point where cold bridging bites hardest.

If your budget is tight and your cabin is heated reasonably consistently, mineral wool will serve you well for decades.

Wood Fibre: Breathable and Forgiving

Wood fibre boards and flexible batts have become a genuine alternative for cabins, and they suit the UK climate well. The material is hygroscopic, meaning it absorbs and releases moisture vapour without losing its insulating value, which buffers the peaks and troughs that cause condensation in intermittently heated buildings.

• Thermal performance: typically 0.038 to 0.045 W/mK, so slightly behind mineral wool for the same thickness. The difference is modest and often offset by better real-world performance in a damp wall.
• Breathability: excellent. It pairs naturally with lime render, timber cladding on battens, and vapour-open membranes.
• Fitting: flexible batts friction-fit between studs much like mineral wool. Rigid wood fibre boards are denser, heavier, and need clean, accurate cuts. A fine-toothed saw or a sharp insulation knife works; a blunt blade tears the edges.
• Watch out for: cost, typically two to three times mineral wool, and availability. Order ahead for smaller projects.

For unheated shelters, shepherd huts and cabins used sporadically, wood fibre is often the more robust choice, because it tolerates the moisture cycles that come with intermittent occupation.

Rigid Boards: Maximum Performance per Millimetre

Polyisocyanurate, phenolic and expanded polystyrene boards deliver far more insulation for a given thickness, with thermal conductivities around 0.022 to 0.030 W/mK. That matters when your stud depth is fixed at 75mm and you cannot afford to lose floor space.

• Thermal performance: a 75mm PIR board can match or beat 140mm of mineral wool.
• Breathability: poor to none. Foil-faced boards act as a vapour barrier, which is useful if placed correctly and a serious problem if placed wrongly.
• Fitting: unforgiving. Every gap must be cut tight and sealed with foil tape or expanding foam. A 5mm gap around a board can cut its effective performance by a noticeable margin.
• Watch out for: sandwiching a vapour-closed board between two vapour-closed layers. In a cabin wall, keep rigid boards to the warm side with a service void inside, or use them as continuous external insulation over the sheathing.

Getting the Fit Right Between the Studs

Whatever you choose, the detailing decides the outcome. A few habits are worth building in from the first bay.

• Measure the bay at the top, middle and bottom. Studs bow, and a single measurement will leave gaps.
• Cut mineral wool and flexible wood fibre batts 10 to 15mm wider than the bay so they compress slightly at the edges and seal themselves.
• Fill the full stud depth. A partly filled bay creates a convection loop inside the cavity that quietly undoes your work.
• Do not let cables, pipes or back boxes interrupt the insulation without thought. Run services in a separate void where you can.
• Seal the top and bottom plates, and around window and door openings, with a vapour-open airtightness tape or membrane.
• Keep a continuous layer outside the studs if possible. Even 25mm of wood fibre or rigid board over the sheathing dramatically reduces cold bridging through the timber.

Choosing for Your Cabin

For a year-round, well-heated cabin, mineral wool between the studs with a continuous layer of wood fibre or rigid board outside is hard to beat on value. For a shepherd hut or off-grid shelter that sits cold for long stretches, lean breathable: wood fibre or mineral wool, vapour-open membranes, and a ventilated cavity behind the cladding. Use rigid boards where space is genuinely tight, but treat them as a precision material rather than a shortcut.

And remember that the walls are only part of the picture. In most small cabins, more heat escapes through the roof, the floor and the glazing than through the frame. Insulate the wall well, then look up and down before you call the job done.