Start with moisture, not just warmth

Most garden cabins in Britain fail not because they are cold, but because they are damp. A timber cabin sits in a climate that swings from soggy Atlantic fronts to still, frosty nights, and the warm moist air you generate inside — kettles, breathing, a fan heater — will always try to travel outwards through the walls and roof. Where that air cools enough to hit dew point, it turns to water inside the structure. That is why the insulation you choose matters as much for how it handles moisture as for its R-value.

The guiding principle is breathability: let the wall dry both ways, keep the inside reasonably airtight, and never place a vapour-tight layer on the cold side of the insulation. Get that right and any of the three main options will serve you well. Get it wrong and even the most expensive board will rot your frame.

Sheep's wool: the forgiving all-rounder

Sheep's wool batts are the natural choice for a cabin that will be used intermittently and is hard to keep perfectly dry. The fibres absorb and release moisture vapour without losing much performance, and wool can buffer a surprising amount of condensation before it becomes a problem. It is pleasant to handle, needs no gloves or masks, and tucks neatly around noggins and cables.

Typical thermal conductivity is around 0.038–0.040 W/mK, so you need roughly 100mm to do the work that 60mm of rigid foam would do. Expect to pay in the region of £10–£16 per square metre for 100mm batts, and slightly less for the loft-grade rolls used in roofs. It is not cheap, and mice do like it, so seal gaps in the floor and around sole plates.

Wood fibre: dense, draught-proof and slow to overheat

Wood fibre boards and batts behave in a similar breathable way but with more density and more rigidity. They are excellent at cutting summer heat gain, which matters if your cabin faces south and doubles as an office or studio. The boards also add a little structural stiffness and are far better than wool at blocking airborne sound, so a cabin near a road or a noisy neighbour benefits.

Conductivity is around 0.038–0.042 W/mK, broadly the same as wool, so budget similar thicknesses. Cost is the sticking point: around £15–£25 per square metre for 100mm. Wood fibre also needs sharp tools and a bit of care to cut cleanly, and it should be kept dry on site — it will happily soak up a leak if you leave it stacked outdoors.

Rigid boards: maximum warmth in minimum space

Polyisocyanurate (PIR) and similar rigid boards are the space-saver's answer. With conductivity around 0.022 W/mK, 50mm of board performs about as well as 100mm of wool or wood fibre. In a cabin where you are squeezing a sofa or desk against the wall, that 50mm is often the difference between usable and cramped.

Foil-faced boards also act as a decent vapour control layer when taped at the joints, which suits a well-sealed cabin with mechanical ventilation or good background trickle vents. The trade-offs are cost — roughly £12–£20 per square metre for 50mm — fiddly cutting around services, and the fact that they trap moisture if they are fitted into a leaky, cold-side-breathing wall. Never leave a foil-faced board with a gap behind it that can collect water.

Walls, floors and roofs each want something different

There is no single winner across the whole build. Match the material to the job:

• Walls: wool or wood fibre between studs, with a breathable membrane outside and a smart vapour control layer inside. In a small cabin, consider 75mm of wood fibre plus a 25mm wood fibre board over the studs to reduce cold bridging.
• Floors: rigid boards win here, because every 25mm of floor build-up costs you headroom and raises the door threshold. Fit 50–75mm PIR between joists over a breathable membrane, and seal the edges.
• Roofs: if headroom allows, wool batts in a warm roof build-up are forgiving and quiet in rain. Where the roof is low, 75–100mm rigid boards between and over rafters keeps the ceiling height intact. Always maintain a 50mm ventilated void in a cold roof, and never block the eaves.

Build in some airtightness whatever you choose. Tape the internal joints, seal around window frames, and fit a decent draught seal to the door. A small, well-sealed cabin holds heat far better than a big, leaky one.

Budget, breathability and the space you actually have

If you want a simple rule: choose rigid boards where space is tight (floors, low roofs, narrow walls), and choose wool or wood fibre where the wall can be thicker and you want a structure that dries out safely on its own. Mixing the two is perfectly sensible — many cabins use PIR in the floor and wool in the walls and roof.

Whichever route you take, plan for ventilation. A cabin sealed like a flask and heated with a fan heater will rain on the inside of the windows by November. Add trickle vents, crack a window after cooking, and check for condensation behind curtains and in corners. Insulation keeps the warmth in; ventilation keeps the structure sound. Do both, and your cabin will still be dry, snug and quiet a decade from now.