How do I insulate a shed or garden office?

Insulating a shed is straightforward. Insulating it without turning it into a rot trap requires two things people skip: an air gap on the cold side of the insulation so the timber can dry, and a vapour barrier on the warm side so moist inside air never reaches the cold cladding.

Safety

What you'll need

• Rigid PIR board or mineral wool to suit the stud depth
• Vapour control membrane or foil-faced board with taped joints
• Breather membrane if the walls have none (optional)
• Battens for the air gap
• Plywood or plasterboard for the internal lining

Step by step

1. Fix the damp and the roof first.Insulation over a leak accelerates rot invisibly. The roof must be sound and the base dry before anything is boarded in.
2. Check the wall build-up.Most sheds are cladding on studs with nothing behind. Add a breather membrane against the cladding if there is none, then battens to hold a 25mm ventilated cavity, then the insulation.
3. Insulate between the studs, tight but not compressed.Cut rigid board a couple of millimetres oversize and friction-fit it, or use mineral wool batts sized to the stud spacing. Gaps at the edges are where all the heat goes.
4. Seal the joints.Foil tape over the joints of rigid board, and expanding foam or a bead of sealant at the perimeter. Small gaps carry a surprising proportion of the heat loss.
5. Insulate the roof with a ventilated gap above.Leave at least 25mm between the top of the insulation and the roof deck, open at the eaves and ridge, so any moisture that gets in can dry out. A fully filled rafter void with no gap rots the deck.
6. Fit a vapour control layer on the inside.A polythene VCL or foil-faced insulation with taped joints, on the warm side of the insulation, sealed at every edge. This keeps inside moisture out of the structure.
7. Insulate the floor.Insulation between the floor joists from below if the shed is raised, or a layer of rigid board and a new floor deck laid over the existing one. A cold floor is a large uninsulated surface and it will be the coldest thing in the room.
8. Line out and keep ventilation.Plywood or plasterboard over the VCL. Keep a trickle vent or an openable window: an insulated, sealed room with people in it produces moisture that has to go somewhere.
9. Leave services accessible.Do not bury cables, sockets or junction boxes behind the lining. Route them in surface trunking or leave access panels.

Tips

• Insulate the roof first if you can only do one element. It is the biggest heat loss and the surface where condensation appears.
• Rigid PIR gives the most insulation per millimetre, which matters when the studs are only 45mm deep. Mineral wool is cheaper and easier to fit around awkward shapes.
• Draught-sealing around the door and window is cheap and gives an immediate improvement, whatever else you do.

Common mistakes

• Filling the rafter void completely with insulation — There is then no path for moisture to dry out of the roof deck, and the deck rots from above the insulation where you cannot see it. Always leave a ventilated gap.
• Omitting the vapour control layer — Warm moist inside air passes into the insulation, hits the cold cladding, and condenses inside the wall. The shed appears fine until the studs are rotten.

If it doesn't work

Questions people ask