Guide
How to insulate a shed
Most sheds sold in the UK are a single skin of 12 mm cladding on a 28 or 44 mm frame. With no insulation, a typical shed wall loses heat six to eight times faster than one with 40 mm PIR and a lining. The calculator above is built on the method below.
Step by step
- Fix leaks and draughts. Re-felt or seal the roof, fix split boards, and seal gaps around windows and doors before you start. Insulation over a leak just hides the rot.
- Measure the frame. Measure the stud depth and the spacing between stud centres on every wall and the rafters. These two numbers decide what thickness of board will fit.
- Batten out the studs. Screw treated battens to the face of each stud so the cavity is deep enough for the insulation plus a 25 mm air gap.
- Fit the insulation. Cut boards 2 to 3 mm oversize and push them between the studs, leaving the air gap against the cladding. Fit insect mesh over the gap at the top and bottom of each wall.
- Seal the warm side. Tape every PIR joint and the edges against the studs with foil tape, or staple a polythene vapour control layer over wool, lapping joints by 150 mm and taping them.
- Line the walls and ceiling. Fix T&G, shiplap or OSB over the insulation, nailing into the battens at every crossing. Never leave PIR or EPS exposed.
- Insulate the floor. Either lay treated battens on the existing floor, fit insulation between them and screw down 18 mm T&G chipboard, or, if you can reach under the shed, push insulation up between the joists and hold it with battens. From below keeps your headroom.
The layers, from outside in
- Cladding. The existing shiplap or featheredge.
- Air gap, 25 mm or more. Lets damp that soaks through the cladding dry out instead of wetting the insulation. Cover the top and bottom with insect mesh.
- Breather membrane (optional). Stapled to the studs. It lets vapour out and stops drips and draughts getting in.
- Insulation between the studs, cut 2 to 3 mm oversize so it grips.
- Vapour control layer on the warm side. Taped PIR foil counts if you also seal the board edges to the studs with foil tape. Wool needs polythene, lapped 150 mm and taped.
- Lining. T&G, shiplap or OSB, fixed through to the battens.
Best insulation for a shed: PIR, mineral wool or EPS
| PIR board | Mineral wool slab | EPS board | |
|---|---|---|---|
| Conductivity (λ) | 0.022 W/mK | 0.035 W/mK | 0.038 W/mK |
| Thickness for the same warmth | 40 mm | 64 mm | 69 mm |
| Sheet size | 2400 × 1200 mm | 1200 × 600 mm | 2400 × 1200 mm |
| Fire | Combustible, always line over it | Non-combustible (A1) | Combustible, always line over it |
| Air gap behind it | 25 mm | 25 mm behind a breather membrane | 25 mm |
| Best for | Shallow studs | Workshops, uneven frames, sound | Tight budgets, deep frames |
Keep heaters and wood burners well away from foil-faced and plastic insulation, and never leave PIR or EPS exposed inside the shed.
How thick should shed insulation be?
Shed studs are shallow, so depth matters more than in a house. The table shows the thickest PIR board that fits with a 25 mm air gap left behind it.
| Stud depth | No battens | With 25 mm battens | With 50 mm battens |
|---|---|---|---|
| 28 mm | none fits | 25 mm | 50 mm |
| 44 mm | none fits (19 mm space) | 40 mm | 50 mm (69 mm space) |
| 70 mm | 40 mm | 50 mm (70 mm space) | 75 mm |
The calculator flags any combination that won't fit and suggests the batten depth you need.
How to insulate a shed roof
Most heat leaves through the roof. Fit PIR between the rafters, leaving a 25 to 50 mm gap under the roof boards so the underside of the felt can breathe. If the rafters are too shallow, batten them down the same way as the walls. Keep the eaves open so air can move through the gap, tape the board joints, then line the ceiling. On an apex roof the slopes are cut to the rafter spacing like the walls; the flat plan shows each slope from inside.
How to insulate a shed floor
Lay treated battens on the existing floor at 400 mm centres, fit insulation between them and screw down 18 mm P5 tongue-and-groove chipboard. If the floor is ever damp, put a polythene layer under the insulation first. Keep the air moving under a raised shed: don't block the gap around the base. A 50 mm build-up plus 18 mm deck raises the floor by 68 mm, so check the door still opens and you keep enough headroom.
Cladding calculator: cover width and laps
Board is sold by its face width, but each board overlaps the next. The width you actually gain per board is the cover width: face width minus the tongue or lap. If you work from the face width you will run about a tenth short. Check your merchant's stated cover width, as profiles vary by 2 to 5 mm.
| Profile | Face width | Typical lap | Cover width |
|---|---|---|---|
| T&G matchboard | 94 / 119 / 144 mm | 10 mm | 84 / 109 / 134 mm |
| Interior shiplap | 119 / 144 mm | 11–12 mm | 107 / 132 mm |
| Featheredge | 100 / 125 / 150 mm | 25–50 mm | 75 / 100 / 125 mm at 25 mm lap |
| OSB3 / ply sheet | 1220 mm | butt joint on a stud | 1220 mm |
Membranes lap too. Lay breather membrane in horizontal runs with each course 150 mm over the one below, so water runs off. Lap polythene vapour barrier 150 mm and tape every joint.
Damp, condensation and ventilation
A heated, sealed shed fills with moisture from breath, drying paint and damp tools. That moisture condenses on the coldest surface it reaches. The vapour control layer keeps it away from the cold cladding, and the vented air gap lets anything that does get through dry out. Inside, open a vent when you work, and run a small dehumidifier in a workshop or garden office. Seal round door and window frames with draught strip, and consider secondary glazing on single-glazed windows: they lose far more heat per square metre than an insulated wall.
Bubble wrap and multifoil
Bubble wrap and thin multifoil are quick to staple up and help a little with draughts and frost. They do far less than the same depth of PIR or wool, so use them for seasonal storage sheds or as an extra layer over boards. They are combustible, so keep them away from heaters and line over them.
How the calculator works
- Studs are placed at your spacing, with extra trimmers each side of every door and window. Insulation is cut into strips the width of each bay and packed onto whole boards, so offcuts are reused before a new board is started.
- Lining boards are laid in courses at the cover width, cut around openings and packed onto the board length you choose. Runs longer than one board get a butt joint.
- Lining is measured on the inside face, so the wall, batten and floor build-up are taken off the shed's outside sizes.
- U-values follow BS EN ISO 6946:2017: standard surface resistances, a ventilated air gap (so the cladding is ignored), and the timber frame as a separate heat path. Conductivities are typical manufacturer declared values.
Shed insulation FAQs
What is the best insulation for a shed?
Foil-faced PIR board gives the most warmth per millimetre (lambda about 0.022 W/mK), which matters in a shed with shallow 28 to 44 mm studs. Mineral wool (about 0.035 W/mK) is cheaper, non-combustible and copes better with uneven framing, but needs a separate vapour barrier.
How thick should shed insulation be?
As thick as fits with a 25 mm air gap left behind it. On 44 mm studs that is only about 19 mm, so most people screw 25 mm battens to the studs and fit 40 mm PIR. 50 mm PIR with battens gives a wall U-value around 0.45 W/m²K, against about 3.8 for a bare shed wall.
Do I need an air gap when insulating a shed?
Yes, for rigid boards. Leave at least 25 mm between the board and the back of the cladding on walls and 25 to 50 mm under the roof so damp that gets through the cladding can dry out.
Which side does the vapour barrier go on?
On the warm, inside face, under the lining. A breathable membrane, if used, goes on the cold side. Never put polythene against the cladding.
How do I work out how much T&G cladding I need?
Use the cover width, not the face width. Cover width is face width minus the tongue or lap: a 119 mm T&G board with a 10 mm tongue covers 109 mm. Divide each wall height by the cover width for the number of rows, then add 10 to 15 percent for waste.
Can I use bubble wrap to insulate a shed?
Bubble wrap or multifoil is quick and cheap and helps a little against draughts and frost, but it does far less than the same depth of PIR or wool. Use it for seasonal storage, or as an extra layer, and never leave it exposed near a heater.
Do I need to ventilate an insulated shed?
Yes. Keep the air gap behind the insulation vented at the top and bottom, keep air moving under a raised floor, and open a vent or run a small dehumidifier if you use the shed as a workshop or office. Sealed, heated sheds without ventilation go damp.
Should I insulate the shed floor?
If you will use the shed in winter, yes. A raised shed floor loses a lot of heat. Lay treated battens on the existing floor, fit insulation between them and add an 18 mm chipboard deck, but check the loss of headroom.
Is a shed insulation kit worth it?
A kit saves you working out quantities, but kits are sized for standard sheds and usually assume one thickness. Measuring your own frame and buying boards to suit, as this calculator does, usually gives a better fit and less waste.