How to Insulate a Shed or Garage: Walls, Roof and Floor
An uninsulated shed or garage is cold in winter, hot in summer and prone to condensation, which is bad news for tools, stored furniture and anyone trying to work in it. Insulating one is a manageable job. This guide covers the main options for walls, roof and floor, from multifoil rolls to PIR boards, with manufacturer figures and live prices so you can plan the job.
Quick answer
- Quick and thin: multifoil such as SuperFOIL SF19+ stapled to the framing and taped at the joints. It also acts as a vapour control layer.
- Best insulation for the money: PIR boards such as Recticel Eurothane GP cut between the studs and rafters, with foil tape over the joints, then lined with OSB or plywood.
- Floor: PIR or EPS boards over a damp-proof membrane on a concrete base, with a floating deck of OSB or chipboard flooring on top.
- Garage conversions into living space usually need Building Regulations approval. Check with Building Control before you start.
Option 1: multifoil insulation
Multifoil rolls are thin, light and quick to fit, which suits sheds with shallow framing. SuperFOIL's published figures:
| Product | Thickness | Core R-value | Wall R-value (inc. air spaces) | Roof R-value (inc. air spaces) | Price (ex VAT, at the time of writing) |
|---|---|---|---|---|---|
| SuperFOIL SF19+ (1.2m × 10m) | 45mm | 1.63 | 2.96 | 2.53 | £95.83 |
| SuperFOIL SF19+ (1.5m × 10m) | 45mm | 1.63 | 2.96 | 2.53 | £119.16 |
| SuperFOIL SF6 (1.2m × 10m) | 30mm | 0.99 | 2.32 | 1.90 | £65.83 |
R-values are in m²K/W. The higher "inc. air spaces" figures depend on having air gaps either side of the foil, so fix battens to create them rather than fitting the foil tight against the cladding and lining. SuperFOIL describes SF19+ and SF6 as a 3-in-1 product combining insulation, a vapour control layer and a radiant barrier. Both are Euroclass F for reaction to fire.
SuperFOIL also makes SF19BB, a breathable multifoil (40mm, core R-value 1.45) for roofs and walls, and SFUF underfloor foil (6mm). Seal all joints in foil insulation with foil tape (£6.08 for 75mm × 45m at the time of writing).
Option 2: PIR boards between the framing
PIR gives the most insulation per millimetre (λ 0.022 W/mK), and general-purpose boards are good value for sheds and garages. Recticel Eurothane GP in 25mm and 50mm suits typical shed studs, and 75mm or 100mm suits deeper garage rafters. Prices are per 2400 × 1200mm sheet, ex VAT, at the time of writing:
| Recticel Eurothane GP | R-value (m²K/W) | Price per sheet |
|---|---|---|
| 25mm | 1.10 | £13.74 |
| 50mm | 2.25 | £21.24 |
| 75mm | 3.40 | £29.58 |
| 100mm | 4.50 | £35.83 |
- Check the outside is weathertight. Fix any leaks in the roof felt or cladding first. Insulation won't cure a leak.
- Keep a gap behind the board where the cladding might let in moisture, so the back of the cladding can dry out.
- Cut the boards to fit tightly between the studs and rafters. Our guide to cutting PIR insulation board shows how.
- Fill small gaps with offcuts or expanding foam, and tape the joints and the edges against the timber with foil tape.
- Line the inside with OSB3 or plywood, which is tougher than plasterboard in a workshop and lets you screw shelves and tools anywhere.
OSB3 prices at the time of writing (per sheet, ex VAT): 9mm £11.24, 11mm £13.33, 18mm £19.99. See all sheet materials.
Insulating the floor
- On a concrete base: lay a damp-proof membrane, then PIR or EPS boards, then a floating deck of CaberFloor P5 chipboard flooring or OSB3. Our PIR vs EPS vs XPS guide compares floor boards.
- On a timber shed floor on bearers: if you can get underneath or lift the floor, fit PIR between the bearers. Otherwise, lay thin boards over the existing floor and add a new deck.
Condensation and ventilation
Once a shed is insulated, warm moist air from inside (from people, drying tools or a heater) can condense on cold surfaces. Keep a continuous vapour control layer on the warm side, either polythene VCL (£64.58 for 4m × 50m at the time of writing), taped foil-faced boards or taped multifoil. Also keep some ventilation. Avoid unflued gas or paraffin heaters, which add a lot of moisture.
Worked example: a 3m × 2.4m shed
With 2m-high walls, the wall area is 2 × (3 + 2.4) × 2 = 21.6m². Allowing about 10% for cutting and ignoring the door, that's 23.8m², or about 8 sheets of 2.88m² PIR. At the time of writing, 8 sheets of 50mm Eurothane GP cost £169.92 ex VAT (£21.24 each), plus the lining boards and foil tape. Our insulation calculator can work out quantities for you.
A single small shed may come in under our £400 minimum order, so it's worth combining it with the roof, the floor or another job.
Frequently asked questions
What's the best insulation for a shed?
For the most insulation in the least depth, PIR board. For speed and a built-in vapour control layer, multifoil. Many people use PIR between the framing and add multifoil or a VCL on the warm side.
Is foil insulation as good as PIR?
Compare manufacturers' figures: the SF19+ core has an R-value of 1.63 m²K/W, and 50mm of Eurothane GP has an R-value of 2.25 m²K/W. SuperFOIL's higher figures (2.96 in a wall) assume air spaces either side. Both can work well. PIR usually gives more insulation for the money in deeper framing.
Do I need a vapour barrier in a shed?
If you'll heat it or use it as a workshop, yes. Fit a continuous vapour control layer on the warm (inside) face, and keep some ventilation.
Do I need Building Regulations for insulating a garage?
Insulating a garage used as a garage usually doesn't. Converting it into a habitable room usually does. Check with your local Building Control.
Order shed and garage insulation
Browse SuperFOIL multifoil, PIR boards and OSB. Prices are shown ex VAT. We are delivery only (no collection) and deliver to mainland England and Wales, with a £400 ex VAT minimum order and free delivery on every order (see delivery information). Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.
Sources: SuperFOIL SF19+ key features and DoP, SF6 flyer and DoP, SF19BB key features, SFUF flyer; Recticel Eurothane GP product page. Figures are indicative and correct at the time of writing.