Insulated OSB Board: What It Is, Where It's Used and the PIR + OSB Alternative
Insulated OSB board is a rigid insulation board, usually PIR, with a sheet of OSB bonded to one face. The idea is to fit insulation and a timber deck or lining in one go, for example over rafters, on loft floors or on internal walls. You can get the same build-up by fitting PIR insulation and OSB3 board as two separate layers, which is how we supply it. This guide explains what insulated OSB is, where it's used, how much the OSB adds to the insulation, and what to check before choosing either route.
Shop this. PIR insulation boards and OSB3 sheets at trade prices.
Quick answer
- What it is: a PIR (or similar) insulation board with OSB bonded to one face. Manufacturers' data sheets we've checked show OSB layers from 8mm to 22mm.
- Where it's used: manufacturers list uses such as over-rafter and under-rafter pitched roofs, loft floors, internal wall insulation and some flat roofs.
- Insulation value: almost all of it comes from the PIR. The OSB adds very little (see below).
- Load-bearing? Not necessarily. One manufacturer's data sheet states its PIR/OSB boards are not load-bearing elements, so check the product's data for your use.
- The alternative: fit PIR boards and an OSB3 deck or lining separately. We don't stock ready-bonded insulated OSB, but we stock both layers.
What is insulated OSB board?
It's a composite board. A rigid foam core (most commonly PIR, with a foil or paper-aluminium facing) is bonded to a sheet of OSB on one side. For example:
- termPIR's AL OSB technical card describes a PIR core with gas-tight facings and an OSB board of 8 to 22mm, bonded with an adhesive layer, with a declared λ of 0.022 W/mK for the PIR.
- Unilin's Utherm Attic L OSB data sheet describes a PIR board faced on one side with 12mm OSB, with tongue and groove edges, made for insulating loft floors.
Names, sizes, edge details and approved uses vary by manufacturer, so always work from the data sheet of the exact product.
Where is insulated OSB used?
- Pitched roofs: over the rafters, where the OSB gives a fixing surface for battens and counter-battens, or under the rafters as a lining.
- Loft floors: insulating and decking a loft in one layer, where the product is designed for it.
- Internal walls: as an insulated lining that gives a timber face for fixing.
- Flat roofs and floors: some products are made for warm-deck flat roofs or floors. Check the waterproofing compatibility and loading in the data sheet.
How much does the OSB add to the insulation?
Not much. R-value (thermal resistance) is thickness in metres divided by thermal conductivity (λ). Unilin's data sheet declares λ 0.130 W/mK for its OSB layer, which we've used below, and the PIR boards we sell list λ 0.022 W/mK:
| Layer | λ (W/mK) | R-value (m²K/W) |
|---|---|---|
| 9mm OSB | 0.13 | 0.07 |
| 18mm OSB | 0.13 | 0.14 |
| 50mm PIR | 0.022 | 2.25 |
| 100mm PIR | 0.022 | 4.50 |
OSB R-values are worked out as thickness ÷ λ; PIR R-values follow our usual table (rounded down to the nearest 0.05). Unilin's data sheet gives 0.092 m²K/W for its 12mm OSB layer, which fits the same sum. So the OSB is there for strength and fixing, and the PIR does the insulating. Work out the PIR thickness you need with our PIR U-value calculator, or read what thickness of PIR insulation do I need?
Insulated OSB vs separate PIR and OSB
| Insulated OSB (bonded) | PIR + OSB3 fitted separately | |
|---|---|---|
| Fitting | One board, one fixing operation | Two layers, so more handling and fixing |
| Choice of thickness | Set combinations from the manufacturer | Any PIR thickness with any OSB thickness |
| Joints | Board joints line up in both layers | You can stagger the OSB joints over the PIR joints |
| Availability | Specialist product, usually ordered in | PIR and OSB3 are standard stock items |
| Check | Data sheet for use, fixings, loads and fire class | Each layer's data, plus the build-up as a whole |
For most domestic jobs, such as a loft floor, shed, garage or garden room, separate layers give you more flexibility. For example, on a loft floor you can lay PIR between or over the joists and fix an OSB3 deck above it, using raised supports where needed so the insulation isn't squashed. Our guide to loft insulation boards and walkable boarding covers that build-up, and shed and garage insulation covers PIR with an OSB lining.
OSB3 sheets we stock
| Board | Sheet size | Price per sheet (ex VAT) |
|---|---|---|
| 9mm OSB3 | 2397 × 1197mm | £11.24 |
| 11mm OSB3 | 2440 × 1220mm | £13.33 |
| 15mm OSB3 | 2440 × 1220mm | £18.33 |
| 18mm OSB3 | 2440 × 1220mm | £19.99 |
All FSC Mix 70%, sold per sheet, at the time of writing (11 November 2026). OSB3 is the EN 300 class for load-bearing use in humid conditions; it isn't designed for permanent exposure to the weather.
What to check before you buy
- The U-value you need for the floor, wall or roof, and the PIR thickness to reach it.
- Condensation risk: foil-faced PIR resists vapour, so ask your designer whether you need a separate vapour control layer and a condensation risk analysis.
- Loads and fixings: use fixings and spans from the manufacturer's or your designer's specification, especially for floors and roof decks.
- Fire: PIR is combustible, so check the fire requirements for where it's going.
Frequently asked questions
What is insulated OSB board?
A rigid insulation board, usually PIR, with a sheet of OSB bonded to one face, so you fit the insulation and a timber deck or lining in one go.
Do you sell insulated OSB panels?
We don't stock ready-bonded insulated OSB at the moment. We stock PIR insulation boards and OSB3 sheets, which you can fit as two layers. Call 03330 908479 if you'd like help planning the build-up.
Does OSB insulate?
Only a little. 18mm of OSB has an R-value of about 0.14 m²K/W at λ 0.13, compared with 2.25 for 50mm of PIR at λ 0.022.
Can I walk on insulated OSB in a loft?
Only if the product is designed and installed for it. Check the manufacturer's data sheet for loading and fixing. Bare PIR isn't a floor; it needs a deck on top fixed to the joists or battens.
Is insulated OSB the same as a SIP?
Not quite. A structural insulated panel (SIP) has an insulation core between two structural facings, usually OSB on both sides, and is designed as part of the structure. Insulated OSB usually has OSB on one face only.
Order PIR and OSB
Browse PIR insulation boards and OSB board. Prices are shown ex VAT. We are delivery only (no collection), with a £400 ex VAT minimum order and free delivery in England. British Gypsum and Celotex products can also be delivered to mainland Wales and mainland Scotland (£400 ex VAT minimum plus £40 + VAT delivery), and FlipFix access panels have their own terms (£15 + VAT delivery on orders under £300). Everything else is delivered in England only (see delivery information). Call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm), email sales@insulationwarehousedirect.co.uk, or order online.
Guide only: figures are typical values for general information. Always check the manufacturer's current data sheets and confirm Building Regulations compliance with your designer, SAP assessor or building control. Insulation Warehouse Direct accepts no liability for decisions made using this guide.
Sources: termPIR AL OSB technical card; Unilin Utherm Attic L OSB data sheet; EN 300 OSB classes. PIR λ values and OSB sizes and prices are from our product listings. Figures are indicative and correct at the time of writing.