Loft insulation boards: using PIR in the loft, including boarding you can walk on

PIR boards can be a good way to insulate a loft where there is not much depth, because they give about twice the thermal resistance of mineral wool for the same thickness. This guide covers using PIR between and over the joists, what the numbers look like, and what to do if you also want to walk on the boarded loft.

Prices correct at 28 October 2026 and may change. All prices are ex VAT; check the product page for the current price.

Quick answer

• PIR has a λ of 0.022 W/mK on the boards below, against 0.044 for a typical mineral wool roll, so half the thickness gives a similar R-value.
• A 100mm layer between the joists plus a 50mm layer over them gives about R 6.8 and costs about £20.09 per m² ex VAT for the boards.
• We do not currently sell PIR boards with a chipboard or tongue-and-groove facing for walking on. If you want to walk on the loft, build a raised boarded platform on top of the insulation.
• Keep the eaves ventilated and ask building control or an electrician if you are unsure about cables and lights.

PIR or mineral wool in the loft?

The usual advice for a mineral wool loft is a total depth of around 270mm (Energy Saving Trust). R-value is thickness in metres divided by λ, so 270mm at λ 0.044 gives about 6.1 m²K/W. PIR at λ 0.022 reaches 6.8 at 150mm. That makes PIR useful where the joists are shallow, where you want to keep headroom, or where you want to board the loft without raising the boarding very far.

Mineral wool rolls cost much less per m², so for an ordinary loft with plenty of depth they are usually the cheaper route. Check the rolls in our loft insulation collection.

Between and over the joists

The usual approach is to fit boards tightly between the joists, then lay a second layer across the joists at right angles so the joists are covered and the cold bridge is reduced. Cut the boards snugly, keep the joints tight and seal gaps in line with the manufacturer’s guidance. The layers below show one way to reach about R 6.8, with the cost of each. Prices ex VAT, correct at 28 October 2026 and may change.

The first row uses 100mm at £36.83 ex VAT per 2.88 m² sheet (£12.79 per m²) and 50mm at £21.03 ex VAT (£7.30 per m²). The mineral wool row uses £31.29 ex VAT per roll, divided by the roll area (13.89 m² and 8.01 m²). The Kingspan Thermapitch TP10 100mm is also available if you prefer that brand. Compare per m² on the product pages and see our PIR price per m² guide.

Check what you are building against Approved Document L with your building control body. For an existing roof being renovated, Table 4.3 in Approved Document L Volume 1 (2021 edition incorporating 2023 amendments) gives 0.16 W/m²K for England. For help choosing a thickness, use our PIR U-value calculator.

Boards you can walk on

Foil-faced PIR is not a floor. Standing on it can crush it, and compressed insulation insulates less. Loft advice from the Centre for Sustainable Energy, SPAB and 21 Degrees is to raise the boarding above the insulation on battens, raised legs or a platform, so the insulation stays at full depth.

We do not stock insulated decking, meaning PIR bonded to chipboard or tongue-and-groove boards for walking on. For the platform itself we do stock West Fraser CaberFloor P5 18mm at £8.74 ex VAT per 2400 × 600mm sheet (£6.07 per m²) and the 22mm board at £10.83 ex VAT (£7.52 per m²). Both are P5 moisture-resistant chipboard with tongue and groove on four edges. They are sold as flooring, so check the manufacturer’s data for the span you need, and ask a structural professional whether your ceiling joists can carry the stored load. The cost of the battens or legs is extra.

Practical points before you start

• Fix any damp or leaks first. Do not cover them up.
• Leave a ventilation gap of at least 25mm at the eaves so the roof can breathe (Centre for Sustainable Energy).
• Think about cables, recessed downlights and pipework before you cover them. If in doubt, ask an electrician.
• Keep boards dry and flat until you use them. Do not use boards that have got wet.
• For a pitched roof where you are insulating between and under the rafters, boards such as Kingspan TP10 are certified for use between, under and over rafters. Check the certificate for your build-up.

Frequently asked questions

Can I walk on PIR boards in the loft?

No. Raise a boarded platform above the insulation so the boards are not squashed.

How thick should loft PIR be?

It depends on the U-value you need and the depth you have. 150mm of λ 0.022 PIR gives about R 6.8. Check with building control.

Is PIR cheaper than mineral wool?

No. On the figures above the rolls cost less per m². PIR is used where thickness matters.

Order loft insulation

Browse our PIR insulation boards, the loft insulation rolls and the loft, roof and ceiling insulation range. Prices are shown ex VAT. Most orders over £400 + VAT are delivered free across England, with delivery to mainland Wales and Scotland too. Mainland Wales and mainland Scotland: Celotex insulation and Knauf loft roll have £40 ex VAT delivery on every order; ceilings have a £400 ex VAT minimum plus £40 ex VAT delivery; British Gypsum plaster and plasterboard have a £400 ex VAT minimum plus £40 ex VAT delivery. We do not deliver to Northern Ireland or the Scottish islands.; see our delivery policy. If you are not sure which board suits your job, call us on 03330 908479 (Mon–Thu 8am–5pm, Fri 8am–12pm) or email sales@insulationwarehousedirect.co.uk and we will help you choose.

Sources: Centre for Sustainable Energy; SPAB; 21 Degrees; Energy Saving Trust; Kingspan Thermapitch TP10 BBA certificate; Approved Document L.