Description
On a warm roof the insulation sits above the deck and below the waterproofing, which means it does two jobs at once: it carries the thermal performance and it keeps the deck warm enough to stay out of the condensation risk zone.
Rigid PIR is the standard board for that build-up because it delivers a given U-value in the least thickness — useful when an extension roof has a fixed height to work to.
- Foil-faced rigid PIR board, 2400 x 1200mm (2.88m² per board)
- Six thicknesses from 25mm to 100mm — build up thickness to hit your target U-value
- Thermal conductivity 0.022 W/mK
- Sold as individual boards, so you order exactly what the roof needs
Specification
| Board size | 2400 x 1200mm (2.88m² per board) |
|---|---|
| Thicknesses | 25mm, 30mm, 40mm, 50mm, 75mm, 100mm |
| Core | Rigid polyisocyanurate (PIR) |
| Facings | Low-emissivity composite foil, both faces |
| Thermal conductivity | 0.022 W/mK |
| Applications | Warm flat roof, pitched roof, wall and floor construction |
| Supplied as | Single boards |
Frequently asked questions
How many boards do I need? Each board covers 2.88m². Divide your roof area by 2.88 and round up, allowing extra for cuts around upstands and outlets — most roofs waste 5–10% on cutting.
Which thickness should I choose? Thickness is driven by the U-value your project has to meet, so check your building control requirement or specification rather than guessing.
Where headroom or upstand height is tight, PIR's low conductivity means you reach a target with less depth than most alternatives.
Can I stack two thinner boards instead of one thick one?
Yes — laying two layers with staggered joints is common practice and helps reduce thermal bridging through the joints, though it adds a little labour.
Does it go above or below the deck? For a warm roof, above the deck and below the waterproofing layer, with a vapour control layer beneath the insulation.
Insulating below the deck instead creates a cold roof, which is a different build-up with its own ventilation requirements.
What fire performance does it have?
Fire classification varies by manufacturer and board type, so we don't quote a rating here — check the datasheet for the board supplied, or ask us before ordering if your project has a specific fire requirement.
Does my roof need a fall built in as well as insulation?
If your deck doesn't already fall to the outlets, our Tapered PIR Roof Insulation Board creates that drainage fall at insulation level instead of using this constant-thickness board.
Building a warm flat roof?
Lay our polythene vapour control layer beneath these boards, then finish with either our EPDM contact bonding adhesive-bonded rubber membrane or our PVC single-ply roofing membrane as the top waterproofing layer, and finish the perimeter with our uPVC gutter drip trim.
If the roof above this insulation is a metal-clad finish rather than a membrane, our standing seam roofing sheet system is a suitable concealed-fix covering for the same warm-roof build-up.
If your build-up needs to breathe rather than seal completely — timber-frame, period or listed reroofs — our wood fibre sarking & insulation board is the vapour-open alternative to this foil-faced PIR.
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