Flat Roof Insulation: Warm Roof PIR Build-Up, Thickness and U-Value
Quick answer
A warm roof (or warm deck) puts the insulation above the structural deck, with the waterproofing layer on top of it. From the bottom up the build-up is: structural deck, vapour control layer (VCL), PIR boards, then the waterproof covering. The thickness you need depends on the U-value you are aiming for. In Kingspan's published tables for its Thermaroof TR24 board, 120mm gives 0.18 W/m²K, 130mm gives 0.17, 140mm gives 0.16 and 150mm gives 0.15. Check your own build-up with our PIR U-value calculator.
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What goes where in a warm deck flat roof
| Layer (bottom to top) | What it does |
|---|---|
| Ceiling and joists, or concrete or metal deck | The structure that carries the roof |
| Vapour control layer | Slows warm, moist indoor air reaching the cold side of the insulation |
| PIR flat roof boards | The insulation, laid so that the deck stays warm |
| Waterproofing | Keeps the weather out. The board's facing must suit the system you choose |
Because the deck sits on the warm side of the insulation, the joists and deck are kept at a temperature close to the room. That is why a warm deck is a common choice when you want no roof void to ventilate. Always have a condensation risk analysis done for the full build-up, which Kingspan states in its TR24 brochure.
How thick does flat roof PIR need to be?
Flat roof boards do not all share the same thermal conductivity. For Kingspan Thermaroof TR24, λ is 0.027 W/mK below 80mm, 0.025 between 80 and 119mm and 0.024 from 120mm. The declared R-values are:
| TR24 thickness | Declared R-value (m²K/W) |
|---|---|
| 30mm | 1.10 |
| 50mm | 1.85 |
| 80mm | 3.20 |
| 100mm | 4.00 |
| 120mm | 5.00 |
| 130mm | 5.40 |
| 140mm | 5.80 |
| 150mm | 6.25 |
Do not borrow R-values from a general-purpose PIR board such as Eurothane GP, which has a different λ. Read the datasheet for the exact flat roof board you are buying. Our roof insulation range includes TR24 alongside other flat roof boards.
Which U-value do you need?
Approved Document L (England) sets the targets. For a newly built roof element, such as a flat-roof extension, the limit is 0.15 W/m²K (Table 4.2). New dwellings use 0.16 (Table 4.1). Where you replace the waterproof covering of an existing flat roof, paragraph 11.2 treats that as renovation, and Table 4.3 asks for 0.16. A note to Table 4.3 says a less demanding standard may be acceptable on a flat roof where load-bearing capacity or upstand height is a problem, so talk to Building Control first. The new edition of Approved Document L takes effect in 2027, so confirm which one applies. Our building regulations guide has the wider picture.
You may see 0.18 quoted for flat roofs. It is not the current England roof figure, but it is a common marker in manufacturers' tables, because 120mm of TR24 on a concrete or timber deck gives exactly that.
Published TR24 U-values by deck type
Kingspan's figures (BS EN ISO 6946 with the conventions of BR 443, two-layer torch-on waterproofing, no packer board) are:
| TR24 thickness | Concrete deck (with ceiling) | Timber deck (with ceiling) | Metal deck (no ceiling) |
|---|---|---|---|
| 100mm | 0.22 | 0.22 | 0.24 |
| 120mm | 0.18 | 0.18 | 0.20 |
| 130mm | 0.17 | 0.17 | 0.18 |
| 140mm | 0.16 | 0.16 | 0.17 |
| 150mm | 0.15 | 0.15 | 0.16 |
The concrete deck is 150mm concrete with a 50mm screed to falls. The timber deck is 18mm plywood on 50 × 150mm joists at 600mm centres. The ceiling is plasterboard with a cavity. Your roof will differ, so treat these as a guide and let your designer or Kingspan's technical team calculate the real build-up.
Vapour control layer
- On new build, Kingspan says to install TR24 over a separate VCL, unless the deck is a sealed metal deck.
- For mechanically fixed boards, the minimum is 1000 gauge (250 micron) polythene, with joints lapped and sealed with double-sided self-adhesive tape.
- For bonded boards, the options are Type 3B felt (BS EN 13707), S1P1 (BS 8747) or a metal-cored VCL.
- Laps are at least 150mm, and the VCL is turned up at the edges (at least 100mm above the insulation height in a reinforced bitumen membrane system).
Laying and fixing the boards
- Lay boards break-bonded, with the long edges at right angles to the roof edge or diagonal, joints lightly butted and no gaps at abutments.
- Bonded systems use a single-part polyurethane adhesive. Ask the waterproofing manufacturer about adhesion centres.
- Mechanically fixed systems must be approved by the waterproofing manufacturer. Kingspan's minimum is 4 fixings for each 1200 × 600mm board (30 to 130mm) and 5 for each 1200 × 1200mm board (140 and 150mm). Wind uplift is assessed to BS EN 1991-1-4.
- Apply the membrane as soon as possible after the boards go down. TR24 is designed for torch-on systems, but do not apply the flame directly to the facing. Use minimum heat and flame or edge guards.
- TR24 takes limited foot traffic. Use promenade tiles where there will be more.
Upstands and edges
Where the roof meets a parapet, the insulation line needs to continue. For TR24, Kingspan's detail uses a minimum 30mm upstand on the inside of the parapet, which rises at least 150mm above the roof insulation, with at least 300mm between the top of that upstand and the underside of the horizontal insulation. Kooltherm K106 or K108 goes above that upstand. Rooflight kerbs need at least 30mm.
Frequently asked questions
Can I use 100mm PIR on a flat roof?
On its own, Kingspan's table puts 100mm of TR24 at 0.22 (concrete or timber deck), which does not reach 0.15 or 0.16. You would add thickness, or add a packer board layer such as TR27, which the brochure lists with its own U-values.
Do I need a VCL on a flat roof?
On new build, TR24 is installed over a separate VCL unless the deck is a sealed metal deck, and a condensation risk analysis should be carried out for every project.
Is PIR fire rated?
TR24's declared reaction-to-fire class is Euroclass F (System 4 in the brochure). The fire performance of the roof as a whole depends on the full system, so ask your waterproofing supplier for the evidence.
Order flat roof PIR
Browse our roof insulation range, including Kingspan Thermaroof TR24 100mm and 120mm. For VCL and tapes, see membranes. We have a £400 ex VAT minimum, with free delivery in England. British Gypsum (including Gypframe), Celotex and Knauf can also go to mainland Wales and mainland Scotland (£400 ex VAT minimum plus £40 + VAT delivery). Access panels can also go to mainland Wales and mainland Scotland, with their own charges (£15 + VAT under £300 ex VAT, free from £300). Other ranges, including suspended ceiling tiles and grid, are delivered in England only. We do not deliver to Northern Ireland or the Scottish islands. Call 03330 908479 if you would like help choosing a board.
Sources: Kingspan Thermaroof TR24 brochure; Approved Document L (GOV.UK). Figures are indicative and correct at the time of writing; they are not a substitute for a calculation by your designer or the manufacturer's technical team, and Building Control has the final say.