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Celotex GA4000 PIR insulation board, now Sopratherm

Insulation Thermal Conductivity (Lambda) Compared: PIR, Phenolic, XPS, EPS, Mineral Wool and Glass Wool

By , Managing Director

Quick answer

Thermal conductivity, called lambda (λ), tells you how easily heat passes through an insulation material. The lower the number, the better it insulates per millimetre. From the products we stock, phenolic board has the lowest declared λ at 0.019 W/mK, then PIR at 0.022, glass and stone wool at 0.032 to 0.044, XPS at 0.033 and white EPS at about 0.038. That means you need more than twice the thickness of loft roll to match a phenolic board. Thinner is not always the goal, though. Mineral wool is non-combustible and absorbs sound, and it is often far cheaper per m², so the right choice depends on where it is going and how much space you have.

What lambda actually means

Lambda is measured in W/mK: watts per metre-kelvin. It is the heat (in watts) that flows through a 1m thick layer of the material, across 1m² of area, for every 1°C (1 kelvin) difference in temperature between the two faces. People sometimes say "per m³" because a metre and a square metre are both in there, but it is not a volume measurement. The kg/m³ figure you see on mineral wool is density, which is a different thing. See our guide to rock wool vs glass wool density for that.

Manufacturers declare λ (λD) under the product standard for each material: BS EN 13165 for PIR, BS EN 13166 for phenolic, BS EN 13164 for XPS, BS EN 13163 for EPS and BS EN 13162 for mineral wool. These are the figures on the datasheets and on our product pages.

From lambda to R-value

The R-value (thermal resistance) is what a particular thickness actually delivers. Higher is better.

R-value (m²K/W) = thickness in metres ÷ λ

So 100mm (0.1m) of PIR at 0.022 W/mK gives 0.1 ÷ 0.022 = 4.55, which Celotex declares as 4.50 m²K/W (manufacturers round down). Turn the formula around and you can work out the thickness for a target R-value: thickness = R × λ. To get R 5.0 from loft roll at 0.044 W/mK you need 5.0 × 0.044 = 0.22m, or 220mm. Our older guide to understanding R-values goes into more detail.

And how U-value fits in

The U-value (W/m²K) is for the whole wall, floor or roof, not just the insulation. It adds up the R-values of every layer (brick, block, cavity, plasterboard, the surface resistances inside and out) and then corrects for things like timber studs, joists, wall ties and fixings that bridge the insulation. At its simplest, U = 1 ÷ (total R). Building Regulations set targets as U-values, so that is the number Building Control will ask about. Use our free PIR U-value calculator for a first look, or see how to calculate a U-value with worked examples.

Insulation lambda compared

Every declared λ, R-value and density below is taken from our product listings, which quote the manufacturer's datasheet or Declaration of Performance. Prices are our live prices, ex VAT, at the time of writing.

Material Typical λ (W/mK) Example IWD product Declared λ (W/mK) R-value at 100mm (m²K/W) Thickness for R 2.5 Thickness for R 5.0 Declared density (kg/m³) Price per m² (ex VAT)
Phenolic 0.019 Kingspan Kooltherm K103 100mm 0.019 5.25 48mm 95mm Not declared £38.24
PIR (foil-faced) 0.022 Celotex GA4100 100mm (now Sopratherm) 0.022 4.50 55mm 110mm Not declared (NPD) £11.91
PIR (foil-faced) 0.022 Recticel Eurothane GP 100mm 0.022 4.50 55mm 110mm Approx. 30 (core) £12.79
XPS 0.033 Kingspan GreenGuard GG300 100mm 0.033 3.00 83mm 165mm Not declared £21.24
EPS (white) 0.038* EPS 70 100mm Not published* 2.60* (typical) 95mm* 190mm* Not declared £6.97
Stone wool slab 0.035 Rockwool RWA45 100mm 0.035 2.85 (calculated) 88mm 175mm 45 £8.74
Stone wool slab 0.038 Rockwool Flexi 100mm 0.038 2.60 (calculated) 95mm 190mm Over 10 (exact figure not declared) £9.57
Glass wool loft roll 0.044 Knauf Insulation Loft Roll 44 100mm 0.044 2.25 110mm 220mm Not declared £2.25
Glass wool loft roll 0.044 URSA 10 DIVERSO 200mm 0.044 2.25 (calculated) 110mm 220mm 10 £4.74 (200mm)
Glass wool partition roll 0.035 URSA ACOUSTIC ROLL 100mm 0.035 2.85 88mm 175mm 22 £3.99
Glass wool cavity slab 0.032 Knauf DriTherm 32 100mm 0.032 3.10 80mm 160mm Not declared £10.75
Glass wool cavity batt 0.032 URSA CAVITY BATT 32 100mm 0.032 3.10 80mm 160mm 32 £9.58

How to read the table:

  • Thickness for R 2.5 / R 5.0 is calculated from the declared λ (thickness = R × λ), rounded to the nearest millimetre for the insulation layer alone. In practice, round up to the next thickness that is made. It ignores facings, air gaps, fixings, compression and the rest of the build-up, so it is a like-for-like comparison of materials, not a design.
  • R-value at 100mm is the manufacturer's declared figure where one is published. Where it says "calculated", we have worked it out from the declared λ and rounded down to the nearest 0.05, which is how manufacturers do it.
  • Density is shown only where the manufacturer declares it. "Not declared" means the datasheet we hold does not give a figure (Celotex lists it as NPD, "no performance determined"). Rockwool states that Flexi exceeds 10 kg/m³ for Approved Document E but does not give an exact figure. Recticel gives the core density of Eurothane GP as approximately 30 kg/m³.
  • *EPS 70: the manufacturer of the unbranded EPS 70 board we sell does not publish a datasheet online, so we do not quote a declared λ for it. The 0.038 W/mK shown is the typical value for white EPS 70 published by the BPF Expanded Polystyrene Group, and the R-value and thicknesses in that row are worked out from that typical figure. Call us for the technical information we hold before you design with it.
  • The URSA 10 DIVERSO price per m² is for the 200mm roll; other prices are for the 100mm product shown.

Each type: pros, cons and where it goes

Phenolic (λ 0.019)

  • Pros: the lowest λ of anything we stock, so it is the thinnest way to hit a U-value. Great where every millimetre counts: floors under screed, internal wall linings and pitched roofs.
  • Cons: the most expensive per m². K103 is Euroclass C-s2,d0 according to Kingspan, so it is not non-combustible.
  • Where: floors (K103), pitched roofs (K107), cavity walls (K106 and K108), timber and steel frame (K112), soffits (K110) and insulated plasterboard (K118). Compare it with PIR in Kooltherm K103 vs PIR floor insulation.

PIR (λ 0.022)

  • Pros: close to phenolic on thickness but a fraction of the price. Rigid, light, foil-faced and easy to cut. Celotex GA4000 and XR4000, Recticel Eurothane GP and Kingspan TP10 all declare 0.022 W/mK.
  • Cons: combustible (Celotex GA4000, Recticel Eurothane GP and Kingspan TP10 are all listed as Euroclass F), so check the fire rules for the job, and it needs tight joints and taping to perform as declared.
  • Where: floors, pitched roofs between and over rafters, flat roofs, walls, and as the go-to board for most extensions and conversions. See PIR insulation boards, Celotex (now Sopratherm) and Celotex vs Kingspan vs Recticel.

XPS (λ 0.033)

  • Pros: closed-cell and highly moisture-resistant, with high compressive strength, so it can go where boards get wet or carry heavy loads.
  • Cons: thicker than PIR for the same R-value, and GG300 is Euroclass F.
  • Where: inverted flat roofs, heavy-duty floors and below-ground insulation. Browse XPS insulation.

EPS (typical λ around 0.038)

  • Pros: the cheapest board per m² we sell, light and easy to handle.
  • Cons: needs much more thickness than PIR, and the manufacturer of our unbranded EPS 70 does not publish a fire classification online.
  • Where: ground floors under screed or slab, void fill and general packing. See EPS 70 boards and PIR vs EPS vs XPS floor insulation.

Stone wool (λ 0.034 to 0.038)

  • Pros: Euroclass A1 non-combustible, excellent at absorbing sound, and the slabs hold their shape between studs.
  • Cons: roughly 60% thicker than PIR for the same R-value, and heavier than glass wool.
  • Where: stud partitions, separating walls and floors, timber frame walls and pitched roofs between rafters. Browse Rockwool slabs.

Glass wool (λ 0.032 to 0.044)

  • Pros: the cheapest way to insulate a loft by far, Euroclass A1 on the products we list, and good sound absorption in partitions. Higher-density glass wool such as DriTherm 32 and URSA 32 products reaches 0.032 W/mK, better than stone wool.
  • Cons: loft roll at 0.044 W/mK needs the most thickness of anything here, so it suits places where depth is not limited.
  • Where: lofts at joist level (loft insulation), cavity walls (cavity wall insulation), stud walls and timber frame. See glass wool insulation and stone wool vs glass wool.

Which is best for…

  • Lofts (joist level): glass wool roll. Depth is rarely the limit, and it costs the least per m². The 270mm guidance comes out at R 6.1 with 0.044 rolls. See what thickness of loft insulation.
  • Walls: full-fill glass wool cavity slabs or batts for new cavity walls; PIR or phenolic for partial fill and internal linings where space is tight; stone or glass wool between timber studs.
  • Floors: PIR for most ground floors, phenolic where the finished floor height is tight, EPS where the budget matters more than thickness, XPS for heavy loads or wet conditions. See what thickness of PIR do I need.
  • Flat roofs: PIR flat roof boards for warm roofs, XPS for inverted roofs.
  • Sound or fire: mineral wool. Rigid foam boards are not acoustic products.

Free calculators

Frequently asked questions

Is a lower lambda always better?

For thermal performance per millimetre, yes. But the cheapest way to hit a U-value is often a thicker, cheaper material if you have the room, and in lofts and partitions mineral wool brings fire and sound benefits that foam boards do not.

Is thermal conductivity measured per m³?

No. It is W/mK, watts per metre per kelvin. The kg/m³ figure on mineral wool is density, which tells you how heavy the material is, not how well it insulates.

Why does my 100mm PIR say R 4.50 and not 4.55?

Manufacturers round declared R-values down to the nearest 0.05 under the product standards, so the figure on the datasheet is slightly conservative.

Can I mix materials?

Yes. Many build-ups combine PIR over rafters with mineral wool between them, for example. Add up the R-value of each layer, then get a full U-value and condensation risk calculation.

Order insulation

Browse PIR boards, Kingspan boards, floor insulation, wall insulation, roof insulation and mineral wool, all at trade prices for everyone. Most products are delivered in England only, with a £400 ex VAT minimum order and free delivery. Celotex and British Gypsum also deliver to mainland Wales and mainland Scotland, with a £400 ex VAT minimum and £40 + VAT delivery. We do not deliver to Northern Ireland or the Scottish islands. Call 03330 908479 for help choosing.

Sources: declared λ, R-values and fire classes are from our product listings, which quote each manufacturer's datasheet, including the Sopratherm (Celotex) GA4000 datasheet, Recticel Eurothane GP datasheet, Rockwool RW Slabs datasheet, Rockwool Flexi product page, URSA datasheets (July 2025) and Knauf Insulation datasheets. Typical EPS 70 value: BPF EPS Group. Figures are indicative and correct at the time of writing; they are not a substitute for a U-value calculation by your designer or the manufacturer's technical team, and Building Control has the final say.

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.

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