Polyurethane: lambda, thickness for R 3.7 and R 6, installed price and fire reaction in 2026

Understanding polyurethane lambda: PUR, PIR and “real lambda” on site #

Polyurethane lambda by range (0.022 to 0.028): what the technical datasheets say and what you actually see in the field #

On technical datasheets, polyurethane shows a declared lambda (standardised value). In practice, the “real lambda” is mostly determined by the installation. A poorly closed joint, and the performance goes up in smoke.

PUR (polyurethane board) vs PIR and facings (foil, kraft): what changes the advertised performance #

At the same thickness, PIR may claim a lower lambda depending on the formulation and blowing agent used in manufacture. Facings (foil, kraft) mainly affect airtightness and rigidity, more than conduction. Be careful not to confuse thermal performance with fire behaviour, which is often a separate topic.

Tolerances, joints, thermal bridges: how to preserve polyurethane insulation performance once installed #

• Plan the layout to limit cuts and gaps. A 2 mm gap becomes a small radiator.
• Treat the ends. Compatible tape, controlled PU foam, continuous vapour barrier if specified.
• Manage thermal bridges. Floor junctions, reveals, fixings and cross walls.

Polyurethane board thickness: achieving R=3.7 and R=6 without losing too much space #

Simple calculation method: thickness = R × lambda (tables for 0.022 / 0.024 / 0.026 / 0.028) #

For a polyurethane board, the rule is straightforward. Thickness (m) = target R × lambda (W/m.K). Then convert to mm.

Common use cases: internal wall insulation, roof slopes, sarking, ground floors (where centimetres really matter) #

For internal wall insulation, targeting R=3.7 often means 80 to 105 mm, which is useful when every cm eats into usable area. For roof slopes or sarking, R=6 is more often in the 130 to 170 mm range. For ground floors, the right compromise depends on the thickness available beneath the slab. Keep in mind that lambda varies by range and facing.

Safety margin: when allowing a little extra thickness avoids nasty surprises at inspection #

On site, take the exact board reference and its declared R value. Choose the next standard thickness up (often +10 mm) to secure cutting allowances, joints and tolerances. In grant applications, inspections check consistency between quote, invoice and technical datasheet. No approximations.

Polyurethane or mineral wool: thickness comparison for R=3.7 and R=6, item by item #

Thickness equivalents versus mineral wool (common lambda 0.032 to 0.040): quantified space savings #

At the same thermal resistance, thickness is easy to calculate. e = R x λ. With low-λ polyurethane, you quickly save a few centimetres compared with mineral wool.

Site impact: window reveals, returns, services and sealing (the “cost of centimetres”) #

Those centimetres saved can reduce the hidden cost on site. But not always.

• Joinery. Reveals, sills, trims and returns may need to be reworked if thickness changes.
• Services. Electrical runs, ducts, radiators and drains may need to be shifted in internal insulation projects.
• Sealing. Thermal bridge treatment, vapour barrier and airtightness remain priorities.

Points not to forget: acoustics, summer comfort and system compatibility depending on the substrate #

Mineral wool often keeps the edge for acoustics and fire reaction. Polyurethane targets thermal performance, but the fire rating depends on the complete system (facing, fixings), a sensitive point in ERP buildings. For summer comfort, also consider thermal mass, time lag and insulation continuity depending on the substrate (masonry, timber frame, roof slopes).

Fire and ERP buildings: what you must check before proposing polyurethane #

Fire reaction of PUR/PIR boards: understanding Euroclasses, smoke and flaming droplets #

For a polyurethane board, fire reaction is read through the Euroclass (EN 13501-1). The letter (A1 to F) describes combustibility. The s1 to s3 indices classify smoke production, and d0 to d2 flaming droplets or particles. Be aware that the stated class applies to a specific system: thickness, facing, bonding or fixing.

Why it gets blocked in ERP buildings: facings, protection and installation solutions to reduce refusals #

In ERP buildings, the issue often arises because of the risk of dense smoke and molten droplets when the foam is exposed. The solution is not to sell a board on its own, but a complete assembly: validated facing (plasterboard, cement board, metal), treated joints, sealed penetrations and, if needed, a thermal barrier or fire protection compliant with the project and the test report.

Documents to request in 2026: test reports, DTA/ATec, installation instructions and traceability to justify your choice #

• Fire reaction test reports for the complete system (board + facing + fixing), valid at the time of use.
• DTA or ATec, or failing that assessment documents and CE marking with Declaration of Performance.
• Installation instructions, detail drawings, and traceability for batches, references and site photos.

Installed price per m² in 2026: pricing polyurethane without “selling performance” blind #

Material price: ranges by thickness, facing, density and brand (plain board, composite, PIR) #

In 2026, polyurethane is priced mainly by thickness and facing. At equal performance, a faced PIR board costs more than a plain board, but it limits thickness.

Installation price: adhesive bonding, mechanical fixing, treatment of special details and finishes (what pushes the bill up) #

The installed price rises on special details, airtightness continuity, and finishes. Adhesive bonding alone is rarely the most realistic scenario on site.

Weighing extra cost vs space gain: comparative estimates with mineral wool at equivalent thermal resistance #

On a wall, polyurethane keeps the advantage when every centimetre counts. For the same R-value, mineral wool needs more thickness, but it is often easier on the budget.