How to Specify Insulation in 2026

Introduction

The UK insulation market in 2026 is being shaped by three pressures at once: tighter energy regulation, stricter fire safety controls, and sharp material price volatility. Approved Document L 2021 remains the current regulatory baseline for most projects in England, but the Future Homes Standard now has a confirmed commencement date of 24 March 2027, followed by a 12-month transition period ending on 24 March 2028 for standard building work.

That transition matters commercially because schemes can continue under the earlier regime only if building-control documents are submitted before the cut-off and meaningful work starts in time. At the same time, post-Grenfell fire requirements continue to narrow the practical product choices for external walls, especially above 11 metres and above 18 metres, so thermal performance can no longer be considered in isolation.

Future Homes Standard

Timing and transition

The Future Homes Standard takes effect on 24 March 2027 for standard building work in England. The main transition period ends on 24 March 2028 — projects that miss that deadline must move onto the new regime. This is especially important for phased developments because transitional protection is no longer treated as a blanket site-wide concession.

From SAP to HEM

The move from the Standard Assessment Procedure toward the Home Energy Model is not simply a software update. HEM models dwelling performance in a more dynamic and time-sensitive way, with stronger treatment of solar gains, heat losses, and the interaction between fabric and heating systems. During the transition, SAP 10.3 remains relevant while the HEM infrastructure is phased in.

U-values, airtightness and performance targets

The most useful framing for 2026 is to separate notional design targets from absolute backstop limits. For FHS-ready detailing, the notional airtightness target is 3.0 m³/h/m² with a tightened backstop of 4.0 m³/h/m² — the previously common 8.0 m³/h/m² figure is no longer appropriate for forward-looking specifications.

Table 1: U-value and airtightness targets — Part L 2021 vs Future Homes Standard 2026
Source: Approved Document L 2021 & Future Homes Standard statutory guidance (gov.uk)

Zero-carbon-ready homes

The Future Homes Standard requires new homes to be designed around low-carbon heating and on-site generation rather than fossil-fuel gas boilers. Heat pumps, solar PV, stronger fabric performance, and tighter airtightness are the default package. Homes built to this standard are designated "zero-carbon ready" — their operational carbon footprint will reduce naturally as the UK electricity grid decarbonises.

Fire Safety and Façade Rules

Over 18 metres

For relevant buildings above 18 metres, EPS, XPS, PIR, and phenolic foam should all be treated as unsuitable in the external wall system unless a very specific and lawful exception applies. Mineral wool (Euroclass A1) and other A1 or A2-s1,d0 products are the only practical compliant route under the Building Safety Act 2022 and Approved Document B.

Between 11 and 18 metres

Combustible façade materials in this range may rely on system-level fire testing (BS 8414-1 or BS 8414-2 with BR 135 criteria), build-up-specific evidence, surface-spread controls, and project-specific justification — not any simple board-level classification claim. Phenolic foam should not be described as a straightforward compliant option for 11–18 metre buildings.

Phenolic foam: the correct fire classification

Products such as Kingspan Kooltherm K5 achieve a Euroclass rating of C-s2,d0 — not Euroclass A1 or A2. Under EN 13501-1, Class C means combustible. This distinction should not be obscured by shorthand marketing wording, particularly in specifications for taller buildings.

2026 second staircase requirement

From 30 September 2026, new residential buildings over 18 metres in England must include a second staircase. This increases façade complexity and the number of junctions where fire integrity and thermal continuity must both be maintained. Secure Information Boxes are required on all buildings over 11m, and Evacuation Alert Systems on all buildings over 18m.

Insulation Materials: What to Specify in 2026

Glass fibre and glass mineral wool

Standard glass mineral wool achieves Euroclass A1 — non-combustible under BS EN 13501-1, the same classification as stone mineral wool, meaning it carries no building height restrictions for most applications. The practical limitation in walls is physical rather than fire-related: its lower density makes it less suitable for high-performance acoustic applications. Current pricing: £3.50–£5.50/m².

Rock / stone mineral wool (Rockwool / Knauf Earthwool)

Rock mineral wool achieves Euroclass A1 non-combustible and is the default choice wherever fire safety and acoustic performance both matter. It is the only straightforward route above 18 metres and the lowest-risk specification for 11–18 metre external walls. Current pricing: £7.00–£11.00/m².

Note on Dri-Therm 37: This product will not meet the FHS 0.15 W/m²K wall target in a standard 100mm cavity. Designers are now specifying wider cavities (typically 150mm) filled with Dri-Therm 32 (λ 0.032 W/mK) or Dri-Therm 34 as the compliant route.

Sheep's wool

Sheep's wool is particularly suited to breathable wall constructions in timber-frame buildings. Pricing has risen modestly to £20.00–£36.00/m² with supply remaining broadly stable.

Expanded Polystyrene (EPS)

EPS has seen significant price increases in 2026, with current pricing at £10.00–£13.00/m². Standard EPS achieves Euroclass E (combustible) and cannot be used in external wall systems on buildings over 18m. It is restricted on 11–18m buildings under the Building Safety Act 2022.

Extruded Polystyrene (XPS)

XPS has proved the most price-stable rigid board in 2026, with current pricing at £5.00–£9.00/m². It is the preferred choice for below-ground and sub-slab applications where moisture resistance is critical. It shares the same Euroclass E fire restrictions as EPS in standard form.

PIR foam boards (Celotex, Kingspan, IKO, Recticel)

PIR boards offer lambda values of 0.022–0.023 W/mK and are highly effective for floors and roofs where thickness is constrained. They cannot be used in external wall systems on buildings over 18m.

Supply alert (May 2026): Celotex has placed all PIR boards on allocation with a 15% price increase. Kingspan PIR has seen a 7.5% increase. Current pricing: £15.00–£35.00/m². Order early and confirm lead times.

Phenolic foam (Kingspan Kooltherm, Ecotherm)

Phenolic foam offers the highest thermal performance of any common rigid board (λ ~0.021 W/mK) but is combustible (Euroclass C-s2,d0). It has a valid role in floors, roofs, low-rise build-ups, and pipe/duct insulation. Current pricing: £25.00–£35.00/m² following a 4–7.5% increase in 2026.

Aerogel (Spacetherm, Proctor)

Aerogel offers lambda values of ~0.015 W/mK — approximately three times better than PIR per millimetre. Spacetherm A1 achieves Euroclass A1 non-combustible with no building height restrictions. Expensive at £77.00–£97.00/m² and best reserved for heritage retrofit and space-constrained applications.

Reflective multifoils and hybrid systems

Modern hybrid products such as Actis Hybris are BBA-certified (Certificate No. 24/7263) with a declared λD of 0.033 W/mK. They should be specified on the basis of certificate-backed build-up performance and have a practical application in sloping roofs and space-constrained retrofits.

Pricing: Q2 2026 Market Conditions

Geopolitical instability and energy cost pressures in early 2026 triggered significant price increases across most insulation categories. The inflation chart below shows only manufacturer-announced percentage increases from formal supplier bulletins — no range-midpoint calculations. XPS remains the most price-stable rigid board. PIR has seen the most significant shift in both price and availability.

Table 2: Insulation material pricing 2025 vs May 2026 (ex-VAT, materials only)
Source: Buy Insulation Online procurement data & supplier bulletins May 2026

Chart 1: Manufacturer-announced price increases Q2 2026 — Celotex, Kingspan, Knauf, BEWI, Sundolitt
All percentages are formally announced figures. Whiskers show the announced range where applicable.

The commercial takeaway is clear: PIR is now both a thermal and a procurement-risk decision. Rock mineral wool has seen more modest inflation and stable availability, strengthening its position as the practical default for walls, façades, and acoustic packages. XPS's relative price stability makes it increasingly attractive where moisture resistance is the primary driver.