Approved Document L for Non-Domestic Insulation: A Specifier’s Guide (2026)

If you specify, build or operate non-domestic buildings in the UK, Approved Document L (volume 2) is the document that decides whether your insulation passes Building Control. Get a U-value wrong by 0.05 W/m²K, miss an airtightness pressure test, or design a junction with a high thermal bridge ψ-value, and the project doesn’t get sign-off — or worse, doesn’t get an Energy Performance Certificate that meets MEES.

This guide walks specifiers and contractors through what Part L (volume 2) actually requires for non-domestic insulation in 2026, what the Future Buildings Standard will change in 2025–26, and how to choose insulation systems that pass the calculation routes specifiers actually have to use.

What Approved Document L (volume 2) is

Approved Document L is the statutory guidance for Part L of the Building Regulations in England — “Conservation of fuel and power”. Volume 2 covers buildings other than dwellings.

The current edition is December 2021, with 2023 amendments. It applies to:

• New non-domestic buildings
• Material change of use of existing buildings to non-domestic
• Major refurbishment / extension of existing non-domestic buildings (the “consequential improvements” route)
• Replacement of building services controlled by Part L

Wales and Scotland have their own equivalents (W and S6). Northern Ireland follows Part F2. The technical content tracks closely but the dates and figures diverge — always check the local document.

The four tests an insulated element has to pass

For any insulated element (roof, wall, floor, glazing) in a non-domestic building, the design has to demonstrate compliance against four limits — not just one. Many failed Building Control submissions hit the U-value, get complacent, then fail on airtightness or thermal bridging.

1. Limiting U-value (the floor)

The “limiting U-value” is the worst-case fabric performance allowed for any element of the same type. New build limiting values for non-domestic from ADL2 (2021/23 edition):

These are limiting values — the design U-values you actually use will usually need to be 10–25 % better than this to make the building pass the SBEM / DSM model overall. Treat the limiting figure as a hard floor, not a target.

For existing buildings undergoing refurbishment, the limiting U-values relax — typically to 0.18 / 0.26 / 0.25 / 1.8 (roof / wall / floor / glazing) — but only if upgrading to the new-build figure is “not technically, functionally or economically feasible”. In practice, most warehouse and industrial refurbishments use the existing-building figures and document the technical case.

2. The Building Emission Rate (BER) vs the Target Emission Rate (TER)

Pass the limiting U-values, and you still have to pass the SBEM (Simplified Building Energy Model) calculation. The model produces a Building Emission Rate in kgCO₂/m²/year. That has to be less than or equal to the Target Emission Rate for a notional reference building of the same shape and use class.

If your fabric is at the limiting U-values and your services are average, you will fail TER. The route through is to either:

• Tighten the fabric below the limiting values (often easier in roofs — drop the U-value to 0.15 with a high-performance closed-cell spray foam)
• Improve the services (high-efficiency boilers, heat pumps, LED, controls)
• Add on-site renewables (PV)

For warehouse and industrial roofs, fabric tightening is usually the cheapest CO₂ saving per pound spent — which is why specifiers default to a U = 0.15 spray foam roof spec on commercial refurbishments.

3. Airtightness

Limiting air permeability for new non-domestic buildings (ADL2 2021/23):

8 m³/(h·m²) at 50 Pa, tested in accordance with ATTMA TSL2.

For some building types (e.g. heated warehouses where airtightness drives heat loss heavily), the BER calculation will require a much tighter design figure — typically 3–5 m³/(h·m²) to pass.

This is the test that catches commercial spray foam in particular — closed-cell spray foam delivers airtightness as a by-product of the application (it bonds to the substrate and seals every junction simultaneously). On a refurbishment where the existing roof / wall has multiple penetrations, no other insulation strategy reaches a tested 3 m³/(h·m²).

4. Thermal bridging (ψ-values)

Each junction in a thermally-broken building has a linear thermal bridge value (ψ — psi). ADL2 doesn’t set a hard limit on individual ψ-values but requires that you either:

• Use Approved Construction Details (ACDs) — a published library of pre-calculated junctions
• Or calculate the ψ-value using a competent person’s analysis (typically ISO 10211)

If you fail to provide either, ADL2 applies a default ψ-value of 0.15 W/m·K to every junction — which usually fails the BER calculation immediately. Spray foam systems generally outperform ACD figures because the application eliminates linear thermal bridges at junctions (no fabric breaks for fixings or laps).

Where the Future Buildings Standard changes things

The Future Buildings Standard (FBS) is the planned 2025/26 update. Consultation closed 2021; final form expected to be in force from late-2025. Headline changes for non-domestic:

• Limiting U-values tightened by approximately 15–25 % across roofs, walls and floors. Indicative new figures (subject to final draft):
• Pitched roof: 0.15 → indicative 0.13
• Flat roof: 0.18 → indicative 0.15
• Walls: 0.26 → indicative 0.21
• Floors: 0.18 → indicative 0.15
• Air permeability target tightened from 8 to 5 m³/(h·m²) at 50 Pa for most non-domestic types.
• CO₂ reduction targeted at 27–28 % below current ADL2 baseline (a similar trajectory to the Future Homes Standard for dwellings).
• Embodied carbon reporting likely required for major projects, separate to operational CO₂.

For specifiers planning building submissions in 2026 onward, design to the FBS figures now — Building Control will accept lower U-values than the limiting figure, but won’t accept anything above. Anchor on U = 0.13 for pitched roof, 0.15 for flat roof, 0.21 for walls and you’ll be FBS-ready on day one.

What this means for specifying spray foam insulation

Closed-cell rigid polyurethane spray foam (Enverge in the systems Duratite installs) carries a published thermal conductivity of around 0.027 W/m·K at typical commercial application densities. To hit the 2026 limiting U-values:

For comparison, mineral wool (λ ≈ 0.035) needs ~180 mm / ~225 mm / ~260 mm for the same targets — roughly 1.3× more thickness for every U-value. On a warehouse with limited soffit clearance, that thickness difference often decides whether a refurbishment is feasible at all.

How fire compliance interacts with Part L

Approved Document L sets the thermal limits. Approved Document B sets the fire limits. For non-domestic, the relevant fire performance figures are:

• Reaction to fire of the insulation: classified to BS EN 13501-1 (Euroclass A1, A2, B, C, D, E, F).
• Spread of flame of internal linings: classified to BS 476 Part 7 (Class 1, 2, 3, 4) — older system, still cited.

Polyurethane spray foam on its own classifies to Euroclass E under BS EN 13501-1 — that is not enough for an exposed application in most non-domestic occupied spaces.

The standard route is to overspray the foam with an intumescent topcoat — IFTI DC315 (International Fireproof Technology Inc). At the published wet film thickness, the system achieves Euroclass B (BS EN 13501-1) and Class 1 / 0 (BS 476 Part 7 / Part 6). That’s the compliance package most non-domestic warehouse, hangar, showroom, retail and educational specifications require.

For a deeper read on the fire side, see our Spray Foam Fire Protection: UK Guide.

Documentation Building Control will ask for

Specifiers and contractors should expect to submit, at building-regulations stage:

1. U-value calculation for every element, in line with BR 443 — independent or competent-person signed.
2. SBEM output showing BER ≤ TER (or, for very small / simple buildings, an Elemental Recipe demonstration).
3. As-built airtightness test report to ATTMA TSL2 from a UKAS-accredited tester.
4. Approved Construction Details library reference, OR ψ-value calculation for non-standard junctions.
5. Reaction-to-fire classification report for the insulation system to BS EN 13501-1, plus any Loss Prevention Standard (LPS) approvals required by the insurer.
6. Manufacturer datasheet and installation method statement for the insulation product.

Skipping any one of these is the most common reason non-domestic projects get held at sign-off.

Quick-reference: what to specify on a UK warehouse roof in 2026

For a typical commercial warehouse refurbishment in 2026, specifying:

• Closed-cell spray foam at 170 mm to deliver U = 0.15 W/m²K (FBS-ready), λ = 0.027
• IFTI DC315 intumescent topcoat at the published WFT to achieve Euroclass B under BS EN 13501-1
• As-built airtightness target ≤ 5 m³/(h·m²) at 50 Pa (TSL2)
• 25-year manufacturer-backed product warranty on the system
• Approved Construction Details for all junctions or, where bespoke, ISO 10211 ψ-values

Will pass ADL2 2021/23 today, and pass the Future Buildings Standard from day one when it comes into force.

Frequently asked questions

Q: Does Part L apply to my warehouse refurbishment if I’m only re-coating the roof?
A: A re-coat alone is not a “consequential improvement” trigger unless you’re also doing a material change of use or > 25 % of the thermal envelope. A pure waterproofing recoat to an existing insulated roof typically falls outside ADL2 — but if you’re insulating the roof for the first time, or replacing > 50 % of an existing insulation layer, you trigger ADL2 and need to meet the limiting U-values.

Q: Are spray foam systems acceptable to UK Building Control?
A: Yes — closed-cell rigid polyurethane spray foam is widely accepted, provided the calculation route, fire compliance, and airtightness test all pass. Each project’s Building Control body may also ask for product certification — check their requirements at pre-application stage rather than at sign-off.

Q: Does Approved Document L cover existing buildings, or only new build?
A: Both. Section 6 of ADL2 covers work on existing non-domestic buildings, with relaxed limiting U-values where compliance with new-build figures is not technically or economically feasible. The relaxed figures still have to be the best achievable on the project.

Q: What’s the difference between BER and TER?
A: BER is the Building Emission Rate — the modelled CO₂ per m² per year for your design. TER is the Target Emission Rate — the figure for a notional reference building of the same shape and use. BER must be ≤ TER for ADL2 compliance.

Q: How does Part L affect MEES (Minimum Energy Efficiency Standard) on commercial leases?
A: They’re separate rules but they reinforce each other. ADL2 sets fabric performance at construction stage; MEES sets the EPC rating threshold for letting (currently E, rising to C by 2027 and B by 2030 under current proposals). A building that scrapes through ADL2 with limiting U-values today will probably need a future fabric upgrade to meet MEES C/B.