Non-Domestic Fabric Standards Under the FBS

The fabric standards in Approved Document L Volume 2 (2026 edition) set the minimum thermal performance that every element of a new non-domestic building must achieve. These are limiting (backstop) values – the worst acceptable U-value for any individual element – and apply regardless of how well the rest of the building performs. They are distinct from the notional building specifications, which set the overall compliance target (AD L2 2026, Section 3).

Limiting U-values for new buildings

Table 3.1 of AD L2 2026 sets out the maximum U-values for new or replacement thermal elements and controlled fittings in non-domestic buildings. These are area-weighted averages – for windows and doors, the U-value applies to the whole unit including the frame (AD L2 2026, Table 3.1).

Opaque elements

• Roof: 0.16 W/m²K
• Wall: 0.26 W/m²K
• Floor: 0.18 W/m²K
• Swimming pool basin: 0.25 W/m²K

Glazing and doors

• Windows, roof windows, glazed rooflights, curtain walling, and pedestrian doors: 1.6 W/m²K
• Vehicle access and similar large doors: 1.3 W/m²K
• High-usage entrance doors: 3.0 W/m²K
• Roof ventilators (including smoke vents): 3.0 W/m²K

For dormer windows, the roof parts are assessed against the roof U-value limit and the wall parts (cheeks) against the wall limit. All values represent the minimum standard – better performance is needed to meet the overall SBEM targets (AD L2 2026, Table 3.1 notes).

FBS 2026 vs Part L 2021 – limiting values

The table below compares the limiting (backstop) U-values under the FBS with those from the Part L 2021 edition. Most values are unchanged, but the overall compliance framework is significantly more demanding because the notional building specifications have been tightened substantially.

Thermal bridging

Thermal bridges – junctions between building elements where insulation is not continuous – must be reasonably limited in both new and existing buildings. AD L2 2026 (paragraphs 3.9–3.14) requires insulation to be reasonably continuous across newly built elements, with particular attention to party walls and junctions that affect Part E (sound insulation) compliance.

Two approaches to accounting for thermal bridging are permitted:

• Option (a) – calculated values: construction joint details calculated by a person with suitable expertise, following BRE's BR 497 methodology. A process flow sequence must be provided to the building control body (AD L2 2026, para 3.11).
• Option (b) – default values: generic linear thermal bridge values from BRE Information Paper 1/06, increased by 0.04 W/(m·K) or 50%, whichever is greater. This is the conservative default where bespoke calculations have not been carried out (AD L2 2026, para 3.12).

In practice, relying on option (b) defaults adds a significant penalty to the SBEM calculation, making compliance harder. Most projects seeking to meet the FBS targets cost-effectively will need bespoke psi-value calculations for key junctions.

Air permeability

Air permeability – the rate of air leakage through the building envelope – is a critical factor in the energy calculation. Under AD L2 2026, the limiting air permeability is the worst allowable performance, while the design and assessed values feed directly into the BPER and BER calculations (AD L2 2026, Section 3).

Pressure testing requirements

All non-domestic buildings must be pressure tested (including extensions treated as new buildings), following CIBSE's TM23 Testing Buildings for Air Leakage. The measured air permeability is used to calculate the as-built BPER and BER – if the tested value is worse than the design assumption, the building may fail to meet its targets (AD L2 2026, Section 6).

Limited exemptions apply:

• Buildings with less than 500 m² total useful floor area – provided the air permeability used for calculations is taken as 15 m³/(h·m²) at 50 Pa, a substantial penalty
• Factory-made modular buildings meeting specific criteria (floor area less than 500 m², planned service life over 2 years but use at one location under 2 years, standard module assembly only)

Pressure testing equipment must be calibrated at least every 24 months by a UKAS-accredited facility. Results of all tests – including failures – must be reported to the building control body (AD L2 2026, para 6).

Fabric standards for existing buildings

When thermal elements in existing buildings are renovated or replaced, they must meet the standards in Table 3.2 of AD L2 2026. Where more than 50% of an element's surface is renovated, or the works constitute a major renovation (over 25% of the building envelope), the entire element should be improved (AD L2 2026, Section 10).

The threshold values in column (a) trigger the requirement to upgrade. Where achieving the improved values in column (b) is not technically feasible or would not achieve a simple payback of 15 years or less, the element should be upgraded to the lowest feasible U-value within that payback period. As a general rule, no upgraded element should have a U-value greater than 0.7 W/m²K (AD L2 2026, Table 3.2 notes).

Practical implications

The tighter limiting values under the FBS mean that specification errors or value engineering that erodes fabric performance carry greater risk. Key practical considerations include:

• Insulation thickness: achieving 0.16 W/m²K for roofs and 0.18 W/m²K for floors requires careful selection of insulation products and thicknesses, particularly where space is constrained
• Junction detailing: thermal bridging at junctions can significantly erode the performance of otherwise well-insulated elements – bespoke psi-value calculations are strongly recommended
• Window specification: while the limiting value remains 1.6 W/m²K, the notional building assumes 1.2 W/m²K – specifying close to the limit will require compensation elsewhere
• Airtightness strategy: achieving 5.0 m³/(h·m²) or better requires an integrated airtightness strategy from the design stage, with clear responsibility for sealing at all penetrations and junctions