SAP & Part L Knowledge Base

SAP (Standard Assessment Procedure) is the UK Government's official methodology for assessing and comparing the energy and environmental performance of dwellings. First introduced in 1993, SAP is now in version SAP 10.2 (2022), which underpins the Part L 2021 Building Regulations for England.

How SAP Works

SAP calculates a dwelling's energy performance by modelling:

• Building Fabric Heat Loss — U-values of walls, roofs, floors, windows, and doors multiplied by their areas, plus thermal bridging (y-value × total envelope area)
• Ventilation Heat Loss — Air infiltration rate (measured or estimated) multiplied by volume and specific heat capacity of air (0.33 W/m³K)
• Solar and Internal Gains — Solar radiation through glazing (by orientation and overshading), plus metabolic, lighting, and appliance gains
• Space Heating Demand — Net heating requirement after subtracting gains from losses, using monthly degree-day method
• Hot Water Energy — Daily hot water demand based on occupancy (related to floor area), distribution losses, and cylinder losses
• Heating System Efficiency — Seasonal efficiency of boiler/heat pump using PCDB (Product Characteristics Database) or SAP Table 4a/4b values
• Primary Energy and CO₂ Emissions — Fuel-specific emission factors and primary energy factors applied to delivered energy

SAP Rating Scale

The SAP rating is an energy cost index from 1 to 100+ (higher is better). A SAP rating of 100 represents zero energy cost. Ratings above 100 are possible with renewables (net energy exporters). The Environmental Impact Rating (EIR) follows the same scale but is based on CO₂ emissions rather than cost.

Part L of the Building Regulations for England covers Conservation of Fuel and Power. It sets minimum energy performance standards for both new buildings and works to existing buildings.

Part L Versions and Applicability

Part L Compliance Routes for Extensions

For domestic extensions in England, two compliance routes exist:

1. 1. Average U-Value Method — Calculate the area-weighted average U-value of all new thermal elements. Compare against the notional average using Part L limiting values. Suitable for most standard extensions.
2. 2. Whole House Method — Required when extensions exceed 50m², have >25% glazing ratio, or fail the Average U-value test. Full SAP calculation comparing DER (Dwelling Emission Rate) vs TER (Target Emission Rate) for the entire dwelling.

U-value measures how effective a building element is as an insulator. Lower values mean better insulation.

Part L Extension Limiting U-Values

All values in W/m²K. Part L 2021 values apply from 15 June 2022 in England.

SAP 10.2 Notional Dwelling U-Values (New Build)

The Average U-Value Method is the primary compliance route for domestic extensions under Part L1B.

Formula

Where U_i is the U-value of each element and A_i is its area in m². The proposed U_aw must not exceed the notional U_aw.

Worked Example

The Whole House Method is required when the Average U-value method fails or the extension is complex.

When Is It Required?

• Extension floor area exceeds 50m²
• Glazing ratio exceeds 25% of extension floor area
• Average U-value method fails (proposed U_aw > notional U_aw)
• Conservatory exemption not met (thermally connected, >30m², or has fixed heating)

Compliance Criteria

The Whole House method requires a full SAP calculation of the entire dwelling (existing + extension). The Dwelling Emission Rate (DER) must not exceed the Target Emission Rate (TER):

This must be assessed by a qualified SAP assessor using accredited software (Elmhurst, STROMA, Bre Dem, etc.).

Thermal bridges are localised areas of the building envelope where heat escapes more rapidly. They account for 20-30% of total fabric heat loss in well-insulated modern buildings.

Types of Thermal Bridge

• Repeating — Regularly spaced elements within a construction (wall ties, mortar joints, timber studs). Accounted for within the U-value calculation.
• Non-repeating (linear) — Junctions between building elements (wall-floor, wall-roof, around windows). Characterised by psi-values (ψ) and calculated separately.
• Point — Discrete penetrations (fixings, brackets, beams). Usually minor but relevant for cladding systems.

Calculating Total Thermal Bridge Heat Loss

Where ψ_j = psi-value of junction j (W/mK), L_j = length of junction j (m), and A_total = total exposed surface area (m²).

Standard psi-values from SAP Table K1 and BRE accredited details. All values in W/mK.

Default = SAP Table K1. Accredited = BRE published. RCD = Recognised Construction Details (brick/block masonry only).

Condensation risk assessment evaluates the likelihood of surface condensation and mould growth at thermal bridge junctions. Assessed using the f-factor (fRsi).

f-Factor Risk Thresholds

Calculation

Where T_si = internal surface temperature at coldest point, T_e = external temperature (-5°C standard), T_i = internal temperature (20°C standard). Standard conditions per BR 497 / IP 1/06.

A Room in Roof requires special SAP calculations to determine the equivalent storey height, heat loss area, and volume. SAP 10.2 Table S6 defines three construction scenarios.

Three Scenarios

Equivalent Storey Height

Where V_roof = calculated volume of the room in roof space and A_floor = floor area at the room in roof level.