Your streamlined solution for Part L Extension Compliance, ensuring your building projects meet regulatory standards with automated Average U-value calculations and clear compliance reports.

SAPPER is a free AI-powered SAP (Standard Assessment Procedure) compliance calculator for UK Building Regulations Part L. It automates data extraction from architectural PDF drawings, calculates thermal bridges, checks Part L compliance using Average U-value or Whole House methods, and generates professional compliance reports. Built by CoreLogic Ai Ltd.

What are the Part L 2021 U-value limits for extensions?

Part L 2021 (England, from June 2022) extension limiting U-values: External walls 0.18 W/m²K, Roofs 0.15 W/m²K, Floors 0.18 W/m²K, Windows 1.4 W/m²K, Doors 1.4 W/m²K, Rooflights 2.2 W/m²K. These are used as notional values in the Average U-value compliance calculation.

How does AI extract SAP data from drawings?

SAPPER uses a multi-pass AI extraction system: Pass 1 classifies drawing types (plans, elevations, sections) and detects scales. Pass 2 extracts dimensional data (floor areas, perimeters, storey heights) from annotations. Pass 3 analyses every elevation for windows, doors, rooflights with orientation detection. Results are validated with sanity checks and achieve 85-95% accuracy.

What are thermal bridge psi-values?

Psi-values (ψ-values) measure additional heat loss at building junctions in W/mK (Watts per metre Kelvin). Common UK junctions include E2 (lintels, accredited ψ=0.20), E3 (sills, ψ=0.04), E4 (jambs, ψ=0.04), E5 (ground floor, ψ=0.12), E10/E11 (eaves, ψ=0.04-0.06). RCD (Recognised Construction Details) values are 20-40% lower for brick/block masonry constructions.

What is the condensation risk f-factor?

The f-factor (fRsi) is the surface temperature factor used to assess condensation and mould growth risk at thermal bridges per BR 497. f ≥ 0.75 = low risk (acceptable), f 0.65-0.75 = medium risk (review needed), f < 0.65 = high risk (mould growth likely, design modification required). Calculated as fRsi = (Tsi - Te) / (Ti - Te).

Is SAPPER free to use?

Yes. SAPPER is completely free with no feature limitations, no credit card required, and no hidden costs. Part L calculator, AI extraction, thermal bridging, room in roof calculator, and PDF reports are all included at no charge.

Thermal Bridges Breakdown

Detailed junction lengths and heat loss calculations

Total Junctions

Total Heat Loss

0.000

W/K

Y-value (estimated)

NaN

Calculated by Elmhurst

Thermal Bridge Junctions

Junction	Type	Length (m)	Psi-value (W/mK)	Heat Loss (W/K)	f-factor	Actions
TOTAL				0.000 W/K

💡 How to use these values:

Go to Elmhurst → Thermal Bridging
Set Method: "Calculate Bridges"
Add each junction with its length and psi-value
Elmhurst will calculate the final Y-value automatically

High heat loss (>1 W/K)

Medium heat loss (0.5-1 W/K)

📘 Junction Type Reference Guide

Common thermal bridge junctions in UK residential construction with accredited psi-values.

E2Lintels

ψ 0.03-0.10

Above windows and doors

Length: Opening width + 300mm bearing

Steel/concrete lintels with perforated base plate. RCD: 0.03-0.05, Accredited: 0.10

E3Sills

ψ 0.04-0.10

Below windows, intermediate floor to external wall

Length: Opening width

Tile/stone/precast sills. RCD optimized: 0.04-0.06

E5Jambs

ψ 0.04-0.08

Sides of windows and doors

Length: Opening height × 2 sides

Cavity closer required. RCD: 0.03-0.05, Accredited: 0.05

E6/E8External Corners

ψ -0.15

Building corners (convex)

Length: Storey height × corner count

NEGATIVE psi = thermal benefit. Detached: 4 corners, Semi: 2 corners

E21Eaves (Ceiling Insulation)

ψ 0.06

Roof to wall junction, insulation at ceiling

Length: Roof perimeter

For cold pitched roofs with loft insulation

E22Eaves (Rafter Insulation)

ψ 0.04

Roof to wall junction, insulation at rafter

Length: Roof perimeter

For warm roofs or rooms in roof

E23Party Wall Eaves

ψ 0.04

Roof to party wall junction

Length: Party wall length at eaves

Only for semi-detached and terraced properties

P1Ground Floor Perimeter

ψ 0.08-0.16

Floor to wall junction

Length: Ground floor perimeter

Varies by floor insulation. RCD: 0.08-0.12, Default: 0.15

P8Intermediate Floor

ψ 0.07-0.13

First floor to external wall

Length: First floor perimeter

Only for multi-storey. RCD: 0.07, Accredited: 0.13

R1Roof Verge (Gable)

ψ 0.05-0.08

Sloping roof edge at gable

Length: Roof slope length × gable count

Typically 2 gables for pitched roof. RCD: 0.05, Default: 0.08

R6Flat Roof Parapet

ψ 0.25-0.40

Flat roof to upstand parapet

Length: Parapet length

High heat loss area. Requires good insulation continuity

🎯 RCD Optimization

RCD (Recognised Construction Details) provide BRE-certified lower psi-values for standard brick/block constructions. Typical improvements: 20-30% reduction in thermal bridging heat loss.

BRE Accredited Psi-Value Database