EPC & Energy Assessment Glossary
Plain English definitions for every term you'll encounter during an EPC assessment, SAP calculation, or MEES compliance review.
Air Permeability
The rate at which air leaks through the building envelope, measured in m³/h/m² at 50 Pascals. The lower the figure, the more airtight the building. Mandatory Part L compliance test for all new builds since 2022.
Air Tightness Testing →Air Source Heat Pump (ASHP)
A renewable heating system that extracts heat from outside air to heat a building. Typically improves EPC ratings by 1–2 bands when replacing a gas boiler.
ATTMA
Air Tightness Testing and Measurement Association. The professional body that accredits engineers to carry out air permeability testing. Level 1 covers domestic; Level 2 covers commercial.
Book an air test →Approved Document L
The part of UK Building Regulations covering energy efficiency. Divided into L1A (new dwellings), L1B (existing dwellings), L2A (new non-domestic), L2B (existing non-domestic).
Band (EPC)
The letter rating given to a property after EPC assessment, from A (most efficient, 92–100) to G (least efficient, 1–20).
BREEAM
Building Research Establishment Environmental Assessment Method. A sustainability framework for commercial buildings, broader in scope than a commercial EPC.
BRUKL Report
Building Regulations UK Part L report. A compliance document produced alongside a commercial On-Construction EPC, demonstrating the building meets Part L2A targets.
Cavity Wall Insulation
Insulation material injected into the cavity between the inner and outer leaves of a wall. One of the most cost-effective EPC improvements for properties built after 1920.
CO₂ Emissions
The carbon dioxide output of a property, measured in tonnes per year. Displayed on EPC certificates as both current and potential figures.
DEA
Domestic Energy Assessor. The qualified professional who carries out RdSAP assessments on existing homes. Must be accredited by a body such as Elmhurst Energy.
Book a DEA visit →Default Value
When an assessor cannot confirm specific data on site, RdSAP software applies a conservative default value. Confirmed data always produces a more accurate result.
DER
Dwelling Emission Rate. The calculated CO₂ emissions for a new dwelling in kgCO₂/m²/year. Must be equal to or lower than the Target Emission Rate (TER) to pass Part L.
DFEE
Dwelling Fabric Energy Efficiency. A metric measuring how efficiently the building fabric retains heat. Must be equal to or lower than the TFEE to pass Part L 2021.
DSM
Dynamic Simulation Modelling. The calculation method for the most complex commercial buildings (Level 5 EPCs), creating a full 3D thermal model.
Elmhurst Energy
The UK's largest government-approved EPC accreditation scheme, responsible for over 50% of all EPCs lodged in England and Wales. All Avorria assessors are Elmhurst-accredited.
EPC
Energy Performance Certificate. A legally required document showing a property's energy efficiency rating from A to G, valid for 10 years.
EPC Lookup →EPC Register
The official government database of all lodged EPCs in England and Wales, accessible at epc.opendatacommunities.org.
Search EPCs →Fabric First
A design approach prioritising the energy efficiency of the building's physical structure before adding renewable technology. Generally the most cost-effective strategy.
Future Homes Standard
A UK government policy requiring new homes to produce 75–80% less carbon than current standards. Will mandate heat pumps or equivalent low-carbon heating in new builds.
G-value
The solar heat gain coefficient of a window, indicating how much solar energy passes through the glass. Used in SAP calculations for new builds.
Heat Pump
A highly efficient heating system that moves heat from one place to another rather than generating it, achieving efficiencies of 250–350%. Significantly improves EPC ratings.
iSBEM
The free government-provided software implementation of SBEM, used by NDEAs for Level 3 and some Level 4 commercial EPC calculations.
Loft Insulation
Insulation installed at joist or rafter level in a roof space. One of the cheapest and most impactful EPC improvements — 270mm mineral wool can add 3–8 SAP points.
MEES
Minimum Energy Efficiency Standards. Regulations requiring rented properties in England and Wales to meet minimum EPC ratings. Current domestic minimum: Band E. Commercial: rising to C by April 2027, B by April 2030.
For Landlords →MVHR
Mechanical Ventilation with Heat Recovery. A whole-house ventilation system that recovers up to 90% of heat from extracted air. Required in very airtight buildings.
NDEA
Non-Domestic Energy Assessor. The qualified professional who carries out commercial EPC assessments using SBEM or DSM methodology, qualified at Level 3, 4, or 5.
OC-EPC
On-Construction Energy Performance Certificate. The EPC issued for a new build property once complete, based on the as-built SAP calculation and air tightness test result.
New Build EPC & SAP →OCDEA
On-Construction Domestic Energy Assessor. The assessor qualified to produce SAP calculations and EPCs for new dwellings and conversions.
Part L
The section of UK Building Regulations covering conservation of fuel and power. See also Approved Document L.
PEA
Predicted Energy Assessment. A document produced from the design stage SAP calculation, showing the predicted energy efficiency before construction. Required by Building Control.
New Build EPC & SAP →RdSAP
Reduced Data Standard Assessment Procedure. The calculation methodology used for EPC assessments of existing domestic properties.
RdSAP 10
The latest version of the RdSAP methodology, launched in 2025. More detailed data collection with improved accuracy for heat pumps, solar, and fabric performance.
What is RdSAP 10? →SAP
Standard Assessment Procedure. The UK government's approved method for calculating the energy performance of new and converted dwellings. SAP score: 1–100+.
New Build EPC & SAP →SAP Score
A numerical score from 1 (extremely poor) to 100+ (net zero or better) representing a property's energy cost efficiency. A = 92+, G = 1–20.
SBEM
Simplified Building Energy Model. The calculation methodology used for Level 3 and Level 4 commercial EPC assessments.
Solid Wall Insulation
Insulation applied to solid walls (no cavity), either externally (EWI) or internally (IWI). More costly than cavity wall insulation but high impact for older properties.
TER
Target Emission Rate. The maximum CO₂ emissions allowed for a new dwelling under Part L. The DER must be equal to or lower than the TER to pass compliance.
TFEE
Target Fabric Energy Efficiency. The maximum allowed fabric energy demand for a new dwelling. The DFEE must be equal to or lower than the TFEE.
Thermal Bridge
A structural element that conducts heat between the warm interior and cold exterior more readily than surrounding materials.
U-value
A measure of how effectively a building element insulates, in W/m²K. Lower U-values = better insulation. Key input in SAP and SBEM calculations.
Valid EPC
An EPC lodged on the national register less than 10 years ago. A property being sold or rented must have a valid EPC to comply with the law.
Zero Carbon Home
A new dwelling that produces no net CO₂ emissions annually. Requires very high insulation, exceptional airtightness, renewable generation, and low-carbon heating. SAP score 100+.