EPC Jargon Buster: Every Term Explained

Energy Performance Certificates are full of technical language. This alphabetical glossary explains every term in plain English so you know exactly what your EPC is telling you.

5 min read

A

Air permeability: A measure of how airtight a building is, expressed in cubic metres of air leakage per hour per square metre of building envelope at a pressure of 50 Pascals. Lower numbers mean a more airtight (and more energy-efficient) home. New builds are pressure-tested; older properties are estimated.

B

Boiler efficiency: The percentage of fuel energy that a boiler converts into useful heat. A modern condensing boiler typically achieves 89-94% efficiency. Older non-condensing boilers may be as low as 60-70%. This figure directly affects the EPC rating.

C

Carbon dioxide emissions: The estimated annual CO2 emissions from the property in tonnes per year, based on its energy use for heating, hot water, lighting and ventilation. This figure drives the Environmental Impact Rating on the EPC.
Cavity wall insulation: Insulation injected or fitted into the gap between the inner and outer leaves of a cavity wall. Most homes built after 1920 have cavity walls. Filling the cavity reduces heat loss by up to 35% and is one of the most cost-effective upgrades listed on an EPC.
Community heating: A district or communal heating system where heat is generated centrally and distributed to multiple homes. Common in blocks of flats. The EPC assesses the efficiency of the overall system, not individual boilers.
Current energy efficiency rating: The property's energy efficiency right now, expressed as a score from 1 (worst) to 100 (best) and a letter band from G to A. Based on the SAP calculation using the property's actual fabric, heating system and controls.
Current environmental impact rating: The property's current CO2 emissions rating on a 1-100 scale, where higher is better (less CO2). Shown alongside the energy efficiency rating on the EPC.

D

Domestic Energy Assessor (DEA): A qualified professional who carries out EPC assessments on residential properties. DEAs must be accredited by an approved scheme and follow the Reduced Data SAP (RdSAP) methodology.
Double glazing: Windows with two panes of glass separated by a sealed gap (usually 16mm) filled with air or argon gas. Reduces heat loss through windows by about 50% compared to single glazing. Triple glazing offers a further improvement.
Dwelling type: The classification of the property: detached house, semi-detached house, mid-terrace, end-terrace, bungalow, flat (ground floor, mid-floor, top floor), maisonette or park home. The dwelling type affects expected heat loss patterns.

E

Energy efficiency band: The letter grade (A to G) on the EPC. Band A (92-100) is the most efficient. Band D (55-68) is the average for existing UK housing stock. Band G (1-20) is the least efficient.
Energy Performance Certificate (EPC): A legal document required whenever a property is built, sold or rented in England and Wales. It shows the property's energy efficiency rating (A-G), estimated energy costs, and recommended improvements. Valid for 10 years.
Environmental Impact Rating (EIR): A separate A-G rating on the EPC that reflects the property's CO2 emissions rather than energy cost. A home heated by a heat pump may score better on the EIR than one heated by gas, even if the energy costs are similar.
Estimated energy costs: The projected annual cost of heating, hot water and lighting the property, shown on the EPC in pounds per year for each category and as a total. Based on standardised occupancy assumptions, not the actual occupant's behaviour.

F

Floor area: The total internal floor area of the property in square metres, measured in accordance with the RdSAP conventions. This is the gross internal area including internal walls and stairwells.

H

Heat pump: A device that extracts heat from a low-temperature source (air, ground or water) and delivers it at a higher temperature for space heating or hot water. Air source heat pumps (ASHPs) and ground source heat pumps (GSHPs) are the most common types. They typically deliver 2.5-4 units of heat for every unit of electricity consumed.
Hot water system: The system used to heat domestic hot water. May be provided by the main boiler (combi or system), an immersion heater, a heat pump, a solar thermal system, or a combination. Efficiency of the hot water system contributes to the overall EPC score.

I

Indicative cost: The estimated cost of implementing a recommended improvement, shown on the EPC recommendations page. These are broad estimates and actual costs may vary significantly based on the property and local labour rates.
Insulation thickness: The depth of insulation material in walls, the roof or the floor, measured in millimetres. The EPC assessor records insulation thickness where visible or uses default assumptions for the age and construction type of the property.

L

Lighting: The EPC records the proportion of fixed lighting outlets that use low-energy bulbs (LED or CFL). Upgrading all lighting to LED is typically listed as a low-cost recommendation and can improve the EPC score by several points.
Loft insulation: Insulation laid between and over the ceiling joists in the loft space. Current Building Regulations recommend at least 270mm of mineral wool or equivalent. Loft insulation is one of the cheapest and most effective energy upgrades.
Low and zero carbon (LZC) technologies: Energy generation technologies that produce little or no CO2 at point of use, including solar photovoltaic panels, solar thermal, heat pumps, wind turbines and biomass boilers. The presence of LZC technologies significantly boosts the EPC rating.

M

Main heating system: The primary system used to heat the property. Typically a gas boiler in the UK, but may also be an oil boiler, electric storage heaters, a heat pump, or a biomass system. The efficiency and fuel type of the main heating system are the biggest single factors in the EPC calculation.
MEES (Minimum Energy Efficiency Standards): Regulations that set the minimum EPC rating for rented properties. Since April 2020, landlords in England and Wales cannot let a property with an EPC below E (unless they have a valid exemption). The government has proposed raising this to C.

P

Photovoltaic panels (Solar PV): Panels that convert sunlight into electricity. Solar PV systems can significantly reduce a property's estimated energy costs and CO2 emissions, boosting both the energy efficiency and environmental impact ratings on the EPC.
Potential energy efficiency rating: The estimated energy efficiency rating the property could achieve if all the recommended improvements on the EPC were carried out. Shown alongside the current rating to illustrate the scope for improvement.
Primary energy: The total energy consumed including generation, transmission and distribution losses. Electricity has a higher primary energy factor than gas because of power station inefficiencies. This is why electric heating often scores poorly on EPCs despite being 100% efficient at point of use.

R

RdSAP (Reduced Data Standard Assessment Procedure): The simplified version of SAP used for existing dwellings. Where the full SAP methodology requires detailed construction data (used for new builds), RdSAP allows assessors to infer values from the property's age, type and visual inspection.
Recommendations report: The section of the EPC that lists suggested improvements, each with an estimated cost range, typical annual saving and the impact on the EPC band. Improvements are ranked by cost-effectiveness.

S

SAP (Standard Assessment Procedure): The government's methodology for calculating the energy performance of dwellings. SAP produces a score from 1 to over 100 based on the property's energy costs per square metre. It considers insulation, heating systems, ventilation, fuel types and renewable energy.
Secondary heating: Any additional heating system beyond the main one, such as a wood-burning stove, electric fire or portable heater. The EPC records the type but secondary heating typically has a limited impact on the overall rating.
Smart heating controls: Programmable or internet-connected thermostats and thermostatic radiator valves (TRVs) that allow more precise control of heating. Upgrading to smart controls can improve the EPC score and is often listed as a low-cost recommendation.
Solar thermal: Panels or tubes on the roof that use sunlight to heat water directly. Different from solar PV (which generates electricity). A solar thermal system can provide a significant proportion of a household's hot water in summer months.
Solid wall: A wall made from a single thickness of brick, stone or concrete with no cavity. Common in properties built before 1920. Solid walls lose heat faster than cavity walls and are more expensive to insulate (requiring internal or external wall insulation).
Solid wall insulation: Insulation applied to the inside (internal) or outside (external) of a solid wall. Internal insulation reduces room size slightly; external insulation changes the appearance. Both options can dramatically improve a property's EPC rating but cost significantly more than cavity wall insulation.

T

Thermal bridging: Points in the building fabric where heat transfers more easily than the surrounding area, typically at junctions between walls, floors and roofs, and around window and door openings. Thermal bridges reduce overall insulation performance and can cause condensation.
Thermal mass: The ability of a material to absorb and store heat. High thermal mass materials (brick, stone, concrete) can help stabilise internal temperatures but are slower to heat up. The SAP methodology accounts for thermal mass when calculating heating demand.
Top-up insulation: Additional insulation added on top of existing but insufficient insulation, most commonly in the loft. If a property has less than 270mm of loft insulation, topping up is one of the cheapest improvements available.

U

U-value: A measure of how much heat passes through a building element (wall, roof, floor or window) per square metre per degree of temperature difference, expressed in W/m2K. Lower U-values mean better insulation. A modern external wall should achieve a U-value of around 0.18-0.30 W/m2K.
Underfloor insulation: Insulation installed beneath the ground floor, either between floor joists (for suspended timber floors) or on top of a concrete slab. Reduces heat loss through the floor and can improve comfort as well as the EPC rating.

V

Ventilation: The movement of air into and out of the building. The EPC considers the ventilation type: natural (windows and trickle vents), mechanical extract, or mechanical ventilation with heat recovery (MVHR). MVHR systems score best because they recover heat from outgoing air.

W

Wall construction type: The construction method of the external walls: solid brick, cavity (unfilled or filled), timber frame, system built, or stone. The assessor identifies the type from the property's age, wall thickness and visual clues. This determines the assumed U-value used in the EPC calculation.
Window energy rating (WER): A rating scheme for windows that combines U-value, solar heat gain and air leakage into a single A++ to G scale. Higher-rated windows let in more useful solar heat while losing less warmth. The EPC assessor records the glazing type and age rather than the specific WER.

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