Decarbonisation Glossary

Air changes per hour (ACH)

Air is continuously exchanged between the inside and outside of a building through mechanical systems, passive ventilation, and infiltration across the . The air change rate, measured in air changes per hour (ACH), quantifies this exchange and is used in ventilation design and heat loss calculations.

Airtightness

Airtightness measures the amount of outdoor air that infiltrates a building. Low-energy buildings require high levels of airtightness to minimize heat loss. It is typically measured in cubic metres per hour per square metre (mat a pressure difference of 50 Pascals, or alternatively, in air changes per hour (ACH) through the external envelope. In both cases, a lower value indicates better airtightness.

Asset register

A plant asset register is a structured system that records all fixed assets - such as plant and equipment - owned by an organisation. It captures key information including location, condition, manufacturer, service history, and other relevant details, supporting effective asset management and maintenance planning.

Biofuels

Biofuels are renewable fuels derived from organic materials such as plants, agricultural waste, or algae. They are used as alternatives to fossil fuels for producing energy, particularly in transportation and heating. Examples include bioethanol, biodiesel and biogas.

Biomass boiler

Biomass boilers use solid, renewable fuels—such as logs, pellets, or wood chips—to generate central heating and hot water. Although burning biomass does release carbon dioxide, its environmental impact is significantly lower than that of fossil fuels like gas or oil. However, if not operated correctly, biomass combustion can emit other harmful pollutants, including nitrogen oxides and particulate matter.

The Boiler Upgrade Scheme is a government initiative that provides grants or incentives to help homeowners and businesses replace old, inefficient boilers with low-carbon alternatives, such as heat pumps, to reduce carbon emissions and improve energy efficiency.

Building fabric

The term building fabric refers to the structural materials, insulation, cladding, finishes, and other components that enclose a building and separate the interior from the external environment. In most buildings, this includes key elements such as the roof, external walls, windows, and doors.

Building management system (BMS)

A Building Management System (BMS), also known as a Building Automation System (BAS), is a computer-based system that monitors and controls a building’s mechanical and electrical systems—such as heating, ventilation, and air conditioning, lighting, security, and fire protection—to optimise overall performance and energy efficiency.

Building services

Building services refer to the systems and infrastructure within a building that support its safe, comfortable, and efficient operation. These include heating, ventilation, plumbing, electrical systems, as well as security, alarm, and fire detection and protection systems.

Carbon conversion factors

Carbon conversion factors are numerical values used to calculate the amount of carbon dioxide (CO₂) emissions produced from a specific activity or amount of fuel consumed. They convert units like energy use, fuel quantity, or distance travelled into equivalent CO₂ emissions, helping to estimate and compare carbon footprints. An example would be converting energy (kWh) used to CO2 to understand the environmental impacts of energy consumed by a building.

Carbon emissions

Under net zero, carbon emissions are generally categorized into three types, often aligned with the Greenhouse Gas (GHG) Protocol scopes. These are defined as follows to help organisations measure and manage their carbon footprint:

• Scope 1: Direct emissions from sources owned or controlled by the organisation (e.g., fuel combustion, company vehicles).
• Scope 2: Indirect emissions from the generation of purchased electricity, heat, or steam used by the organisation.
• Scope 3: All other indirect emissions that occur in the organisation’s value chain, such as emissions from suppliers, business travel, waste disposal, and product use.

Carbon footprint

A carbon footprint is the total amount of greenhouse gas emissions, mainly carbon dioxide, generated directly or indirectly by an individual, organization, product, or activity, usually measured in tonnes of CO.

Cavity wall insulation

Cavity wall insulation is a method of improving a building’s energy efficiency by filling the gap (or cavity) between the inner and outer layers of a cavity wall with insulating material. This reduces heat loss through the walls, helping to maintain indoor temperatures, lower energy bills, and reduce carbon emissions.

Display energy certificate (DEC)

DECs rate the actual or operational energy performance of a building against established benchmarks and take into consideration the ways in which occupants use the building. The operational rating on a DEC illustrates how efficiently the building is using energy. Since July 2015 DECs need to be prominently displayed in buildings over 250m2 that are occupied by public authorities and frequently visited by the public. DECs for buildings between 250-1000m2 have a 10-year validity and DECs for building over 1000m2 have a validity of 1 year.

District heating network

A district heating network is a system that distributes heat generated in a central location—such as a power plant or a dedicated heating facility—to multiple buildings through a network of insulated underground pipes. It provides space heating and hot water efficiently to residential, commercial, or industrial users, often using renewable or waste heat sources. This avoids the need for individual boilers or electric heaters.

Embodied carbon

Embodied carbon is the total greenhouse gas emissions linked to all materials and construction processes throughout a building’s lifecycle before use. It includes emissions from raw material extraction, manufacturing, transport, construction, maintenance, and end-of-life disposal or recycling.

As buildings become more energy-efficient, embodied carbon forms a larger share of their overall emissions. Reducing it involves choosing low-carbon materials, reusing components, efficient design, and using transparent environmental data. Managing embodied carbon is key to meeting net zero targets in construction.

Energy consumption

Energy consumption refers to the total amount of energy used by a building, device, system, or process over a specific period. It includes all forms of energy such as electricity, gas, fuel, and heat required to perform activities like heating, cooling, lighting, and operating equipment.

The difference between regulated and unregulated energy consumption lies in the types of energy use considered under building regulations and assessments.

• Regulated Energy Consumption - Energy used for fixed building services that are controlled by building regulations. This includes heating, cooling, ventilation, hot water, lighting, and fixed appliances. Typically covered in energy performance assessments like EPCs or building codes.
• Unregulated Energy Consumption - Energy used for activities not controlled by building regulations. This includes plug-in appliances, office equipment, kitchen appliances, lifts, and occupant devices. Not usually accounted for in regulatory energy calculations but can be significant in total energy use.

Energy performance certificate (EPC)

An EPC is an official document that rates the energy efficiency of a building or property on a scale from A (most efficient) to G (least efficient). It provides information on energy use, typical costs, and recommendations for improving efficiency to reduce energy consumption and carbon emissions.

EPCs are required for both residential and non-residential buildings, but listed buildings are generally exempt from the requirement to have an EPC when they are sold or rented.

Energy use intensity (EUI)

Energy Use Intensity (EUI) is a metric that measures the energy consumption of a building relative to its size. It’s usually expressed as energy used per square metre (or square foot) per year (e.g., kWh/m²/year).

EUI helps compare the energy efficiency of different buildings, track performance over time, and identify opportunities for energy savings. Lower EUI values indicate better energy efficiency.

External wall insulation (EWI)

A system that involves fixing a layer of insulating material to the outside surface of a building’s external walls, then covering it with a protective finish such as render or cladding. The can be covered in a variety of materials eg. render, brick slips, or cladding.

G value

The g-value, or solar heat gain coefficient, is a measure of how much solar radiation passes through a window or glazed surface into a building. It represents the total solar energy transmitted, including both directly through the glass and the heat absorbed and re-radiated inside. It's a value between 0 and 1, where 1 means all the sun's heat passes through and 0 means none of it does. A lower g-value indicates better solar heat reduction and reduces cooling needs.

Geothermal

Heat that comes from the Earth’s internal energy. This natural heat can be harnessed for various uses, such as heating buildings, generating electricity, or hot water supply. Geothermal energy is a renewable, sustainable resource typically accessed by tapping into hot rocks or water reservoirs beneath the Earth's surface.

Glazing ratio

The proportion of a building’s external wall area that is made up of windows or glazed surfaces. It’s usually expressed as a percentage and helps assess natural light, solar gain, and thermal performance of a building’s façade. Also called window-to-wall ratio, it is a key variable in façade design affecting energy performance in buildings.

Heat emitter

A heat emitter is a device or system that releases heat into a space to maintain a comfortable indoor temperature. Common examples include radiators, underfloor heating pipes, fan coils, and convectors, which transfer heat from a heating source (like a boiler or heat pump) to the room.

Heat gain

Heat gain refers to the increase in indoor temperature caused by heat entering a building from external sources. This can come from sunlight (solar gain), warm outdoor air, electrical equipment, occupants, or lighting. Managing heat gain is important for maintaining comfort and reducing cooling energy needs.

Heat loss

Heat loss is the transfer of heat from the inside of a building to the outside environment, causing the indoor temperature to drop. It occurs through walls, windows, roofs, floors, doors, and ventilation. Minimizing heat loss is essential for improving energy efficiency and maintaining indoor comfort.

Heat pumps

Heat pumps are devices that transfer heat from one place to another, typically extracting heat from the air, ground, or water outside a building and moving it indoors to provide heating. They can also work in reverse to cool buildings by removing heat from inside. Heat pumps are energy-efficient alternatives to traditional heating systems because they move heat rather than generate it by burning fuel.

Common types of heat pump include:

• Air source heat pump (ASHP)
• Exhaust air heat pump (EAHP)
• Ground source heat pump (GSHP)
• Domestic hot water heat pump (DHW)

Refer to separate fact sheet on Technology & Innovation for more information on each type.

Indoor air quality (IAQ)

Indoor Air Quality refers to the condition of the air inside and surrounding buildings, particularly in relation to the health and comfort of occupants. Managing IAQ involves identifying and controlling indoor pollutants to reduce the risk of health issues. It is influenced by several factors, including the presence of contaminants, the effectiveness of ventilation, humidity levels, and indoor temperature.

Infiltration

Buildings are never completely airtight; they allow air to enter unintentionally—a process known as infiltration. The infiltration rate refers to the amount of outdoor air that leaks into a building through cracks, gaps, and other unintended openings, typically driven by wind pressure and the stack effect (the movement of air due to temperature differences).

Buildings with solid walls often have relatively low insulation values. As there is no cavity to fill, one option is to apply internal wall insulation (IWI) to the inner side of external walls to improve thermal performance, covered by plasterboard. A variety of materials can be used and therefore the thicknesses required to reach Building Regulations varies but will usually be between 40mm and 100mm.

Life cycle assessment (LCA)

A life cycle assessment is a systematic process that evaluates the environmental impacts of a product, building, or system throughout its entire life cycle—from raw material extraction, manufacturing, and construction, through use and maintenance, to disposal or recycling.

LCA considers various environmental factors such as energy use, carbon emissions, resource depletion, water consumption, and pollution to provide a comprehensive understanding of overall environmental performance.

Buildings of special architectural or historic interest with legal protection. Any works to a listed building, whether internal or external, that could affect its special architectural or historic character, generally require Listed Building Consent. This includes alterations, extensions, and even some repairs. It's a criminal offense to carry out unauthorized works to a listed building.

Mechanical ventilation with heat recovery (MVHR)

MVHR (Mechanical Ventilation with Heat Recovery) is a whole-building ventilation system that extracts stale air from a building and recovers its heat to warm incoming filtered, fresh air. It improves indoor air quality while reducing heat loss and energy consumption.

Net zero

Net zero refers to the balance between the amount of greenhouse gases emitted and the amount removed from the atmosphere. A building, organisation, or country reaches net zero when it produces no more emissions than it can offset or absorb through methods like renewable energy, carbon capture, or offsetting projects. Achieving net zero is essential for limiting climate change and meeting global climate targets.

Offsetting

Offsetting is the process of compensating for carbon emissions by contributing, usually financially, towards solutions that reduce or remove an equivalent amount of emissions elsewhere. Offsetting is often used by individuals, companies, or organisations to help achieve net-zero or carbon-neutral goals when direct emissions reductions are not fully possible.

Operation & maintenance manuals (O&Ms)

In construction and facility management, "O&M" stands for Operations and Maintenance. O&M Manuals form part of the usual handover documentation and usually include operation and maintenance instructions along with manufacturers literature, as built drawings and signed test and commissioning sheets.

Passive design uses layout, fabric and form to reduce or remove mechanical cooling, heating, ventilation and lighting demand. Passive design strategies are decided based on the climate of the place. Examples of passive design include optimise spatial planning and orientation to control solar gains and maximise daylighting, manipulating the building form and fabric to facilitate natural ventilation strategies and making use of thermal mass to help reduce peak internal temperatures.

Passivhaus

Passivhaus is a rigorous energy efficiency standard for buildings that aims to reduce energy use and create a high level of indoor comfort. Originating in Germany, the Passivhaus standard focuses on very high levels of insulation, Airtight construction, high-performance windows, Heat recovery ventilation, and minimised thermal bridging.

Buildings designed to the Passivhaus standard use up to 90% less heating and cooling energy than conventional buildings, making them ultra-efficient and environmentally friendly.

Peak Demand

Peak demand refers to the highest level of energy consumption (both electricity and gas) observed over a specific period—typically during times of maximum usage in a building, facility, or energy grid. It is a critical factor in designing and managing energy systems, as it determines the capacity required to meet the maximum load without failures or shortages. Reducing peak demand through energy efficiency or load shifting can lower energy costs and reduce strain on the power grid.

Photovoltaics (PVs)

Refer to Solar Panels

Plant

In construction and building services, "building plant" refers to the machinery and equipment installed within a building to perform essential functions such as power supply, heating, cooling, and ventilation. This equipment is typically housed in a dedicated area known as the plant room.

Power purchase agreement (PPA)

PPA is a contract between two parties, one which generates electricity (the generator) and one which is looking to purchase electricity (the consumer). While under contract, the consumer receives a significantly reduced electricity tariff, offsetting the costs from their utility provider, while the generator receives income from the sale of the generated electricity.

Renewable Heat Incentive (RHI)

The Renewable Heat Incentive is a government subsidy scheme that is designed to incentivise the installation of renewable heating measures, such as biomass boilers and ground source heat pumps. Under this scheme, households are paid for each unit of renewable heat that they generate. This significantly increases the payback of renewable heating measures.

Retrofit

Retrofit, in the context of buildings, refers to the process of upgrading or modifying existing structures with new technologies or features to improve their energy efficiency, performance, or functionality. This often involves installing more efficient heating systems, better insulation, and incorporating renewable energy sources. Retrofitting aims to make buildings more comfortable, sustainable, and cost-effective to operate.

Levels or approaches to retrofit vary based on the extent and depth of the upgrades:

• Shallow Retrofit – Involves minor, incremental improvements targeting specific issues eg. adding insulation to a single wall, replacing windows, or upgrading lighting.
• Deep Retrofit – Comprehensive upgrades covering multiple building elements and systems eg. upgrading insulation, installing energy-efficient heat source and ventilation systems, and improving air tightness simultaneously.
• Stepped Retrofit – Retrofit carried out in stages over time, spreading costs and disruption. Each phase targets different parts of the building or systems, gradually moving towards a deep retrofit.

Solar panels

The two main types of solar panels are:

• Photovoltaic (PV) Panels convert sunlight directly into electricity. These panels contain photovoltaic cells that generate a flow of electrons when exposed to light, producing electricity. This electricity can then be used to power homes, businesses, or feed back into the electricity grid.
• Solar thermal panels use solar energy to generate heat. These panels absorb sunlight and transfer the heat to a fluid (water or antifreeze solution), which then heats water in a storage tank to be used for space heating or hot water.

Space heating demand

Space heating demand refers to the amount of energy required to maintain comfortable indoor temperatures in a building by heating the interior spaces. It depends on several factors, including, building insulation and airtightness, outdoor temperature and climate, building size and layout, occupancy and internal heat gains, glazing and thermal bridging.

Thermal bridge

A thermal bridge, also called a cold bridge or heat bridge, is an area in a building’s envelope where heat flows more easily due to a break or weakness in the insulation, often caused by materials with high thermal conductivity (like metal or concrete). This leads to increased heat loss, cold spots, and potential condensation or mould issues, reducing the building’s overall energy efficiency.

Thermal envelope

The thermal envelope of a building refers to the continuous barrier—comprising walls, roof, floors, windows, and doors—that separates the heated or cooled interior from the outside environment. It is designed to control heat flow, air leakage, and moisture, helping to maintain comfortable indoor temperatures and improve energy efficiency.

Thermal performance

Thermal performance in buildings refers to how well a building maintains a comfortable internal temperature, minimizing energy use for heating and cooling. It's about the building's ability to resist heat transfer, keeping heat in during winter and out during summer. This is achieved through factors like insulation, air tightness, and the thermal properties of materials used in the building's construction.

U-value

U-values measure how well a building material insulates. A lower U-value indicates better insulation, meaning less heat is lost through the material. They are expressed in watts per square meter per degree Kelvin (W/m²K) and represent the rate of heat transfer through a material. 

Whole life carbon

Whole life carbon refers to the total greenhouse gas emissions associated with a building or product over its entire life cycle. This includes:

• Upfront carbon - emissions from material extraction, manufacturing, and construction processes.
• Operational carbon - emissions associated with energy used by a building in use over its life cycle.
• End-of-life carbon - emissions related to demolition, disposal, or recycling.

Whole life carbon assessment provides a comprehensive view of a building’s environmental impact, helping to identify opportunities for reducing carbon emissions throughout its lifespan.

WUFI

A complex computer software tool used for simulating heat and moisture transfer in building components and assemblies. It helps architects, engineers, and builders analyse how moisture moves through walls, roofs, and floors under various environmental conditions, aiding in the design of durable, energy-efficient, and healthy buildings.

Zonal heating system

A zonal heating system divides a building into separate areas or “zones,” each controlled independently for heating. This allows different rooms or sections to be heated to different temperatures based on their use and occupancy, improving comfort and energy efficiency.

Each zone typically has its own thermostat and control valves to regulate heat flow.