Retrofit Glossary

Our retrofit glossary is intended as a quick-reference guide to some key retrofit and energy efficiency terms. We’ll be adding to these over time, but in the meantime don’t hesitate to call if you stumble across something on our website or out in the wider world that you need explained (retrofit-related of course!).


The Energy Company Obligation (ECO) – The Energy Company Obligation (ECO) is a Government subsidy scheme paid for through a levy on UK households’ energy bills. It provides grants towards in the installation of energy efficiency measures, such as solid wall, loft and cavity insulation, with a focus on fuel poor and “hard to treat” homes.

The Feed-in Tariff (FiT) – The Feed-in Tariff (FiT) is a Government subsidy scheme that is designed to incentivise the installation of measures that generate renewable electricity, such as solar panels. Under ths scheme, households are paid for the each unit of electricity that they generate, as well as being able to sell excess electricity to the grid. This significantly increases the payback of renewable heating measures.

The Green Deal – The Green Deal is a Government scheme designed to help finance the installation of retrofit measures. It provides a form of loan which is paid back via the household’s energy bills.

Private Rented Sector Energy Efficiency Regulations – Under the Energy Act 2011, legislation was put in place that will require private landlords to bring their properties up to a minimum energy efficiency standard (an EPC rating E). This is expected to have a significant impact on fuel poverty.

The 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 ths scheme, households are paid for the each unit of renewable heat that they generate. This significantly increases the payback of renewable heating measures.


BREDEM – The BRE Domestic Energy Model (BREDEM) is a methodology for calculation of the energy use and fuel requirements of dwellings based on their characteristics. It shares some features with the SAP methodology, but allows users to adjust inputs which are fixed in SAP, making it better suited to certain analysis tasks. The current edition is BREDEM 2012.

CROHMOur CROHM assessments make use of unique property data in conducting analysis that accurately reflects the retrofit potential of stocks of domestic properties.The primary intention is to establish how an organisation’s specific objectives can most cost-effectively be achieved through retrofit – whether concerned with increasing SAP scores, reducing CO2 emissions, tackling fuel poverty or simply protecting asset values.

Green Deal Assessment – You must get a Green Deal Assessment (GDA) of your property to use the Green Deal.  The GDA involves a visit to your property, by a qualified green deal assessor, to talk to you about your property and your energy use.  After the visit you’ll get a document, called a Green Deal advice report, that helps you decide if you could benefit from Green Deal improvements. We can offer GDAs where required.

Home Energy Masterplan – Our Home Energy Masterplan assessments are the UK’s leading advice tool for those wishing to improve the energy performance of their home. Using a combination of comprehensive physical property and behavioural data, Masterplans provide analysis of the best measures available to suit your home, budget and lifestyle.

RdSAP – RdSAP is the method used to produce Energy Performance Certificates (EPCs). Rd stands for Reduced Data, and the method is designed to allow surveys to be completed more quickly and therefore more cheaply than a full SAP survey at some cost of accuracy. RdSAP underpins both Green Deal assessments and the minimum standards regulations. We can offer EPC assessments if required.

SAP – SAP (Standard Assessment Process) is a method for assessing the energy performance of houses using a standard methodology specified by the UK government. The current version of SAP is SAP 2009, and it calculates a ‘SAP rating’ as well as an estimate of energy bills and CO2 emissions associated with the estimated energy use. The SAP calculations are based on building dimensions, construction (and therefore energy performance) of building elements such as walls and windows, details of the heating and hot water systems and controls, and any installed renewable technologies including solar PV panels.



Cavity wall insulation – from around 1920, houses in the UK were typically built with a cavity in between two layers of brick wall. Heat loss through this type of wall can therefore be reduced using cavity wall insulation, which is installed by blowing insulating material into the gap.  Where applicable, it  is often one of most cost effective energy saving measures.

Loft insulation – loft insulation reduces the heat loss rising from the main heated areas in the house below.  The insulation is typically laid over the floor of the loft, with a recommended thickness of 270mm.  It  is often one of most cost effective energy saving measures, especially where little or none is currently present.

Internal wall insulation – older walls, especially solid walls, have relatively low insulation values. As there is no cavity to fill, one option is to apply internal wall insulation (IWI), which is then 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. Although internal wall insulation will reduce the room size it is worth remembering that it is only applied to external walls. IWI can be expensive, but can often attract a grant/subsidy.

External wall insulation – External wall insulation (EWI) is another solution for solid walls or hard to treat cavities. Although room sizes aren’t affected, the outside appearance of the building will be affected and roof eaves may need to be extended. The insulation is applied to the walls and then covered in a suitable material – usually render but brick slips, pebbledash or cladding is also available. EWI can be expensive, but can often attract a grant/subsidy.

Heating systems

Combi boiler – a combi boiler is both a water heater and a central heating boiler, combined (hence the name) within one compact unit. Therefore, no separate hot water cylinder is required.

Condensing boiler – condensing boilers use heat from exhaust gases that would normally be released into the atmosphere through the flue. As a result, they tend to be more efficient than non-condensing boilers.

Heating controls – effective heating controls let you take charge of when, where and at which temperature your heating is operating – including varying these across different zones or rooms in the house. Improving these, or simply just changing the way you use them, could save you money.

Thermostat – a thermostat is a component of a heating control system which ensures the temperature is maintained at a set level. Your bills will typically fall – all other things being equal – for each degree C you drop your thermostat’s temperature setting.

Thermostatic radiator valves – a thermostatic radiator valve (TRV) is a self-regulating valve fitted to hot water heating system radiator, to control the temperature of a room by changing the flow of hot water to the radiator.

Underfloor heating – there are two types of underfloor heating; “wet” and electric. Wet underfloor heating can be linked to a gas boiler or heat pump, and can offer savings over a conventional radiator system. Electric underfloor heating  is only really appropriate where there is no wall space for radiators, as it is generally very expensive to run.

Zoning – when the heating system of a house is set up or changed such that different areas are heating at different times and to different temperatures it is called zoning. The saving benefits can be large as you are essentially reducing the house volume every time you aren’t heating a room.


Air source heat pump – air source heat pumps (ASHPs) takes low-grade heat from the outside air and convert it into heat for use in providing central heating and hot water. They are best suited to “off-gas” properties. They are eligible for the renewable heat incentive (RHI, see above).

Biomass boiler – biomass boilers take solid fuel such as logs, wood chip, or pellets and burn them to provide central heating and hot water. Their main advantage is in their use of renewable fuel, which results in lower CO2 emissions. They are best suited to “off-gas” properties. They are eligible for the renewable heat incentive (RHI, see above).

Ground source heat pump – similar to their air source cousins, ground source heat pumps (GSHPs) takes heat from the ground and uses this to provide central heating and hot water. As, below a certain level, the ground stays at a fairly constant temperature all year round, these can achieve a higher Coefficient of Performance (COP) than air source heat pumps (which work better in milder temperatures). They are eligible for the renewable heat incentive (RHI, see above).

Solar thermal panels – solar water heating systems use solar panels, called collectors, fitted to your roof to convert heat from the sun into hot water. A boiler or immersion heater is often used as a back-up to heat the water further when the sunshine is insufficient to reach the temperature required. They are eligible for the renewable heat incentive (RHI, see above).


Cost-effect – Cost-effectiveness analysis (CEA) is a form of economic analysis that compares the relative costs and outcomes (effects) of two or more courses of action. The measure that delivers greater benefit (e.g. energy saving or carbon reduction) per unit cost would be described as the more cost effective.

Fuel Poverty – There are two well-known definitions of fuel poverty: “10% definition” and the “Hills definition”. The “10% definition” simply states that, a household is said to be in fuel poverty when they  have to spend more than 10% of their household income on fuel to keep their home in a ‘satisfactory’ condition. Under the “Hills’ definition”,  households are considered to be fuel poor if they have required fuel costs that are above average (the national median level) and were they to spend that amount on fuel, they would be left with a residual income below the official poverty line.

Off-grid – Off-grid is a term mainly used in terms of not being connected to the main or national gas/electrical grid. The higher costs associated with fuels such as coal and LPG means that these properties often have significant scope for cost effective retrofit.

Payback Period – The period of time required for an investment to recover its initial outlay in terms of income and/or savings generated.

Retrofit – Retrofitting refers to the addition of new technology or features to older systems. With regard to home energy retrofit, retrofit refers to the installation of  technologies used improve the performance of existing buildings in terms of their energy efficiency.

Smart meter – As the next generation of gas and electricity meters, smart meters have an accompanying in-home display to help residents keep track of the energy used in their homes, cutting out the need for meter readings.

Stock assessment – In the retrofit indsutry, stock assessment refers to analysis undertaken to look at the current energy performance of portfolios properties, and the options for improving it. Our CROHM assessments are en example of this kind of service.