The ability of a material to absorb radiant heat. It is expressed as a fraction of the energy absorbed by a body compared to that absorbed by a black body at the same temperature


The extent to which a material can absorb heat (thermal mass) is measured in terms of admittance

Aligning layers

Where two material layers contain the same material of the same thickness, this material is aligned across the two layers


Biomass is biological material derived from living, or recently living organisms. In the context of biomass for energy this often used to mean plant based material, but biomass can equally apply to both animal and vegetable derived material


Building Research Establishment


The BRE's preferred methodology for calculating the total fuel running costs for a dwelling (space heating, domestic hot water heating, lights, appliances and cooking)


The BRE's preferred methodology for calculating the space heating and domestic hot water heating costs for a property

Building fabric

What the building is made of - ie. its physical construction


Combined Heat and Power. A unit that simultaneously generates heat and electricity.

Cold bridging

Where a material of high thermal conductivity passes completely through a layer then the insulation in that area is said to be bridged.

Cold roof

Where thermal insulation is installed at the ceiling level and the remainder of the roof stays at a low temperature


The transfer of heat energy through a material without the molecules of the material changing their basic positions.


The transfer of heat energy through a material/body vai the movement of particles.


Combined primary storage units - a boiler with a thermal store all in the same casing


Directive Implementation Advisory Group. The UK advisory group for implementing the European Energy Performance of Buildings Directive

Dwelling Emission rate (DER)

The estimated annual CO2 emissions per square meter due to space heating, water heating, ventilation and internal lighting, minus any C02 emissions saved by the generation of electricity


The ability of a material to give off radiant heat. It is expressed as a fraction of the energy radiated by a body compared to that radiated by a black body at the same temperature


The Energy Performance of Buildings Directive


Home Condition Report

Heat Exchanger

Heat exchangers offer a large surface area contained in a small space, through which warm air passes. The heat from the warm air is transferred, via the surface, to the air which is to be warmed. The most common application is in advanced ventilation systems, where outgoing warm, stale air is used to pre-heat the fresh, cold air brought in from the outside.

Heat Pumps

Electrically powered heating and cooling systems. A heat pump operates on the same principle as a refrigerator, extracting heat from a low-temperature source (the outside air, a body of water or the earth), and concentrating it to a higher, more useful temperature. A pump using lkW of electricity can produce 3kW of heat. The process can be reversed, to draw heat out of a building in hot weather.


Home Information Pack


Heat loss parameter


The total additional heat loss due to all of the non repeating thermal bridges

Incidental gains

Sources of heat that contribute to the space heating requirements of a dwelling - but the heat is incidental to the main function or activity. Eg. Heat from occupants of the dwelling, electrical appliances and lighting.

Interstitial condensation

Condensation that occurs within one of the construction materials.

Inverted roof

A roof constructed with the damp proof membrane below the insulation so the insulation can be affected by water.

Linear thermal transmittance

This is the additional heat loss associated with a thermal bridge, and is the rate of heat flow per degree per unit length of the bridge. It is usually measured in W/mK.

Load compensator

A device, or feature within a device (such as a boiler energy manager), which adjusts the temperature of the water circulating through the heating system according to the temperature measured inside the building.


A measure of air leakage, the LSO is the air flow rate required to maintain a pressure of 50 pascals during a pressure test.

Multiple occupation

Where more than one occupant lives in the dwelling

Multipoint (gas water heater)

A type of instantaneous water heater which provides hot water to a number of outlets (such as sinks, baths and basins) rather than just one.


Mechanical ventilation with heat recovery - systems that extract stale air from kitchens and bathrooms and deliver fresh air to living rooms and bathrooms. The heat loss associated with the air changes is reduced by a heat recovery mechanism.


National Home Energy Rating


Nitrous Oxide - a bi-product of burning fossil fuels. NOx has a global warming potential 270 times that of carbon dioxide

Passive Solar Design

The use of the sun's energy for the heating and cooling of living spaces.

Primary heating system

The system that provides heat to most rooms in the dwelling. Where there are two systems and one also provides hot water, this system will be the primary system.


Photovoltaics - a system that uses cells to convert solar radiation into electricity


The transfer of heat energy by electromagnetic waves.


Reduced data SAP - the survey system that the UK Government has developed for Energy Performance Certificates in existing homes. It is a reduced data set from the SAP conventions as many SAP data items cannot be seen in an existing home.

Relative Humidity

The amount of water vapour in the air as a proportion of the maximum amount of water vapour the air can hold at a given temperature

Resistivity (Thermal resistivity)

A measure of the ability of a material to impede the flow of heat, typically in units of KT/W

Robust Construction Details

Method of building to reduce risks and potential problems that can arise as a result of building to higher energy efficiency standards (eg. such as increased condensation risk)


Thermal resistance, a measure of the opposition to heat transfer offered by a particular body.


A number between 1 and 100 based on the annual space and water heating and internal fixed lighting cost per square metre for the property


Seasonal efficiency of Domestic Boilers in the UK - the average annual efficiency achieved in typical domestic conditions

Solar Collector

Solar collector Panels that collect heat from the suns radiation

Specific loss

The specific loss is a measure of the heat loss (in watts) of the dwelling per degree centigrade difference in temperature between the inside and outside of the dwelling

Target Emission Rate (TER)

To meet one of the criterion for compliance with Part L1A, the dwelling C02 emission rate (DER) must be no greater than a target emission rate based on a notional dwelling of the same size and shape.

Thermal break

A layer of insulating material in the frame of a metal framed opening. This insulation significantly reduces the heat loss through the frame so the U-value is adjusted according to the thickness of insulation.

Thermal bridging

Where a material of relatively high thermal resistance has a material of lower thermal resistance crossing through it thus providing a path for heat loss (see also cold bridging). Eg steel wall ties through insulation.

Thermal conductivity

A measure of the rate at which heat is conducted through a particular material under specific conditions - it is measured in W/mK and often denoted k

Thermal efficiency

The efficiency of a boiler based on the ratio of heat absorbed to total heat input. This does not include heat loss from the boiler shell.

Thermal loss

Heat loss

Thermal mass parameter

The sum of the areas multiplied by the admittances of each element in the construction, divided by total floor area. This measures the extent to which a building can absorb heat

Thermal performance

The physical properties of a home which affect the rate of heat loss from it - including insulation, physical dimensions, ventilation, glazing, orientation etc.
Thermal resistance "A measure of the opposition to heat transfer offered by a particular component in the building element (units = m2K/W)"

Thermal store

"Basically a hot water store providing mains pressure instant hot water and acts as a buffer for the space heating. At least 70 Itrs of store volume must be available to act as a buffer to the space heating demand, if not then it is treated as a conventional boiler and hot water cylinder."

Transmittance 'g' factor

Amount of solar energy transmitted through windows. Varies depending on type of glazing

U values

A measure of the rate of heat transfer. Units = W/m2oC

Warm roof

Where thermal insulation is placed immediately beneath the waterproof covering so the roof space remains on the warm side of the insulation.

Weather compensator

"A device, or feature within a device (such as a boiler energy manager), which adjusts the temperature of the water circulating through the heating system according to the temperature measured outside the building. "