Target emission rate (TER)

The calculated target CO2 emission rate for new buildings other than dwellings, expressed in kgCO2/m2/year. The predicted emissions from the actual building design are known as the building emission rate (BER) and may not exceed the TER.

Terrestrial ecotoxicity

The impact of toxic substances emitted to terrestrial ecosystems.

Thermal break

An element of low thermal conductivity placed in an assembly to reduce or prevent the flow of thermal energy between conductive materials. A typical example would be that found in a metal window frame to reduce the conduction of heat from the inside to the outside.

Thermal bridge

A thermally conductive material which penetrates or bypasses an insulation system; such as a wall tie, metal fastener, concrete beam, slab or column. Thermal bridging lowers the overall thermal insulation of the structure by creating areas where heat loss is greater in one area than it is for another. The effect is to reduce the overall u-value of the construction element. The heat loss per unit length of thermal bridge is known as the Ψ-(psi) value and is measure in W/mK.

Thermal bypass

Heat transfer enabled by the uncontrolled air movement within and through walls. A recently identified example of thermal bypass occurs within cavity walls acting as separating walls (party walls) between adjoining houses or flats. Cavities in these instances are not normally insulated thus allowing warm air to enter the cavity and by means of convection to rise through the space and escape into an attic or through the roof covering. An excellent account by Mark Siddall of the nature and effects of thermal bypass was published in Summer 2009 edition of ‘Green Building Magazine’ (

Thermal (storage) capacity

The ability of the constituent materials in a building to store heat, for a given rise in temperature, measured in units of kWh/K for a whole building or in Wh/K.m2 to indicate the building's thermal capacity per unit floor area.

Thermal conductivity (K-value)

A measure of the rate at which heat is conducted through a particular material under specified conditions.

Thermal (heat) emitter

A material or device that emits heat. The term is commonly used with reference to space heating where for example a radiator is a thermal emitter as is a concrete floor used as thermal mass. Another example of a (unwanted) thermal emitter is where a balcony acts as a thermal bridge between an internal floor slab and the exterior; The large surface area of the balcony renders it as an efficient emitter.

Thermal envelope

The insulated external fabric of the building.

Thermal flanking

Heat loss by means of thermal bridging around areas of insulated construction.

Thermal mass

The ability of construction materials to absorb, store and release heat. Thermal Mass can be used effectively to absorb daytime heat gains (reducing cooling load) and release the heat during the night (reducing heat load), thereby maintaining a constant level of comfort through stable temperature. Materials of high thermal mass include water, stone, earth, brick and concrete. More recent innovations include ‘phase change’ materials that store energy whilst maintaining constant temperatures. The quality of thermal mass is usually described in terms of ‘admittance’. Admittance is the ability of a material or construction such as a wall to exchange heat with the environment when subjected to a simple cyclic variation in temperature. For buildings, this is 24 hours. Admittance is measured in W/m2K, where temperature (K) is the difference between the mean daily value and actual value within the space at a specific point in time. Key variables that determine admittance are thermal capacity, conductivity, density and surface resistance. (note that 'K' is used in a slightly different way from that involved in the calculation of u-value) 

Thermal resistance (R-value)

Thermal resistance is the measure of a component’s ability to restrict the passage of heat across its thickness. The R-value is calculated by combining the lamda value (thermal conductivity, or 'k-value') and the thickness of the material. Hence R=t/λ, where 't' is the thickness. Units are measured in m2W/K. Used in connection with insulation, the higher the R-value, the more effective the insulation. The R-value is also used to calculate the U-value (see below)

Thermal store (heat bank)

A way of ‘saving’ heat for future use as space or water heating. Heat is added to water contained in a cylinder (resembling a regular hot water cylinder) from sources such as a solar hot water collector or boiler and then withdrawn for space or domestic hot water heating at a later time.
Another form of thermal store is the ‘rock store’. Containers containing rock or gravel are located beneath the ground floor. Air warmed in an adjoining sun space is directed into the store where it heats up the rock for later release as warm air or underfloor heating.

Thermal transmittance (U-value)

Thermal transmittance is a measure of the overall rate of heat transfer, by all mechanisms under standard conditions, through a particular section of construction. This measure takes into account the thickness of each material involved and is calculated from R-values of each material as well as constants accounting for surface transmittance (Rsi and Rso, inner and outer surfaces respectively) and also for a small standard air gap (Rso). Thermal transmittance is measured in W/m2K

Timber certification

A process which results in a written statement (a certificate) attesting to the origin of wood raw material and its status and/or qualifications, often following validation by an independent third party eg FSC. (see also: The Forest Stewardship Council (FSC) )


The Timber Research and Development Association (TRADA) is a source of information on the use and specification of timber and wood products in construction. (