Daylight transmittance

The ratio of the amount of light transmitted through a window divided by the amount of light incident on its outside surface. 


Sometimes referred to as ‘construction in reverse’, deconstruction is the selective dismantling of a building for re-use, recycling and waste management. The technique is in contrast to ‘demolition’ which is where a building is removed from a site by the most expedient means. 

Decrement delay

Refers to the time it takes for heat generated by the sun, to transfer from the outside to the inside of the building envelope and affect the internal conditions. Materials affording higher rates of decrement delay will have a low lambda (thermal conductivity or k-value) value, high specific heat capacity and high density.

Insulation materials offering a high decrement ‘factor’ include cellulose fibre (7.3 hr), wood fibre insulation board (11.3 hr); whereas materials with a low decrement factor would include low-density mineral fibre (3.7 hr) and polyurethane/polystyrene.

Decrement delay can be useful in the design of timber frame buildings. Insulation with a high decrement factor can be used to limit solar over-heating in particularly warm climate conditions. For example, installing cellulose as a roofing insulation will likely slow-down the heat transfer from a sun-heated roof surface, through to the inside by around 7 hours – or by evening time.

Decrement factor

The time taken (measured in hours) for heat to transfer between opposite surfaces of a material. (see above)


Is the clearance of naturally occurring forests by the processes of logging and/or burning of trees in a forested area.

Degree days

Is the rate of heat loss from a building, (related to the building fabric) and the temperature difference between the inside and outside of a building - the greater the temperature difference the more heat will be lost. A heating degree day is counted for each degree below 15.5oC reached by the average daily outside temperatures.

Delivered energy

The amount of energy which is supplied to final users, e.g., households, office buildings, schools and factories.


Is the degradation of land in arid and dry sub-humid areas, resulting primarily from man-made activities and influenced by climatic variations.

Design for Deconstruction (DfD)

Anticipating the end-of-life of a building at the design stage can help optimise the reuse and recycling of components in the event of its deconstruction. 

Diffuse pollution

Is the release of potential pollutants from a range of activities that individually may have no effect on the water environment, but at the scale of a catchment can have a significant impact (ie reduction in water quality, decrease in wildlife, etc.).

Diffusion Open

Usually applicable to timber frame construction - a membrane / layer that permits the passage of vapour – normally the ‘breather membrane’ is included in the outer layers of the construction.

Diffusion Tight

Usually applicable to timber frame construction - a membrane / layer that inhibits the passage of vapour – usually the ‘vapour control layer’ is applied to the warm side of the insulation layer.

Distributed generation

Is the generation of energy at smaller scales, usually close to where it is used, than that of traditional power stations. Amongst other advantages, distributed generation reduces the amount of energy lost through transmission. 

District heating

Is the use of a centralised boiler installation to provide heat for a number of buildings. This can use heat from only a boiler, or the heat from a combined heat and power (CHP) plant.

Diurnal heat flow

The heat that flows in and out of a building from daytime to night-time.

Diurnal temperature variation

The daily temperature shift that occurs from daytime to night-time.


A material that loses its original function / value through recycling, but continues to be used in its downgraded format.