D

Daylight
What we see as visible light has a wave length in the range 0.38 to 0.76 microns (a micron is a thousandth of a millimetre) - or 380 to 780 nanometres (a nanometre is a millionth of a millimetre). The colour of the 380-420 nanometre band of radiation is deep violet and that of the 660-760 nanometre band is dark red.

Visible light falls into the middle of the total spectrum of electromagnetic radiation. The complete spectrum ranges from cosmic rays (with wave lengths around a millionth of a nanometre) at the lower end to radio waves (with wave lengths around a hundred kilometres) at the upper end. The speed of light is 299,820 kilometres a second.

Daylight is composed of of direct and diffuse light. Direct light beams are, by the time they reach the earth, effectively parallel. Diffuse light is received from the sky after it has been reflected by the gases and water droplets in the atmosphere. For daylighting, the sky can be regarded as the integration of an infinite number of finite point sources.

Daylight factor
The ratio of indoor illuminance at a reference point on a horizontal plane (commonly at desk height) to the simultaneous illuminance received from an overcast sky on a horizontal plane outdoors.

Delivered energy (kWh or MJ)
The heat content of the fuels (gas, oil, electricity, etc.) purchased by households and used to produce useful heat or work; the amount of fuel required to produce a given amount of useful work is equal to the useful energy produced times the reciprocal of the conversion efficiency of the appliance used for the task.

Density (kg/m3)
The mass of a unit volume of material.

Design heat loss (W)
The sum of the products of surface areas of building elements surrounding spaces in a building by their U-values and by the temperature difference between indoor spaces and the outside (for external elements) - or by the difference in temperature between adjacent rooms (for internal elements) - plus the rate of heat loss due to air change for ventilation summed for the whole building. The value used for the outdoor temperature in the calculation of the design heat loss is a mean minimum temperature. The result of such calculation is used as a basis for sizing heating appliances; further increments may be added to allow for intermittent heating and domestic hot water.

Design temperature
The coldest ambient temperature that, per the local building code, a building must cope with in guaranteeing that a required room temperature (i.e. 20°C) is achieved. The design temperature under night conditions (no solar) is a basis for sizing the capacity of the heating plant for a given building design.

Diffuse radiation (kWh or kWh/m², MJ or MJ/m²)
The solar radiation that reaches a surface from all directions except the immediate region of the sun's disc.

Direct radiation (kWh or kWh/m², MJ or MJ/m²)
Solar radiation from the solid angle of the sun's disc.

Direct storage
When solar radiation strikes a material - either directly or after transmission through glass - part of it is absorbed, transformed into heat and stored in the mass of the material. The material heats up progressively by conduction as the heat diffuses through it.

Heat penetration is quickest in materials with a high thermal diffusion coefficient. This increases with increasing conductivity. Thermal diffusion in the material prevents the surface temperature from rising rapidly when radiation falls on it and causes the temperature of the entire mass to increase. Materials with high heat storage capacity such as concrete, brick and water heat up and cool down relatively slowly. Thermal insulating materials such as glass fibre and foam, usually because of their open or cellular structure, form poor heat stores and diffuse heat very badly. The insulating layer of a wall minimises heat exchange between the solid layers adjacent to it.

Double flow ventilation with heat recovery
Controlled double flow mechanical ventilation centralizes the inlet of fresh air and the extraction of waste air. It is useful to provide a heat recuperator, since the average temperature of extracted air is close to 20°C. During heating periods, this heat may be recovered and transmitted through a plate exchanger, a heat duct or a heat pump, to the fresh air arriving from the outside at a lower temperature. There is no mix between the incoming air and extracted air.

It is preferable to install the aeraulic circuit in the heated volume and/or to insulate the extraction and blower ducts, in order to encourage heat recovery. Apart from the recovered calories, new air blown in may be heated or cooled by batteries or a reversible thermodynamic system.

A double flow system avoids air inlets on the façade, which protects the rooms from external noise and pollution.

Draught
A current of air perceived by a person as a source of higher heat loss from the skin than under still conditions.