Internal conditions

Internal conditions

Air temperature: The general recommended range for internal air temperature is between 19°C and 23°C in winter and less than 27°C in summer. Air temperature gradient between head and foot level is also important for comfort. The temperature at foot level should generally not be less than 4°C below that at head height.

Radiant temperature: This is a measure of the temperature of the surrounding surfaces, together with any direct radiant gains from high temperature sources such as the sun. The mean radiant temperature is the area-weighted average of all the surface temperatures in a room. If the surfaces in a space are at different temperatures then the perceived radiant temperature in a space will be affected by the position of the person in relation to the various surfaces, with the closer or larger surface areas contributing more to the overall radiant temperature. Comfort can be affected by radiant asymmetry.

Relative humidity (RH): The RH is a percentage measure of the amount of vapour in the air compared to the total amount of vapour the air can hold at that temperature. The RH of a space will affect the rate of evaporation from the skin. When temperatures are within the comfort range of 19–23°C, RH should be within the range 40–70%. At high air temperatures heat loss is important to maintain comfort and higher RH values inhibit this heat loss process.

Air speed: This is a measure of the movement of air in a space. People begin to perceive air movement at about 0.2 m/s. Air speeds greater than 0.2 m/s produce a 20% greater comfort dissatisfaction due to perceived draught. For most naturally ventilated spaces the air speed will be less than 0.1 m/s, away from the influence of open windows. For mechanically ventilated spaces, the air speed is generally greater than 0.1 m/s and could be greater than 0.2 m/s in areas close to air supply devices.

Light levels: The provision of good quality lighting, especially to maximise the use of daylight is important to achieve a satisfactory visual environment. Some degree of contrast for visual interest is desirable. Excessive uniformity and glare from electric lights, reflective surfaces and solar radiation should be avoided. Glazing solutions that optimise the use of daylight should also be able to provide control of solar gains if glare and high radiant heat gains are to be avoided.

Internal conditions light levels

A daylight factor (DLF) value is commonly used as a measure of the quantity of daylight entering a space. It is a measure of the light at a work surface compared with the total amount of daylight available outside at the same time, expressed as %. An average daylight factor of 2% over 80% of the occupied spaces is generally considered to be achievable by good daylighting design with best practice providing an average of 4%.   


internal conditions

WLGA - CLILC Llywodraeth Cynulliad Cymru / Welsh Assembly Government