Calum Maclean, Senior Research Engineer, BSRIA Sustainable Construction Group
The thermal comfort of an occupant can affect his or her wellbeing in a number of ways and I will go through some of them here; however first I will describe thermal comfort and how to quantify it.
The thermal comfort of a person is described as “that condition of mind that expresses satisfaction with the
thermal environment and is assessed by subjective evaluation”. The thermal comfort of an individual is personal and varies greatly from person to person. The subjective evaluation usually suggests a survey is needed to get the personal input from each of the occupants of a building. The large range of conditions and number of people required to give proper averages make this impractical in the majority of cases and cannot be done pre-occupation. A different approach consists of measuring environmental conditions and then calculating the thermal comfort indices, which relate to the measured values and calculated indices as if a range of people were surveyed.
The standards BS EN ISO 7730 and ASHRAE 55 give methods for taking the environmental measurements and subsequent calculations to give quantified numbers to compare thermal comfort between buildings and different conditions within the same building. The measured values required by the standard are air speed, turbulence intensity (using the standard deviation of air speed), air temperature, black globe temperature/operative temperature and relative humidity.
The thermal comfort calculations provide values for the Draught risk, Predicted Mean Vote (PMV) and the Predicted Percentage Dissatisfied (PPD). The Draught risk is an indication of the percentage of people that would perceive a draught given the measured conditions and uses the air temperature, air speed and turbulence intensity. The PMV is a prediction of the average vote of a large group of people occupying the space as if a survey was conducted on a scale of -3 to +3; where “-3” indicates feeling cold, “+3” indicates feeling hot and “0” is comfortable. The PPD is an estimate of the percentage of people who would find the space uncomfortable, based on the PMV results.
The calculations of PMV and PPD take into consideration the insulation provided by clothing (CLO) and the activity of the people working/living in the space (MET). The CLO is a measure of the average clothing insulation and the MET is a measure of the heat output from an average person doing a stated task in the space. The standards give methods to calculate the CLO and MET for a space, however, the actual individual values can change drastically depending the occupiers of the space, e.g. an office space versus a supermarket or factory floor where the activity level is significantly different, therefore the specific CLO and MET must be chosen carefully to give true indicative values of PMV and PPD.
One thing to note is that the PPD has a calculated minimum of 5 per cent as for the same CLO and MET there will always be some people who will feel either hot or cold however this is normally compensated by changing an individual’s CLO, i.e. if a person is in an office and wearing a jacket, and the dress code allows, they will remove it when feeling hot, etc.
The effect on wellbeing of the thermal comfort is becoming a focus of study and coming to the attention of building owners and occupiers. The reason for the focus is that detrimental thermal comfort can have a large effect on the morale and in some cases even the mental and physical health of the occupants of any building. Any problems with morale or health can affect the productivity of the occupants. A measurement of the thermal comfort, either pre-occupation by heat-load testing or of the occupied building, can indicate any problems or show that the building conforms with the expected comfort levels required, e.g. the BS EN ISO 7730 standard has classifications of the space depending on the PMV, PPD and other factors.
The distraction caused by adverse thermal comfort can be significant and lead to occupants feeling the space is uncomfortable even if/when the conditions in the space have improved, e.g. either through changes to the ventilation system, or by moving an individual to a more suitable thermal environment. If a space is felt to be too hot or too cold for too long and no actions are taken, the perception of the occupants of their thermal comfort can become biased. Perceived long term thermal discomfort can be hard to dispel and productivity can be negatively affected.
The effect of thermal comfort on wellbeing of people can be significant and if ignored, it may affect the morale, health and productivity of people in the space. Thought should be made to the thermal comfort of the occupants of a space prior to and during occupancy to minimise any adverse effects on wellbeing.