According to the Chartered Institution of Building Services Engineers (CIBSE), the overheating of buildings is now a key problem. Mike Sopp investigates.

A combination of climate change and the need to reduce energy consumption has created a challenge for both building designers and managers in terms of finding a good thermal balance.

Overheating buildings can result in building occupiers experiencing thermal discomfort, with the potential for consequent ill health and reduced productivity.

With no official upper temperature limit and differing individual perceptions of what is comfortable, determining — and achieving — what is deemed to be good thermal comfort will require consideration through the risk assessment process.

Defining thermal comfort

According to the CIBSE, “overheating within a building occurs when the actual indoor temperature for any given day is hot enough to make the majority of people feel uncomfortable”.

Thermal comfort is not just related to air temperature alone. It takes into account a range of other environmental and personal factors including radiant temperature, air velocity, humidity, clothing insulation and metabolic heat. These factors make up what is known as the “human thermal environment”.

Thermal comfort is actually defined in British Standard (BS) EN ISO 7730 as “that condition of mind which expresses satisfaction with the thermal environment”. Conversely, thermal discomfort is where people start to feel uncomfortable, ie they are too hot or too cold, but are not necessarily made unwell by the conditions.

The Health and Safety Executive (HSE) website notes that an acceptable zone of thermal comfort for most people in the UK lies roughly between 13°C (56°F) and 30°C (86°F), “with acceptable temperatures for more strenuous work activities concentrated towards the bottom end of the range, and more sedentary activities towards the higher end”.

Most individuals will not suffer medical symptoms due to thermal discomfort, beyond irritability and tiredness. However, in some circumstances, exposure to excessive heat can result in more severe conditions such as heatstroke and dehydration, leading to dizziness, fainting, confusion and heat cramps.

As well as potentially affecting the health of employees, thermal discomfort in the indoor workplace can affect worker morale, reduce productivity, increase absenteeism and cause mistakes to be made.

Studies by the Centre for Economics and Business Research (Cebr) support this and estimate that during the 2006 heatwave, work levels dropped by almost a third when temperatures rose to more than 30°C. This resulted in UK employers losing an estimated £168 million a day in productivity.

Assessing the risks

The HSE website notes that, through a suitable and sufficient risk assessment, “the temperature of the workplace is one of the potential hazards that employers should address to meet their legal obligations”.

However, the HSE suggests that prior to the completion of a risk assessment, the employer can undertake a survey of employees and/or their representatives by the completion of a “thermal comfort checklist”.

This is based around the six factors noted above that can influence the overall thermal comfort of an individual, with the HSE stating that “thermal comfort is not measured by room temperature, but by the number of employees complaining of thermal discomfort”.

With thermal comfort, it is recognised that the perceptions of individuals and their own needs will differ based upon the factors noted above. In addition, psychological parameters such as individual expectations will also affect thermal comfort.

Therefore, the HSE finds that the “best that you can realistically hope to achieve is a thermal environment that satisfies the majority of people in the workplace”. An historical indicator would be if 10–15% of employees complain of discomfort.

Where this is the case, the HSE website suggests that, generally, simple measures may improve the thermal comfort of the majority but that a more detailed risk assessment may be required.

Assessing the risks can be challenging but, in summary, the approach should:

  • identify the problems and objectives (how overheating risks can be managed and thermal comfort maintained)

  • establish decision-making criteria and performance targets for thermal discomfort and heat stress

  • assess the risk using qualitative and quantitative criteria

  • identify and evaluate the adaptation options for keeping premises cool that are reasonable and cost-effective

  • implement and monitor the effectiveness of the adaptations.

Key to the process is determining what would be deemed to be an acceptable temperature that should not be exceeded for any given period of time, be it number of hours or percentage of the year (eg summer time).

This is an important factor as the assessment process should be used to determine thermal comfort over a set period so that short-lived “heatwave” periods do not give false perceptions of the thermal comfort issues.

With no legal upper temperatures provided in building or health and safety legislation, much research has been undertaken into what would be deemed to be “comfort threshold temperatures”.

The “overheating criterion” often cited in professional guidance in offices, for example, was that temperatures should not exceed 25°C for more than 5% of the year and/or exceed 28°C for more than 1% of the year.

However, CIBSE now states that “while this advice is basically sound, the assumption that there is a single indoor temperature limit irrespective of outdoor conditions is no longer considered sufficient”.

Therefore, a new approach is being considered based upon methodologies contained in BS EN 15251:2007, which suggests the following criteria:

  • the number of hours that the operative temperature can exceed the threshold comfort temperature

  • the severity of overheating within any one day

  • an absolute maximum daily temperature for a room, beyond which the level of overheating is unacceptable.

Put in simpler terms, CIBSE states that “indoor operative temperatures over 28°C for long periods will result in increased dissatisfaction and reduced productivity”.

Risk control measures

Decisions will have to be drawn as to whether the thermal discomfort is likely to create an unacceptable risk, based upon the benchmark criteria. CIBSE suggests that, when making decisions, the following should be considered.

  • To what extent will passive measures be able to improve thermal comfort and ameliorate the increased propensity for overheating?

  • How effective (including cost-effectiveness) will different approaches to comfort cooling continue to be under the changing climate?

  • What are the energy use implications of the various strategies?

CIBSE states that “overheating happens in a building either through bad design, poor management or inadequate services” and any remedial measures should be taking these factors into consideration.

It is unlikely that one single measure will be sufficient; rather a “mixed mode approach” will have to be adopted. The action to be taken can be based around a number of areas, such as:

  • safe systems

  • management measures

  • provision of equipment

  • adaptation of building elements.

Under health and safety legislation, the employer is required to do what is reasonable to reduce the risk to as low as reasonably practicable.

Certainly some straightforward systems can be introduced that will enable the thermal comfort of occupiers to be improved but these tend to be short term in nature and react to thermal discomfort rather than eliminating the issue. As such, they may be useful for example during heatwave periods when thermal comfort issues arise for short periods of time that do not necessitate long-term solutions. Measures could include:

  • planning or rescheduling work times, practices and rest schedules

  • relocation of work (eg home working)

  • rotation of workstations

  • flexible working

  • relaxation of dress codes.

Provision of equipment and facilities may also be considered, particularly where longer-term solutions are required. This will include adequate blinds, provision of portable fans or air conditioning units, access to water and even the relocation of workstations.

In addition, management should provide sufficient information and instruction to employees about the policies and procedures on thermal comfort/hot temperature conditions and how they are to be applied.

As mentioned previously, an important factor can be psychological. A key element of thermal comfort is for management to give employees some form of control over their working environment.

This can include the systems factors noted above but will also include allowing employees to use any equipment or facilities provided.

CIBSE notes that buildings are designed to last for a significant period and will remain significantly unchanged, which imposes severe limitations on how the building can be modified to take account of overheating issues. Clearly, the ideal scenario is to eliminate conditions that create thermal discomfort, particularly where the risks are significant throughout summer periods, for example.

Mechanical ventilation systems not forming part of the original design of the building under assessment are highly unlikely to be cost-effective to install and will impact on energy consumption issues. Therefore, they are unlikely to be reasonable except in the most extreme of cases.

Where mechanical ventilation systems are installed, the associated plant and equipment needs to be examined, tested and maintained in accordance with the manufacturer’s guidance to ensure it remains effective.

However, other options can be applied to limit heat gains to spaces so as to reduce the need for mechanical comfort cooling. These include employing solar shading, reducing the density or power output of lights and machines, insulating pipework and ensuring natural ventilation is available.

Whatever measures are employed, it is important that they remain under review. A simple indication of success is that a building can be said to have performed its task if occupants are not conscious of its temperature.

Further information

Health and Safety Executive

British Standards Institution

  • BS EN ISO 7730:2005 Ergonomics of the Thermal Environment. Analytical Determination and Interpretation of Thermal Comfort Using Calculation of the PMV and PPD Indices and Local Thermal Comfort Criteria

  • BS EN ISO 10551:2001 Ergonomics of the Thermal Environment. Assessment of the Influence of the Thermal Environment Using Subjective Judgement Scales

  • BS EN 15251:2007 Indoor Environmental Input Parameters for Design and Assessment of Energy Performance of Buildings Addressing Indoor Air Quality, Thermal Environment, Lighting and Acoustics

Chartered Institution of Building Services Engineers

  • TM52 The Limits of Thermal Comfort: Avoiding Overheating in European Buildings

  • Guide A Environmental Design

  • Keeping Cool in a Heatwave 1: Top Tips for Facilities Managers

  • KS16 How to Manage Overheating in Buildings

Last reviewed 13 February 2017