What is overheating?

Overheating occurs when the heat generated by humans isn’t able to dissipate to allow thermal equilibrium with the surroundings therefore the environment becomes uncomfortable.

What is thermal comfort?

Thermal comfort is when an occupant is satisfied with the thermal environment. The level of thermal comfort is determined by a combination of factors including; air temperature, relative air speed, radiant temperature asymmetry, vertical air temperature difference, surface temperatures, humidity, direct sunshine, clothing levels, level of physical activity, gender, age and health. The combination of these to a satisfactory level is subjective and will be influenced by whether the space is air conditioned/cool or naturally ventilated.

Thermal comfort in naturally ventilated spaces

In naturally ventilated spaces the outdoor climate will influence the indoor climate and humans can adapt to these variations over the year (to some extent). Acceptable temperature will not only be dependent on the day’s climate but also the previous few days’ climate e.g. if there have been a few hotter days in a row then higher temperatures are more likely to be acceptable.

There are three main types of adaptations- behavioural, physiological and psychological. These include adaptations such as changing in clothing, access to environmental controls and shifts in occupant expectations.

There are three criteria (from CIBSE TM52) which are used to define the level of overheating in naturally ventilated spaces:

  1. The first criterion sets a limit for the number of hours that the operative temperature can exceed the threshold comfort temperature (upper limit of the range of comfort temperature) by 1K or more during occupied hours of a typical non- heating season (1 May- 30 September)
  2. The second criteria deals with the severity of overheating within any one day, which can be as important as its frequency, the level of which is a function of both temperature rise and its duration. This criterion sets a daily limit for acceptability.
  3. The third criterion sets an absolute maximum daily temperature for a room, beyond which the level of overheating is unacceptable.

Thermal comfort in air conditioned/cooled spaces

Within a cooled building, the climate can be controlled throughout the year. Overheating can occur if the ventilation system is undersized or poorly controlled. To check the unit capacity Fanger’s PMV/PPD (Percentage Mean Vote and Percentage People Dissatisfied) method is used to ensure that most people are satisfied for majority of the time. The PMV is dependent on the humidity, air speed, air temperature, clothing and activity level. If the PMV is over 0.5 with PPD>10% then the space will be classed as overheating.

How do we know if a building will overheat?

The four most critical factors to predicting thermal discomfort are air temperature, radiant temperature, humidity and air movement.

  1. Temperature is determined by the balance of gains into and losses out of the building/room. Gains include heat from people, lighting, equipment and solar. Losses are a combination of heat loss from the walls, roof, floor, windows, thermal bridges and air losses through infiltration and ventilation. When the gains outweigh the losses then this will cause overheating and hence passive or active cooling will be required. (When the losses outweigh the gains this difference is the heating required)
  2. Radiant temperature i.e. the amount of radiant heat transferred from a surface will depend on the material and will vary depending on the surrounding surface’s temperature and emissivity. The experienced mean radiant temperature will largely be influenced by how much of the occupant is in direct sunlight.
  3. Relative humidity is the ratio of water vapour in the air to the amount of water vapour that air could hold at that specific temperature and pressure. At a higher relative humidity, i.e. the air is holding nearly the maximum amount of water then the evaporation of sweat from the skin is a less effective heat loss mechanism. (Low relative humidity will affect health- e.g. dry skin.
  4. Whilst air movement can be pleasant in some circumstances it can be unwanted and become uncomfortable. It is particularly noticeable on uncovered body parts such as head, neck and arms. The acceptable level of air speed will vary based on the activity and clothing levels e.g. higher air speeds in gyms are likely to be more acceptable whilst not in spaces with sensitive or fragile people such as nursing homes.

These factors all need to be determined and assessed to ensure that the environment is thermally comfortable and provide a more holistic view compared to the simple method of assessing the overheating from the number of hours above a set air temperature.

A building’s overheating risk can be reduced, however this will require a good design approach at a very early stage. Involvement from an overheating specialist can help reduce the amount or cooling required or avoid active cooling altogether. There are numerous strategies that designers can use to mitigate overheating e.g. reducing window area, having larger openings, adding shading, optimising orientation. An integrated, balanced approach provides the best results. Please see “10 ways to stay cool: secrets to reducing overheating” for more information or speak to one of the team at Greengauge for more advice.

For more information see CIBSE Technical Memorandum TM 52.

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