Overheating in buildings
The term 'overheating' refers to discomfort to occupants caused by the accumulation of warmth within a building. It is considered to be a growing problem in the UK due to climate change, the urban heat island effect, electronic equipment, increasing amounts of glazing and so on.
CIBSE define overheating as: 'conditions when the comfortable internal temperature threshold of 28C is surpassed for over 1% of the time.' They also define 35C as the internal temperature above which there is a significant danger of heat stress.
Overheating can result in serious health issues, particularly amongst the elderly or young children. The summer heat wave of 2003 recorded an increase of 2,000 deaths in the UK due to heat exposure. It is predicted that by the 2080s this figure could have risen to as much as 5,000.
 Design causes of overheating
Some of the reasons for overheating are thought to be modern building fabric standards that aim to keep buildings warm in colder climates. The House Builder's Association have been reported as saying that 'the ever exacting standards' of Building Regulations Part L cause overheating by stipulating airtightness levels that are too high.
Overheating may be caused by a single predominant factor or as a cumulative effect of different factors. These include:
- Solar radiation passes through glass and heats internal surfaces, which re-radiate long wave infra-red radiation that cannot pass through glass. This is known as the greenhouse effect.
- Double-glazed windows reduce heat losses through conduction.
- Increasingly high levels of insulation reduce heat transmission across the building fabric.
- The activities of occupants such as cooking, bathing, showering all generate heat. Electrical appliances generate heat when in use.
- Occupants themselves generate heat, the amount of which is dependent on their activity level.
- If a site is in close proximity to airborne noise, pollution or odour from busy roads, railways or industrial sites, occupants will be reluctant to open windows and so heat will accumulate inside.
- Urban heat island effect is primarily caused by the replacement of natural surfaces with hard impervious surfaces that are generally dark and absorb large amounts of solar radiation. Urban hard surfaces are significant in the built environment in the form of roads, paved areas, roof tops and so on.
- Buildings oriented with south facing glazing may accumulate high levels of solar gain.
- Overheating problems may be contributed to by heat gain from boilers, hot water storage and distribution and other building services systems. This can be a particular problem in apartment buildings that have community heating pipework.
- Increasing levels of airtightness can reduce the amount of 'fresh' air entering a building.
 How to deal with overheating
- Orientation and footprint can be designed to minimise solar gain and maximise opportunities for cross ventilation, stack ventilation and so on.
- High-performance glazing such as low-e glass, smart glass and so on can reduce heat gains. Smart glazing can be manually or automatically adjusted to control the amount of light, glare or heat that passes through. Whilst the price is decreasing it is still a very expensive option and as such is most likely to be found in commercial developments.
- Adjustable blinds can allow some internal control over solar gains.
- External shading such as canopies, louvres and shutters may help to prevent overheating. Intelligent facades can give control over climate exposure.
- Glazing ratio: Large expanses of glazing increase solar gain and make a building more susceptible to overheating. However, large windows also admit natural light. Glazing ratios try to achieve an equilibrium between these two positions.
For more information, see Preventing overheating.
 Related articles on Designing Buildings Wiki
- Comfort in low energy buildings.
- Cool roofs
- Evolving opportunities for providing thermal comfort.
- Heat stress.
- Overheating - assessment protocol.
- Preventing overheating.
- Thermal comfort.
- Thermal indices.
- Thermal pleasure in the built environment.
- Urban heat island
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