Overheating: Introduction and Causes

What is overheating?

Why is it becoming an issue?

1 Summers are becoming warmer

Thanks to The Wine lands of Britain



2 Buildings are becoming more air-tight

Background ventilation in old buildings came from chimneys and naturally air-leaky construction

Higher sealing standards, such as Passivhaus, drive down construction air-leakage to avoid heat loss .


3 Modern buildings are heavily insulated to retain heat


Some Causes of Overheating


1 Uncontrolled Solar gain through windows / glazing


Solar radiation passes through the glazing and heats objects and surfaces - in-turn, they emit heat to raise the overall room temperature.

2 'Caravan Syndrome':  
Summer overheating through conventional construction

Holiday caravans rapidly overheat when exposed to constant summer solar radiation

A contemporary shaded or un / intermittently  sun-lit timber frame wall

Within the context of the current climate, conventional walls cause few overheating problems.


The same wall circa 2050 subject to increasingly constant solar radiation

• Walls of conventional timber frame construction including mineral fibre infill insulation are likely to be problematic as climate change results in more intense summers.

• In addition to high ambient summer temperatures, constant exposure to solar radiation heats up the surface of the brickwork and, through conduction, transfers the heat to the space within.

• Time taken for the heat to appear on the inside of the wall will be typically 7 hours - resulting in overheating during the late afternoon and early evening.

• In transferring the heat to the interior during the warm daylight hours, the construction type provides inadequate thermal buffering / lag /decrement delay


Even conventional masonry cavity walls will be subject to overheating


Chronic overheating is more pronounced in roof spaces

• Loft spaces have always been subject to high temperatures in summer
• The Sun's angle of incidence is closer to the perpendicular - causing more intense radiation
• Rooms in the roof have accentuated the problems of heat transfer through lightweight fabric and structure


3 Internal Gains

People, Appliances, Lighting

Building Services


4 Inadequate or poorly controlled Ventilation

Some buildings have no, or ill-conceived, ventilation systems

The noise and fumes from adjacent traffic inhibit the opening of windows

Ventilation rates through the century:

Home ventilation rate circa 1900 = 2.0 air changes per hour (ACH)
Home ventilation rate circa 1970 = 0.7 ACH
Home to Passivhaus standard 2015 controlled ventilation rate = 0.4 ACH


5 The urban 'Heat Island Effect'

  • Heat absorbed by the urban fabric during the day is re-radiated during the night - providing little respite from a hot atmosphere.
  • In turn, purging ventilation through buildings works barely at all where there is so little difference between inside and outside temperatures.


6 Orientation

Windows to the front elevations of these two identical 1960s houses are subject to very different amounts of solar gain during the day

  • House types are often repeated throughout a 'housing estate'.
  • It is likely that one type template will be located facing in a number of different directions.
  • Windows facing the SW and W are likely to experience higher levels of solar gain penetration than other orientations late in the day.


6 Exposed thermal mass without the potential of night time 'purging' ventilation

  • The sun's radiation is absorbed into exposed thermal mass eg stone tile flooring
  • When the room temperature drops at night the heat is re-emitted
  • If ventilation is unavailable the heat will be trapped overnight in a well-insulated, well-sealed building.