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.