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

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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.

 








 

 

 

 

 

 

 

 

 

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