The document discusses the risks and factors associated with overheating in UK dwellings, highlighting the importance of benchmark temperatures and thermal comfort. It explains the sources of heat, both internal and external, and factors influencing overheating, such as building fabric and ventilation effectiveness. Additionally, it addresses the growing challenge of excess heat due to increased insulation and air tightness in modern buildings.

1. Part of the BRE Trust
Overheating in UK Dwellings Mich Swainson
BRE
5th November 2014
2014 | BRE – HVAC Engineering

2. Overheating and excess heat

3. CIBSE Overheating risk (CIBSE Guide A 2006)
–Summertime thermal performance of buildings is usually measured against a benchmark temperature that should not be exceeded for a designated numbers of hours or a percentage of the annual occupied period. The benchmark temperature is usually related to the likelihood of discomfort, although it may be related to other factors, such as productivity or health. When the benchmark temperature is exceeded the building is said to have ‘overheated’ and if this occurs for more than the designated amount of time the building is said to suffer from ‘overheating’. Accordingly, a design target for the assessment of overheating risk is set and this is called the overheating criterion.
Overheating & excess heat – the difference
CIBSE Design Guide A 2006

4. –Assessment of risk of overheating through SAP
–Prior to the 2005 version of SAP, no overheating calculation was undertaken
Purge ventilation = 4 ach in each habitable room
–For hinged windows that open 30o or more, the height x width of the opening should be at least 1/20 of the floor area of that room
–For hinged windows that open 30o or less, the height x width of the opening should be at least 1/10 of the floor area of that room
Overheating & excess heat – the difference

5. Excess heat and heat stress
–As temperatures rise, thermal stress increases, initially triggering the body’s defence mechanisms such as sweating. High temperatures can increase cardiovascular strain and trauma, and where temperatures exceed 25°C, mortality increases and there is an increase in strokes. Dehydration is a problem primarily for the elderly and the very young.
Excess heat is the elevation of internal temperatures within a building over a sustained period. The evidence suggests that the key to recovering from a hot day is a cool night, and a good sleep. If this is disturbed over a long period then we have moved from overheating and thermal comfort criteria to a potential hazard to health.
Overheating & excess heat – the difference

6. Sources of heat

7. Heat is either generated inside the building or is transmitted (and drawn) through the building fabric from outside.
External sources of heat
–The outside air temperature around the dwelling is greater than that internally. This will result in conduction of heat through the building fabric and increase the internal air temperature as this air is used for ventilation.
Sources of heat
–Solar gains, direct through glazing or indirect through opaque elements of building fabric. Internal sources of heat
–Occupants and equipment used by occupants.
–Heat liberated from systems in dwelling – DHW cylinders, etc.
–Heat liberated from communal heating systems.

8. Conduction of heat through the opaque fabric of the building
–This heat transfer is a direct function of the U Value of each of the fabric elements, However the heat transferred is not only a function of the outside air temperature – incident solar radiation heats the outside surface of the fabric raising the temperature. Therefore colour of the outside surface is also important.
Ventilation
–During hot periods of weather the outside air temperature is above that internally and ventilation results in the internal air temperature increasing rather than reducing.
Sources of heat – External

9. Direct solar gains
– The heat transmitted to the inside of a
building occurs through glazing. The
proportion of heat incident on the glazing
is a complex function of the glazing
properties, orientation and sun time and
the level of over-shading.
– Most manufactures provide detailed
information on the performance of glazing
products and this includes light and heat.
– Note the levels of light transmission
– Use CIBSE Tables to obtain solar
irradiance and calculate solar gains for a
given glazing.
Sources of heat – External

10. Sources of heat – Internal
Normal occupancy gains and household
equipment used by occupants
– Occupants ~ 80 W per person.
– Lights.
– IT and audio/visual.
– Cooking, cleaning, etc.
SAP uses an assumption based on the floor area
for both occupancy and realistic values of internal
heat gains.
SAP 2005 9-81 Heat gains

11. Sources of heat – Internal
DHW system – losses from cylinder,
distribution system, etc.
Suggested insulation of primary heating system pipes – reproduced from TIMSA HVAC Compliance Guide, March 2006.

12. Sources of heat – Internal
–Internal gains from communal heating systems 051015202530353/6/105/6/107/6/109/6/1011/6/1013/6/1015/6/1017/6/1019/6/10 Air temperature (ºC) Living RoomExternal airBed roomHeat exchangercupboard

13. Factors influencing the risk of overheating

14. The location of building
–Rural or sub-urban house
–Urban house or flat
–Deep urban flat
Factors:
–Local environment – noise, pollution, crime, etc. and acceptability of leaving windows open for ventilation at night.
–Security
–Urban heat island and micro climate effects.
Factors influencing the risk of overheating

15. Urban heat island effect
–London’s heat island intensity, 31 May to 31 August 1999 (all data, mean ± standard deviation)
Factors influencing the risk of overheating

16. The building fabric
–U Value of walls, roof, windows, etc.
–Exposed walls, loft insulation, loft conversion and upgrade of insulation, etc.
–Glazing type and area
–Glazing orientation
–Very important to note here that the same dwelling with a different orientation may perform totally different due to solar loads.
Factors influencing the risk of overheating

17. Glazing orientation
– The west elevation in summer get low sun for a long period into the
evening
Factors influencing the risk of overheating
NF44 NHBC Overheating – A quick Guide

18. Solar gains
–The glazing is the key to minimising transmission, but solar shading can reduce the incident solar radiation. But it needs to be effective across a wide range of sun angles
–Solar shading provided for summer sun – but not effective for lower sun in mid seasons
Factors influencing the risk of overheating
13:00
13:00
13:00
14:30

19. Factors influencing the risk of overheating
Solar gains
–Solar shading provided for summer sun – but not effective for lower sun in summer or mid seasons
South
West
East

20. Factors influencing the risk of overheating
The effect of thermal mass
– Thermal mass is a term used to describe the change in temperature of a
structure when heated or cooled. High thermal mass results in a low
change in temperature. This can be used to reduce overheating risk, but,
all heat stored in thermal mass must be rejected or it will slowly build up,
potentially exacerbating overheating.
BRE IP 6/01
The impact of solar gain on the internal
temperature in a naturally ventilated
heavyweight building
The impact of solar gain on the
internal temperature in a naturally
ventilated lightweight building

21. Ability to achieve effective ventilation
–Type of windows
–Ability to achieve purge ventilation overnight
Factors influencing the risk of overheating

22. The local micro climate – the source of the ventilation air drawn into the building. The sol-air temperature not only increasing the heat transmission through the fabric but gives an indication of the temperature of the air being drawn into the building for ventilation.
Factors influencing the risk of overheating
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13:00
18:00

23. Factors influencing the risk of overheating
Natural ventilation
–In a single storey, single sided flat natural ventilation is driven by the wind.
–Ventilation rate required to remove heat – an order of magnitude greater than that for normal ventilation - IAQ
– SAP puts the actual ventilation rates likely to be achieved into context.

24. –Why is excess heat an increasing problem? With increasing fabric insulation and air tightness, if ventilation is not effective - the internal and external environment are totally divorced from each other
Modern buildings

25. Thank you
swainsonm@bre.co.uk
2014| BRE – HVAC Engineering