Overheating in UK dwellings

Part of the BRE Trust

Overheating in UK Dwellings

Mich Swainson

BRE 5th November 2014

2014 | BRE – HVAC Engineering

Overheating and excess

heat

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

– 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


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.


Sources of heat

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.

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

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

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


DHW system – losses from cylinder,

distribution system, etc.

Suggested insulation of primary heating system pipes – reproduced from TIMSA HVAC Compliance Guide, March 2006.


– Internal gains from communal heating systems

0

5

10

15

20

25

30

35

3/6/10 5/6/10 7/6/10 9/6/10 11/6/10 13/6/10 15/6/10 17/6/10 19/6/10

Air

te

mp

era

ture

(ºC

)

Living Room

External air

Bed room

Heat exchangercupboard

Factors influencing

the risk of overheating

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

Urban heat island effect

– London’s heat island intensity, 31 May to 31 August 1999 (all data,

mean ± standard deviation)


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.


Glazing orientation

– The west elevation in summer get low sun for a long period into the

evening


NF44 NHBC Overheating – A quick Guide

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


13:00

13:00

13:00

14:30


Solar gains

– Solar shading provided for summer sun – but not

effective for lower sun in summer or mid seasons

South West East


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

Ability to achieve effective ventilation

– Type of windows

– Ability to achieve purge ventilation overnight


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.


11:00

13:00

18:00


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.

– 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

Thank you

[email protected]

2014| BRE – HVAC Engineering

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Overheating in UK dwellings

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