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Last edited 04 Jul 2016

Preventing overheating

This article has been extracted from the guidance document Overheating in Dwellings, originally published on BRE Buzz in June 2016. Reproduced with kind permission of BRE.

It was written by:

  • Andy Dengel (BRE)
  • Mich Swainson (BRE)
  • David Ormandy (Warwick Medical School, University of Warwick – BRE Trust Research Fellow)
  • Véronique Ezratty (Service des Etudes Médicales, EDF, Levallois-Perret, France)

You can download the full document here.


Contents

[edit] Introduction

Due to the complexity of heat transfer and dynamics of heat storage, combined with the need to assess a range of different heat rejection measures, no single solution can be considered as solving all cases of excess heat. Added to this the appropriateness of many solutions will differ between different buildings as for example, the cost, planning restrictions, etc. may make some solutions on some buildings inappropriate or prohibitively expensive. The following should be considered:

[edit] Insulation

The provision of additional thermal insulation to the walls and loft (roof) will help prevent solar gain through the structure. However, external wall insulation is problematic for solid wall construction, particularly where the dwelling abuts the pavement (as is the case for many English terraces in urban areas).

Where heat gain is identified as being from communal heating systems, insulation of pipes, reduction of boiler flow temperatures, ventilation of service voids (spaces within the structure for service pipe work, such as gas, electricity, water and drainage) must all be considered.

[edit] Shading, reflection and protection

There are various options to provide shading to limit solar heat gain through windows facing South through to West. Internal shutters can provide some protection, as can curtains, but it is preferable to provide external protection so preventing the sunlight entering the building.

External protection can be provided by a brise-soleil or an awning. These are most suited for South facing windows and walls, giving protection from high level sun. Vertical shading is more suited to windows facing East or West, giving protection from low level sun.

External blinds (e.g. roller blinds) and shutters will provide the most protection, but have the disadvantage of restricting daylight and views of the outdoors.

[edit] Means of ventilation

Ideally, ventilation should be passive, so avoiding the use of additional energy needed for fans or air conditioning. A disadvantage of reliance on fans or any form of a mechanical system is that they use energy, which may mean that they are not a viable option for dwellings occupied by a household on a low income.

However, for ground floor dwellings and those in apartment blocks with windows opening onto access balconies where there is a need for security, and dwellings in noisy locations, window opening may not be practical or appropriate. In such locations an effective mechanical ventilation system may be necessary. Whatever the system it should be capable of achieving the high levels of air change rate required for purge ventilation without impacting the residents, i.e., it must be acoustically attenuated and sized appropriately.

Where security alone is an issue, an option on refurbishment is the installation of windows that incorporate fixed secure louvered panels.

[edit] Occupier behaviour

Limiting heat gain and reducing indoor temperatures requires the active participation of the occupiers. This includes use of any means of shading from the sun, and understanding appropriate day and night ventilation.

If possible, occupiers can gain relief and cooling where there is a cool room, such as a North-facing room, within the dwelling (or building). However, influencing behaviour has been found to be difficult.

[edit] Natural ventilation

Ventilation during the day by opening windows is only useful where the outdoor temperature is lower than the indoor. Ventilation at night with high air change rates, to replace warm indoor air with cooler air from outdoors is important to ensure residents can sleep and heat built up over the preceding days is liberated.

Typical background ventilation rates in UK dwellings are approximately 0.5 air changes per hour (ach). Many mechanical ventilation systems have an ability to provide a boost level of ventilation. On site measurements have revealed that this may only be an increase of 25 to 50%. Doubling the ventilation rate to 1 ach would be very unusual for a typical mechanical system.

Purge ventilation is considered to be at least 4 ach, i.e. eight times greater than the normal background ventilation rate, which no mechanical system could achieve unless specifically designed to do so. Achieving purge ventilation through opening windows assumes that the windows are wide open.

[edit] Air movement

As mentioned above, as well as air temperature, the risk from overheating is also affected by other factors, including air movement. Air movement helps the body cool principally by evaporation, depending on the Relative Humidity.

There have been studies and a recent review on the effectiveness of fans to provide air movement during heat waves (Gupta et al, 2012) and, a recent study involving eight healthy males, found fans were effective during hot and humid periods (Ravanelli et al, 2015). However, fans do not replace the requirement for adequate ventilation.

[edit] Comfort cooling

Installing a comfort cooling system is, in terms of solving an overheating problem, a fail-safe solution, in that whatever is the scale of the problem, a system large enough could be installed. However, this solution should only be considered when all measures to minimise the heat gains have been taken and the remaining options for heat rejection can be demonstrated to be insufficient to provide a safe internal environment.

Refrigeration systems are not 100% reliable and the impact on occupants of a failure must be considered. Any mechanical system has an on-going maintenance and running cost. These facts alone may make this option inappropriate for households on a low income.

The wider impact on the local environment must also be considered as heat rejected from one household will tend to increase the air temperature in the local micro-environment, increasing the risk of overheating in adjacent households. These systems may also generate significant noise when running and this may not be acceptable to adjacent households leaving windows open at night to reject heat build-up.

[edit] Assessing potential effectiveness of remedial measures

There is currently no large body of evidence showing the results of undertaking a variety of remedial measures to overcome cases of excess heat. The number of cases is increasing and remedial measures are being implemented, but it will be some time before a 'catalogue' of solutions is available that can be reviewed for appropriateness for any given case of excess heat.

In the absence of this knowledge base it is suggested that a mix of judgement and mathematical modelling is used to assess the effectiveness of any given combination of remedial measures.

If the source of the heat gains can be very clearly identified, for example; poorly controlled storage heaters and gains from poor thermal insulation of the domestic hot water cylinder, then the remedial measures can be clearly identified without the need for further assessments.

However, where the source of heat gains is less clear, and cannot be largely removed, the effectiveness of reducing gains combined with increasing heat rejection must be assessed. This requires that the assessment method captures the dynamics of the thermal processes within the dwelling and its surrounding micro climate. It is suggested that this can only be effectively undertaken through the use of full dynamic modelling.

Great care needs to be exercised when undertaking modelling to ensure that sufficient detail is used to accurately capture the dynamics of the key elements of the building, but it should not become an exercise in modelling every last detail. Over complicating the modelling will significantly increase the time and cost of modelling. The key aim of modelling is to allow the relative effectiveness of a range of remedial solutions to be assessed, both individually and then as a combination.

In this way a solution can be identified that is appropriate for a dwelling, which may differ significantly from the appropriate solution for the adjacent dwelling, with for example, a different orientation.

[edit] Heat transfer process

[edit] Sources of heat gain

Overheating1.jpg


[edit] Potential measures to minimise heat gains

Overheating3.jpg


[edit] Thermal mass and the effectiveness of night ventilation

Overheating2.jpg

If 0.5 ach = 25 l/s and the inside/outside air temperature difference is as above, cooling capacity at night = 300 W. At 4 ach, cooling capacity = 2,400 W.

Useful levels of heat rejection only occur when inside/outside air temperature difference is significant. Therefore during the day, gains are not rejected but result in internal air temperature rising above that outside. Heat is also stored in building mass.

Typical internal heat gains:

  • DHW cylinder 3.0 kWh/day = 125 W (continuous).
  • Occupants 60-80 W each (continuous).
  • Fridge/freezer up to 2.0 kWh/day = 83 W (continuous).
  • Cooking 1.6 kWh/day (intermittent).
  • Lights 30-200 W (intermittent).
  • IT and audio/visual up to 250 W (intermittent).

These figures show clearly that normal background ventilation may at best only reject on-going heat gains, not heat stored in the thermal mass of the building.

[edit] Recent data on internal heat gains for UK dwellings

Overheating4.jpg



--BRE Buzz

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