The real reasons for overheating in residences – and why improved insulation standards aren’t the problem
Recent media coverage of overheating conditions in residences, particularly in new homes, has helped to raise public awareness of a serious issue increasingly affecting many occupants. However, the lack of general media analysis of the real causes has meant that journalists have speculated as to why more homes are experiencing overheating conditions. Unfortunately, this doesn’t help to stimulate an informed debate and the real reasons need to be addressed.
In September The Sunday Times highlighted ‘a series of misery tales about eco homes, which have left many owners complaining about overheating’. The paper implies that residences designed to high energy efficiency standards are the main reason for the overheating phenomenon. It might seem logical that since modern homes have better thermal insulation standards that it’s the main reason for homes to capture the heat in summer and stay hot. The truth is more complex, as evidence gathered by the Zero Carbon Hub, BRE and the NHBC Foundation has shown.
We know that overheating conditions can significantly affect occupant comfort and wellbeing. Some housing associations, for example, are recording room air temperatures above 30˚C for prolonged periods in the summer and home owners in private apartments are threatening developers with litigation unless conditions can be improved. Guidance from CIBSE (based on field study data) advises that thermal comfort and quality of sleep begin to decrease if bedroom temperature rises much above 24˚C and living rooms should have limited periods above 26˚C.
Global warming and the urban heat island effect are contributing factors. The recently published Zero Carbon Hub report, ‘Impacts of Overheating’ (2015), states: Studies agree that the number of heat-related ‘excess deaths’ are expected to increase in the future. A number of estimates have been made, but the most recent suggests a tripling of current levels, from approximately 2,000 to 7,000 heat-related deaths per year by the 2050s, as a result of climate change and a growing and ageing population.
Housing developments are increasingly constructed in dense urban locations with little or no green landscaping provision, which would naturally moderate peak summer temperatures. Dark road surfaces, pavements and the close proximity of adjacent buildings can mean that new homes suffer the consequences of warmer temperatures arising from this unfavourable microclimate. The urban heat island effect can increase city temperatures to several degrees above suburban and rural areas, especially at night time. It doesn’t help an occupant trying to get respite from a hot day when the evening air temperatures remain high. In very dense locations the heat rejection from commercial chilling equipment can be a particular problem in raising ambient temperatures. Ironically, installing cooling equipment in the home may seem the only solution to coping with hot external conditions, which further worsens the atmosphere as the equipment expels exhausted heat into it.
Building form and orientation, thermal mass and the amount of glazing in new homes also affect overheating risk. Traditionally most homes had double aspect elevations with opportunities for cross-flow ventilation. This was great for daylight penetration as well as enabling high natural ventilation rates (when the occupants opened the windows on both elevations). Many new developments give apartments only a single aspect, which significantly reduces natural ventilation. Mechanical systems can help provide air movement within the dwelling but they tend to be specified to provide minimum ventilation rates for building regulation compliance rather than the higher rates needed to mitigate overheating. It should be noted that Part L only provides ‘a method for assessing the propensity of a house to have high internal temperature in hot weather’. This does not mean Part L is an adequate method for assessing overheating risk. The Part L check is simplistic, treating the dwelling as a single zone, taking insufficient account the of proposed occupancy patterns for a dwellings, and its treatment of solar gains is simplistic. It does not consider the urban heat island effect or climate change.
Comfort conditions can be checked more effectively using dynamic thermal modelling – a tool that can be used at the concept or pre-planning stage to assess whether a proposed building form, orientation, thermal mass and glazing configuration would result in overheating. It should be stressed that this isn’t a mandatory requirement but we’ve used it on many residential projects to help inform and optimise design proposals. Some clients are now asking for comfort modelling as a standard assessment, so that overheating risk doesn’t become a problem that is discovered only once the dwelling is sold or let. Thermal modelling can also be used to test whether solar shading should be installed, how glazing specifications can be optimised or whether balconies provide sufficient self-shading to limit solar gains.
Operational issues can also be a contributing factor to overheating. For example, poorly insulated communal heating pipes in shared corridors running at high temperatures can warm up corridors all year round. Heating systems should be designed with lower temperatures and thicker insulation, as specified in many other European countries that don’t find this to be a problem. Another performance gap factor can be that mechanical ventilation systems aren’t delivering the intended air flow rates, so heat built up within the dwelling isn’t discharged effectively. Better attention to installation methods, testing, commissioning, and good user guidance can help.
In summary, it’s essential that we take overheating concerns seriously in residential design, but we need to understand and evaluate the many factors influencing comfort and not accept that higher fabric standards are the predominant cause.
Ashley Bateson is partner and head of sustainability at Hoare Lea and a member of the RIBA Sustainable Futures Group
Environmental Design (Guide A), Chartered Institute of Building Services Engineers, 2015
Overheating Evidence Review, Zero Carbon Hub, 2015
Understanding Overheating-Where to Start, NHBC Foundation, 2014