Avoiding overheating

Rising summer temperatures and improved air tightness of buildings mean that heat gain is a challenge as well as heat loss.

Jubilee library, Brighton

The CO2 challenge for new buildings is likely to become much more about controlling heat gain than heat loss in the relatively near future given the energy efficiency of new buildings.

Climate change is expected to result in considerably warmer and drier summertime conditions and wetter winters, with increased incidence of extreme weather. Much of the existing housing stock is inherently air leaky and thermally massive, and will operate adequately at higher external temperatures, but still needs to improve its performance in winter months. However, there is a risk that highly insulated new housing without adequate thermal mass will overheat internally to dangerous levels, a problem that has already been experienced in some energy efficient developments.

Recent evidence from mainland Europe has shown that heatwaves can have deadly effects, particularly on the more vulnerable citizens living in older housing. The 30,000 heat-related deaths across western Europe in Summer 2003 brought this sharply into focus. This problem is likely to become worse and it is therefore important that adaptation measures are introduced. Without careful management, the UK could soon emit more CO2 in summer than in winter in the future as the need for cooling increases.

At the site and building scale, it will become increasingly important to model the internal temperatures of new development arising under different climate scenarios. Buildings developed over the next few years will be expected to stand for over 100 years, so we need to be confident that they will still be comfortable to occupy within that timeframe.

It is possible to maintain comfort and cool buildings through passive measures that reduce the need for air conditioners or fans. Increasing the thermal mass properties of buildings is likely to be central to creating sustainable buildings in the future. The role of thermal mass in preventing increased energy demand for cooling is an important benefit, allowing a building to support a more comfortable temperature over a 24 hour period.

Alternative approaches include adding insulation on the outside of thermal mass which slows the flow of heat so buildings can maintain stable internal temperatures rather than using mechanical heating and cooling to ‘fight‘ diurnal variations with the weather outside.

Different building types will need different approaches, depending on their patterns of use and the desirable temperatures of operation. Optimum levels for different building types should be used to steer design solutions. Considering a building’s operation should include controlling heat gains from equipment, particularly computer servers and systems which create significant heat and often require cooling to operate at an optimum.

Buildings such as that at 160 Tooley Street in London have used thermal mass as a major driver in the design strategy. However, it needs to be recognised that the urban heat island effect can reduce diurnal temperature variation, reducing the potential for cooling of the mass during the night hours, and therefore making interventions like this less effective.

In buildings which are naturally or passively ventilated there is a need to control ventilation when external temperatures are higher than internal temperatures. In addition, the security of night time ventilation must be considered to ensure a building can operate as the design intended.

The Jubilee Library in Brighton is another example of how external and internal features can be designed to reduce the potential for overheating. It uses heavy concrete mass to achieve thermal stability while local climatic conditions are relied upon to drive passive environmental control.

Across a site other design solutions can include:

• passive cooling using planting for shade
• changes to the size and positions of windows
• using roof overhangs to provide shade for high-sun angles
• use of smart glazing materials.

The use of shading in a site’s design can exclude up to 95 per cent of solar gain, thereby significantly limiting direct heat gain from the sun. Such an approach needs to be well balanced so as to not reduce natural daylighting. CIBSE’s Climate change and the indoor environment: impacts and adaptation provides guidance and case studies on how to limit heat gain through maximising shading and other approaches.

Across a city, interseasonal thermal storage could have significant benefits. This approach allows a large capacity for thermal storage and can therefore help to smooth out temperatures across seasons not just days or weeks. It is used in The Netherlands, where a relatively simple regulation states that over a two-year period a scheme must not take or reject heat to the ground or groundwater. This means schemes always have to be designed in thermal balance and systems are always efficient without the need to be more prescriptive.

The Environment Agency takes a similar regulatory approach in its policy document GP3 (policy 10-1).

Evidence and data on the conditions that designers will need to take into account should be fed into the evidence base for local development frameworks and guide development control documents and discussions. The production of a local climate impacts profile can be useful in this regard.

The role of public spaces in supporting comfortable microclimates should be considered alongside the internal design and operation of buildings. Designing public spaces to cope with extreme weather – both heat and rain – can help both provide shade and cooling for adjacent buildings as well as shaded and protected space to alleviate the impact of hotter summers in a neighbourhood.

Given that temperatures will be increasing as a result of climate change, there is a need to provide future temperature headroom in building design. This is particularly important when it comes to maximising the future natural ventilation potential of buildings, instead of locking them permanently into air conditioning or operation only at a specific range of external temperatures. Extensive vegetation around buildings on a site is part of this approach – thereby buffering the building to increasing temperatures.

Priority: reduce energy demand
Tags: energy, buildings and spaces