How should practices go about building for the weather of the future? By Bill Gething
Despite government hesitation in pushing the construction industry towards zero carbon in this economic climate, there is only one direction that it can head in. As part of the global efforts to avoid catastrophic climate change, a commitment to reduce the energy consumption and associated carbon emissions of buildings is an essential component of the government’s legally binding strategy to reduce UK carbon emissions.
The previous government’s original roadmap, set out at the end of 2006, triggered a period of rapid change in design and construction practice, as the industry responded to the ramping up of energy performance standards at an unprecedented rate. The drive to reduce carbon emissions is embedded in regulation and correspondingly ingrained in the industry.
Designs must anticipate change
However, the parallel climate change agenda, adaptation, is less well embedded. We have been slower to recognise that the climate is already changing and that, because buildings last a long time, we need to design differently to take projected changes into account (see box). That said, although impacts have yet to be reflected in building regulations and standards, planning policies increasingly require submissions to address the changing climate, and some clients with a long-term view are starting to ask for future climate to be taken into account in building design. The legal profession has also floated the issue as a potential area of future litigation – because as a generally accepted phenomenon climate change should be taken into account in design.
The Technology Strategy Board’s Design for Future Climate programme, launched in 2010, aims to help the industry explore how design and construction practice should respond to the challenge of adaptation. This is summarised under the three headings of Comfort (particularly summer overheating), Construction (the impact of changing conditions on structure, materials and detailing) and Water (too much and too little).
The TSB programme funded the design teams of 50 live projects to consider how climate change should affect their design proposals and what measures their clients might take forward in the projects. In doing so, it has brought this emerging issue to the attention of a wide cross-section of the industry and enabled a significant number and range of projects to act as pathfinders. The teams, many collaborating with academics, investigated the range of potential change and how this might affect their proposals over the time frames relevant to their clients. They explored the available climate data and how it has been translated (or not) into usable design information, identifying gaps in data and guidance and considering how they might be filled. They developed ways of communicating issues and adaptation strategies to clients, and the cost benefit of any measures proposed.
The teams’ findings are available through the TSB _connect website (https://connect.innovateuk.org) and are necessarily tailored to the individual projects, but there are some common themes that are worth noting.
Airtightness equals summer overheating
The overwhelming focus for design thinking across the projects was how to deal with summer overheating, perhaps driven by inter-related circumstances. For many building types, summer comfort has not been a particular design driver (my environmental design textbook at university did not mention it once – though admittedly this was in the 70s…), there being an assumption that by opening a few windows, buildings would look after themselves. As we improve insulation and airtightness, and increase glazing to improve daylighting (to reduce artificial lighting), all in the name of energy conservation, it is perhaps not surprising that buildings overheat in summer. There is evidence that this problem is growing; an unintended consequence of our focus on reducing space heating and the trend to increase densities of occupation and the use of IT etc just adds to the problem. This is also one area where basic climate data has been translated into the weather files that industry-standard dynamic simulation models can use to analyse environmental performance, so the teams could use their familiar software tools to identify problems and test solutions. The teams relished the architectural possibilities of developing new design solutions – of form, layout, materials and facade treatment – to meet the challenge passively rather than rely on mechanical services, however efficient.
‘By definition, past climate records are out of date. Just to cope with current conditions, we should be using 2020, not historic data’
Yesterday’s weather is old news
The availability of future weather data also highlighted the inadequacy of using averages of past records to design buildings in a changing climate. By definition, past records are out of date – after all, in terms of climate data (which averages values over a 30 year period), we are already in the 2020s (the 30 year period centred on 2025). Just to cope with current conditions, we should be using 2020, not historic data.
Design data to reflect changes in patterns and intensity of rainfall and wind to inform materials selection and detailing are less well developed. Climate statistics for rainfall, for example, are expressed as daily or hourly volumes rather than the short burst intensity (a couple of minutes) needed to design rainwater systems (see box). This was found to be typical of many construction related design issues. The teams found that the necessary regulatory frameworks and standards are in place; they just need reviewing to reflect the speed and magnitude of projected change. Quite how and when this will happen remains to be seen.
Some useful methodologies were explored that could form the basis of future standards, but climate change is a moving target with multiple layers of uncertainty that make definitive judgements difficult. Each project has to grapple with these uncertainties and reach its own conclusions on what allowances to make on an individual basis. This cannot be efficient or consistent. What the industry needs is ‘reasonable’ minimum standards based on consensus, and the Design for Future Climate projects provide a rich source on which to build that consensus.
Design for Climate Change by Bill Gething and Katie Puckett aims to provide some background to the issues, to show how the TSB teams selected the conditions to design for and to draw out lessons from across the projects illustrated by examples from them
How big should a gutter be?
The intensity of a rainfall event is described in terms of its expected frequency, or return period.
For example, in an analysis of 100 years of rainfall data, a 1-in-20 year event would be expected to occur five times within that dataset, on average once every 20 years. In any given year, the probability of such an event occurring would be 1 in 20, or a 5% chance. A 1-in-1000-year storm would obviously be much more intense, but has only be a 0.1% probability of happening.
To work out exactly how intense an event a building should be able to withstand, designers must decide how severe the consequences of the system overflowing would be, and choose one of four risk categories. A building with conventional eaves and gutters that can overflow with no significant impact would fall in a lower category than a building with internal gutters, where an overflow could cause significant damage or affect some critical process. For each category, an associated multiplier is applied to the anticipated life of the building to give an appropriate return period on which to base the design.
A series of contour maps in British Standards provide design rainfall intensities in litres/second across the UK for return periods of 1, 5, 50 and 500 years. An additional map shows maximum probable intensities for buildings which require the highest level of security. The map with a return period that is equal to or greater than the design return period should be chosen: for a building with an anticipated life of 30 years, the design return period is 45, and so the 50-year map should be used to set the appropriate design rainfall intensity.
Extract from Design for Climate Change