In the third and final part of AJ’s three-part mini-series, exploring how the profession could tackle global warming, we examine UK building regulations to see if they are set up to accommodate a warmer climate.

Overheating is a serious risk for many architects and masterplanners. Though some professional organisations have produced helpful technical guides, there are vast differences in the UK’s approach.

So, how far has existing legislation progressed on handling the country’s increasingly hot climate, and how much further will it need to go?

‘Prior to 2017, there was no standardised methodology for addressing overheating risk at all,’ says Becci Taylor, director and retrofit leader at Arup. 

In the meantime, guidance has been developed by professional bodies. Taylor was involved in the TM59 project, which began in May 2017. Chartered Institution of Building Services Engineers Document (CIBSE), which defines how to assess the risk of overheating in homes. It built on earlier guidance outlined in CIBSE’s TM52 (2013) on avoiding overheating and TM49 (2014) on designing for summer years in London. 

In 2019, the available guidance was further fleshed out by the Good Homes Alliance’s (GHA’s) overheating risk tool This guide provides early guidance on the design of new homes. It scores them based upon inputs like site characteristics and typology. 

Only in 2021, did the government introduce their own Overheating: Approved Document O, marking the first technical performance requirements to ‘protect the health and welfare of occupants of [a] building by reducing the occurrence of high indoor temperatures’.

‘What Part O has done is to take industry-interested documents – which some people did use voluntarily to [create] good design – and make them statutory,’ explains Barny Evans, sustainability director at planning and development consultancy Turley.

Part O’s new-build legislation includes limits on glazing to reduce solar gains (dictated by façade orientations), minimum free areas for cross-ventilated buildings, and requires thermal modelling calculations to be carried out using either its ‘simplified’ or ‘dynamic’ model (incorporating CIBSE’s TM59). 

Its catalogue of ‘acceptable strategies’ for reducing overheating risk include ‘fixed shading devices’ like awnings or shutters, adaptations to orientation, g-value or depth of glazing, balconies, and shading from existing structures.

But experts claim that Part O contains significant loopholes and needs to be addressed in order to protect UK building residents from dangerously high temperature, particularly when tackling the existing stock. 

Anna Bardos, principal sustainability consultant at Max Fordham engineers, explains Part O ‘relates only to new build residential, leaving other building types subject only to the limited solar gain guidance in Part L’. 

‘Expanding Part O to cover non-residential buildings and retrofits would be a good start’

‘Expanding Part O to cover non-residential buildings and retrofits would be a good start,’ says Bardos, who insists many local authorities are retrofitting their housing stock to meet net zero commitments on thermal performance, but overheating considerations ‘can be entirely absent’.

Furthermore, Bardos says existing legislation mostly uses ‘2020 weather files’ to calculate performance, covering the years 2010-2040. This ‘does not look far enough into the future, considering buildings’ expected lifespans’, says Bardos, who typically uses ‘2050 and 2080 weather files’ to analyse a project’s climate resilience at Max Fordham. 

Evans agrees Part O’s thermal modelling methodology could be more sophisticated, particularly the ‘simplified’ method, which he says clients find ‘buggy’ and therefore unhelpful to aid compliance with the regs. However, Evans says the dynamic model is much more robust, as it can ‘evolve’ based on a weather file input which can be updated with changing climate projections. 

The dynamic model is currently reserved for if ‘the standards of the simplified method cannot be met’, for example when concerns such as high noise levels clash with overheating mitigation, because an occupant would be unlikely to open their windows. 

‘Requirements for overheating, fire, daylight and sunlight, and acoustics [are] often in direct competition’

Simon Henley, principal at Henley Halebrown architects, says current overlapping regulations are ‘incredibly complex’ for architects, with ‘requirements for overheating, fire, daylight and sunlight, and acoustics often in direct competition with each other’, which can drive power-hungry cooling solutions to the fore. 

But Bardos insists designing for overheating alongside these considerations is ‘certainly possible’, as well as ‘necessary’, and can help drive ‘excellent passive environmental design’ like Max Fordham’s Stirling Prize-shortlisted Central Somers Town project, which features moveable external shutters to control thermal gains, and St John’s Hill Peabody Estate project, which uses acoustic louvres to achieve passive ventilation while filtering out noise.

Local plans and planning policies are guided by the National Planning Policy FrameworkThe challenge of overheating mitigation continues to grow. 

Planning requirements for overheating ‘vary greatly’ between local authorities, explains Bardos, and can take years or even decades to update, leaving existing building stock at risk. 

And Evans says many local plans are ‘vague’ about minimising overheating risk, failing to offer clear guidelines. He points to the Greater London Authority (GLA), which now uses the GHA’s overheating tool, and requires completion of an overheating checklist as part of an outline planning application, as a good example of an authority planning well for new buildings. 

As for retrofit, Evans suggests a planning policy rule whereby planning a refurbishment within a ‘high-risk’ area – say, a city’s urban heat island – automatically triggers a mandatory requirement to address overheating, or else a more ‘qualitative approach’ such as speaking directly to residents about their thermal comfort. 

Evans says planners, architects and engineers also need to open up a conversation about how British design norms ought to change to meet the heat – for example, attitudes towards external wooden shutters, which he says ‘are probably the single best answer to overheating’, but barely have a market in the UK. Evans cites residential heat networks as another example of a heating solution that is popular, but not efficient. 

Bardos also encourages more communication between planners, developers, and design teams, particularly at early design stages, ‘so that considerations of passive environmental design shape a scheme from the outset’. 

She explains: ‘Local plans often state a “preference” for dual-aspect and passive design solutions but, once a scheme has progressed, these may be presented as unachievable.’

Planners also have a responsibility to ensure ‘aesthetic considerations don’t preclude the passive environmental design solutions which deliver improved occupant experience and low energy use,’ says Bardos – especially in heritage settings. 

Tom Fox, Senior Associate at Masterplanners We Made That, says local authorities ultimately ‘need to be able to rebalance the short-term focus on maintenance costs with longer-term value’, thinking creatively about temperature-moderating devices like street trees, soft landscape and blue infrastructure in cities. 

‘Local authorities are well placed to meet these challenges and implement changes at scale,’ says Fox, and ‘stand to gain from the economic, health and social benefits that come from more comfortable, inclusive and lively places’ by executing the changes in an equitable way.

Perhaps controversially, Evans has a final warning to architects, engineers and planners, to not get ‘too obsessed’ with overheating. He is concerned that designers may focus too much on overheating, at the expense of other design features. 

As Evans points out, ‘You could design a building which is almost impossible to overheat but terrible to live in.’

And he warns against ruling out mechanical cooling methods altogether – lest we risk leaving ourselves unprepared  for temperatures so high (like 2022’s record 40C) that you can’t simply ‘passive your way out’.

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