Building Facades - Can they mitigate rising urban temperatures?

Brian Murphy | Green Building Encyclopaedia

A recent announcement anticipated temperatures of 48 degrees C in London in 2050, 14 degrees higher than the Midlands peak of 34 degrees C in July 2016. Urban climate specialists argue that building façade design needs to respond to alleviate the consequences of future high temperatures.

Today’s 100% glazed sealed facades, favoured by letting agents and developers, (and often essential in the noisy trafficked and polluted streets of urban areas) means air-conditioned interiors are essential in office buildings, to cope with solar gains on the east, south and west facades. Air-conditioning puts a large financial burden upon businesses and additional burden on the urban climate by adding more heat to urban air for the self-same air conditioning to compete against.

With significant differences between expected external summer urban temperatures and internal chilled temperatures of 20 degrees there will be heat flow inwards by conduction through the façade, worsened by the high U values of most glass facades. So what are the solutions?

Reducing the external air temperature

One approach is to throw stable or rising heat away from the surface of the building to reduce the temperature differences between external and internal. Smooth flat thin facades do little to help with this, but there are a number of techniques that can achieve it.

Undulating the surface of the façade, stepping the surface of the façade or adding deflecting or diverting devices are all techniques that separate the warm air from the façade and keep it away. Undulating the surface in the vertical plane is achieved by pushing the façade in and out relatively to the structure grid; the façade then includes special profiled parts to accommodate sloping planes.

Steps in the façade can be achieved by sloping façades in the vertical plan with horizontal connecting planes. Deflecting and diverting devices could include cantilever solar shading usually above windows or cantilever light shelves, usually at clerestory height through glazing transom bars.

What are the barriers preventing hot air separation from the façade?

Solar shading usually consists of cantilever frames supporting solar shading blades, which are usually arranged sloping down and outwards on a southerly façade. From below the same blade configuration slope upwards and inwards. As any warm air rises up a façade it may encounter these solar shading blades and be redirected back towards the glass, undoing the good work of an undulating façade. Light shelves may be more effective in the form of a horizontal plane that pushes the warm air away and up.

There are a lot of poorly considered external solar shading systems which are either non-functional at mid-day, cause more glare in the gaps between the shading blades adjacent to columns, or stop and start at window edges and leak sunlight and glare at their ends.

There are also the ‘architectural frippery’, (e.g. Blue Fin building Southwark) types of solar shading systems, which many architects are now copying with varying details - sometimes applied on all facades including the north where the sun does not shine!

Developers and letting agents want good net to gross ratios in floor areas and will not want to create sloping and thicker walls. Undulating walls may be measured at the widest point of the wall slope from the structure or, at best, half way up the wall to half the thickness.

Quantity Surveyors allegedly looking after the interest of their client will ‘value engineer’, anything that is more expensive than the most basic installation, and we do not yet have Quality Surveyors who can engage robustly with Whole Life Costs or TOTEX (Total Expenditure TOTEX = Capital Expenditure CAPEX and Operational Expenditure OPEX).

Issues with open facades

Open facades with opening casements allow ventilation and cross ventilation, smoke venting and replacement air intake. Opening casements can be in many different ironmongery configurations each having different ventilation, weathering and acoustic properties. One simple approach is top hung open out. This is good for weathering, but warm air rising will be scooped into the building; also street level generated noise will rise up the building and bounce off the sloping glass into the space behind these windows.

The next approach is bottom hung and open in, again good for weathering and less likely to scoop warm air in, but will allow the street noise in. Undulating facades with opening casements will need careful thought to maintain the integrity of the façade and achieve and maintain the rejection of the warm air rising away from the glass and opaque panels.

The latest ironmongery development is where the casement is detached top and bottom and pushed away from the casement, parallel with the vertical façade. This allows external air to flow unhindered past the opening, between the façade and the casement, in any direction and potentially sucking out heat as it flows past or providing ventilation to exchange air, release humidity and refresh poor indoor air.

Cross ventilation may be problematic at low level; if the external paving and wall fabric is heated by solar gains and the outside air temperature is raised, it may be hotter than the internal shaded air and end up warming the building interior instead of cooling it.

Other design points

In shaded areas of the building below the urban roof-line, facades need to be low thermal mass (masonry is no longer the ideal wall construction at low level). Any 100% glass facades need thin, high thermal mass materials. Light coloured facades can create more reflected light glare for any building opposite. Above the urban roof-line opaque walls and roofs need high thermal mass or decrement delay characteristics to prevent solar heat gain through lightweight opaque materials.

The common approach to adding additional floors to tops of buildings is lightweight construction with metal cladding or roof tiling. This is highly risky. Building Regulations address limiting heat loss in winter but ignore solar heat gains in summer; the consequence is that solar heat can get in through an opaque lightweight external envelope when insulation materials are chosen with only their U value in mind.

Once inside the heat is trapped with the result that the top floors of buildings often overheat. This happens already and will only get worse as summer temperatures increase. Architects need to be designing smart and effective facades right now.

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© GBE NGS ASWS Brian Murphy aka BrianSpecMan 14th August 2016 - 19th November 2016

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