Wind effects around large buildings

As buildings increase in size, the wind has to work harder to find routes around and through them as it crosses an area. Higher buildings disturb more of the higher, faster wind flow, creating downdraughts towards street level as the air seeks to find the easiest routes around the obstruction. This can be particularly noticeable as the changes in wind direction create added turbulence as well as additional speed, especially as the wind passes around building corners.

The design of larger buildings should consider and aim to mitigate the impact of the redirected wind that these buildings create, particularly where it directly affects people.

Wind impacts can be predicted and then used to inform design amendments by using physical wind tunnel testing or computational fluid dynamics (CFD) modelling (see Figure 11.2). Wind tunnel testing tends to give more reliable results and was in the past used for large projects, while the falling price of CFD means that its popularity is increasing, albeit with relatively little verification in practice. CFD is not as effective at predicting the gusting that can make a place noticeably uncomfortable.

Observations of annual regional wind data for a number of years provide the basis for predictions of average and extreme wind events. This information is used in a sufficiently large model to show how the local topography and built environment modify the wind. All wind directions should be considered, given that in the UK non-prevailing winds occur for much of the time. The results present the impacts of wind due to the new development on pedestrians both inside the development, and in the surrounding area. Ideally this should include planned future developments for their cumulative effects.

Results are normally presented using the Lawson criteria for ‘acceptability’¹² (see Figure 11.3). This suggests a range of tolerable conditions for different activities with an exceedance across a year. The criteria relates mainly to public safety and has relatively little to do with today’s perceptions of comfort for such activities as sitting outside in a cafe. For example, the Lawson ‘sitting’ criteria permits wind speeds up to 2.5m/s, but in terms of comfort this would have a chill factor of some 5°C below the local air temperature, (see Figure 11.4). So if the aim is for a space offering a reasonable level of year-round outdoor comfort, such design studies should also integrate solar access and orientation, as well as wind-chill factors, wind speed and local turbulence.

Figure 11.1 More redirected wind is concentrated at gaps and around edges as buildings become higher and wider.

Figure 11.2 A wind assessment, with red areas being the strongest wind. These areas will be in different locations depending on wind direction, so it is important to test for different potential conditions.

Street pollution flushing

As buildings get larger and more continuous, the deeper street canyons they create tend to trap pollutants, which in turn is likely to compound the UK’s continuing problems with air pollution.

Major urban vehicle routes, where most pollutants are generated, benefit from a controlled level of wind flushing, at least until such time as policies promoting pollution-free electric vehicles are implemented.

Methodologies for predicting flushing rates based on street massing and orientation are used in various countries worldwide, often based on specialist CFD modelling. However, this is not generally a policy requirement in the UK. Particular care is needed with specialist input because many CFD software packages are not well suited to this analysis. In simpler terms of street massing, broad guidelines are available from the planning guidance prepared for other cities.

Reconciling conflicting objectives

Addressing the desire to keep pedestrian wind impacts low, while providing a good level of street flushing, can create a conflict of objectives. However, careful design of building massing can be used to harness both of these objectives, as shown in Figure 11.5. Wind can be used to generate indirect eddy flushing in streets below, while its main air flow passes above the street from and between higher towers. However, the higher and more continuous the street canyon becomes, the more difficult this is to achieve. This reduced flushing might be acceptable if the level of pollution is lowered by reducing vehicular access. For wider streets, higher wind speeds tend to dominate, so more pedestrian protection is needed.

Figure 11.3 Lawson criteria thresholds for tolerable conditions. These relate to health and safety, not comfort.

Figure 11.4 Wind-chill factor reduces the comfortable temperature below the measured air temperature by the amount shown in the table. ¹³

Dwelling indoor daylight

Access to daylight in planning terms relates to building massing and fenestration, providing a consistent level of daylight indoors. Daylight availability is largely governed by the amount of sky view unobstructed from a window. Other buildings, particularly of large massing and grouped for high density, will have a substantial impact on daylight accessibility.

Figure 11.5 Using building forms to harnessing eddies to flush pollutants from the street but keep the main air movements away from pedestrians.

Ambient daylight varies with the time of day and weather conditions, so a standardised diffuse overcast sky is assumed for the assessment. For consistency and quality for daylight, light from the north is generally regarded as most favourable. Assessment of daylight is provided by the BRE guide Site Layout Planning for Daylight and Sunlight: A Guide to Good Practice, which provides metrics for quantifying different aspects of daylight access.

Although the BRE guide suggests minimum room daylight criteria, these are only guidelines and many desirable homes in urban areas, for example in 19th-century mansion blocks, have lower levels than recommended by BRE in some rooms. Most new urban developments have a proportion of dwellings that do not satisfy current guidelines too. Over the years precedents have been used by developers to progressively decrease the proportion meeting these criteria, without discussion about whether this cumulative reduction in amenity is reasonable. So while in an urban development 15 years ago some 95% of apartments might have satisfied the BRE guide, in some developments today as few as 50% might. Specifying the BRE guide in planning policy without a pass standard, how much of a development should meet or exceed the guideline levels, does not deliver any particular expectation of daylight availability.

Figure 11.6 A retrofitted wind downdraught canopy partially reduces a tall building’s impact on pedestrian areas.

In practice, good design could reverse this trend. Instead of assuming that windows are of constant size across the facade, their size could increase where daylight availability is less and, conversely, be more modest where daylight availability is higher (and reduces the risk of overheating). Forcing such design thinking would need clearer expectations of minimum daylight levels in policy.

The BRE guide suggests average daylight factor criteria of 1% for bedrooms, 1.5% for living rooms and 2%for kitchens. In practical terms, for high-density apartments window area is prioritised for living rooms and bedrooms. The kitchen is typically set deeper into the building and hence has less daylight. In a modern kitchen, work surfaces are located below wall units and have their own lighting, and the standing position shields the window daylight, so the BRE guide’s kitchen criteria is probably now obsolete.

The England and Wales legal ‘right to light’ differs from a planning assessment of daylight. This right to light is relevant only for existing buildings, where it is gained only after 20 years of uninterrupted use. It also uses different and much lower pass criteria.

Sunlight for dwellings

Sunlight is dealt with as an amenity entirely separately from daylight access. Once again the BRE guide offers a metric to quantify this amenity, namely the annual probable sunlight hours (APSH). It suggests the standard that habitable rooms facing between east, through south to west should achieve a minimum APSH of 25%. Just as for daylighting, although the assessment may be specified, there is no defined standard on what proportion of rooms should have this orientation or should satisfy the BRE guide’s minimum standard.

Various other countries do have defined minimum solar access standards. For example, China goes as far as specifying that habitable rooms should have an hour or so of sun access potential in December. This has a significant impact on building form and massing.

Direct sunlight has clear amenity value, particularly for balconies. On the other hand its winter heat contribution becomes less significant as thermal insulation standards improve. Instead, summer overheating is becoming an important issue. Stacked balconies positioned in front of living-room windows work well in providing sunlight access to the amenity space as well as solar overheating protection for the internal space.

Sunlight for outdoor spaces

Sunlight is a valuable amenity for public and private open space. The higher and more dense the development, the greater the challenge to get sunlight down into these spaces (see Figure 11.9).

The BRE guide suggests a criterion for public amenity spaces and rear gardens. Where new developments influence existing open space, the BRE guide suggests that a 20% loss in the existing amenity is unlikely to be noticeable.

Once again, there is an absence of any defined minimum criteria for planning purposes. Although the BRE guide’s assessment is carried out, that at least two hours of sunlight potential on 50% of public amenity space for the sun path of 21 March,¹⁴ this may not deliver a particular expectation of sunlight access. Given the importance of direct sunlight for attractive public spaces in our generally cool climate, solar access should be an important part of the planning policies and application discussions.

Light pollution

Light pollution is caused by unintended light emissions, both at the city and the street scale. Detrimental to biodiversity and often a nuisance, light pollution is a waste of energy and a source of unnecessary carbon emissions. In planning terms, metrics for defining a reasonable level of light pollution are set out in the Institute of Lighting Professionals (ILP) publication Guidance for the Reduction of Obtrusive Light.¹⁵ Different recommendations are provided for rural through to urban centre ‘environmental zonings’.

From a design perspective, best practice is to have well-defined illumination shining only on to the surfaces required, with the minimum of losses in other directions. For the external illumination of buildings, best practice is to avoid blanketing surfaces with light, but instead to highlight certain features and make use of the contrast against darkness.

The ILP guidance has its limitations. It considers only large outdoor light sources, and excludes small light sources or light escaping from indoor light sources through large

Figure 11.7 Cutting back the height of a neighbouring facade, or moving it back, can improve the amount of light entering a building.

Figure 11.8 Access to daylight within a room makes it more usable and pleasant.

Figure 11.9 The relative width of a public space compared to the height of surrounding buildings will affect sunlight penetrate and hence the positioning of appropriate public uses.

Figure 11.10 Buildings heated during the day by direct sunlight can emit heat at night, worsening the urban heat island effect.

window areas. Lighting technology has recently changed dramatically with the use of multiple small LEDs, which enables far more control over lighting direction and spill.

Renewable energy generation

The NPPF places an obligation on planning authorities to identify the most appropriate local renewable generation. In the urban environment, as a general rule the most suitable form of renewable energy is photovoltaic (PV) electrical generation. Providing PV on roofs will help decarbonise the electricity supply, a UK national policy requirement irrespective of building energy efficiency standards. Consequently, planning new roof areas and making use of existing ones becomes a valuable strategy, either for installing PV now or futureproofing for later installation.

Energy efficiency in buildings

It is national policy to reduce UK carbon emissions by at least 80% for 2050. In the meantime, almost every new building is increasing rather than reducing UK emissions. Adding just 1% to the building stock each year would mean almost a third more buildings being built by 2050. Focusing on retrofit has also proved more problematic than expected, and the Committee on Climate Change has indicated that higher standards are needed for both new-build and retrofit if the 80% target is to be met. So the original principle of zero-carbon new-build to avoid adding to carbon emissions, and using such techniques learned in the retrofit market, is still sound. Thus there is scope for the planning system to improve on the building regulations’ ‘worst permitted’ levels where this is unlikely to discourage development (for example, where local prosperity levels are relatively high). Locally supported pathfinder and demonstration projects can increase knowledge and skill levels, with a view to subsequent national rollout as the scale increases and costs come down.

The idiosyncrasies of UK building energy labelling make understanding proposed development carbon emissions a challenge. Unfortunately the ‘A-G’ energy performance certificate (EPC) scale is not intuitive: a grade B does not mean that the building emits less carbon than, say, a grade C. Indeed, it could emit double that of the lower grade C, simply because these are comparisons against different benchmarks. The unintended consequence of this is to disincentivise lower-carbon-emitting natural ventilation in favour of air-conditioning, simply because just as good an EPC rating can be achieved, even though the carbon emissions are twice as high. Asking for a simple measure of annual carbon emissions per floor area (kg/m2/yr) would enable greater transparency.

High buildings tend to emit more carbon than lower ones, largely because they tend to be air-conditioned, and the above-mentioned EPC idiosyncrasy has encouraged this trend. Countries with more transparent energy labelling, such as Germany, have developed a far greater number of more naturally ventilated towers. This tendency for higher carbon-intensive servicing solutions, and the challenges of maintaining and replacing cladding and other building components at height, mean that the building use should be one that can afford the continuing extra running costs.

Embodied energy

As building operation energy use reduces, the energy embodied in the building materials forms a larger proportion of total building carbon emissions. While the methodologies for assessing embodied energy are still at an early stage, the direction of travel is clear.

An embodied energy hierarchy for buildings and infrastructure should focus on:

1. Reducing the volume of material used (for example, using half the amount of materials halves the amount of embodied energy).
2. Reusing, by designing for longer useful building life (for example, doubling useful life can halve the amount of embodied energy).
3. Calculating, to increase understanding of the development’s embodied energy content using industry best-practice tools. This builds evidence for defining future policy direction.

Certain building types inherently use more materials. Typically a 50-storey tower will use 65% more materials for its floor area than a low-rise building. So to reduce its embodied energy to levels comparable to those of a low-rise building, a tall building should perhaps be designed, and have components selected, for a 65% longer life.

Dwelling overheating

As global temperatures rise, creating relatively cool summer indoor environments with a minimum dependence on energy-consuming systems becomes a priority. But many modern homes already overheat.

Despite the myths, thermal insulation and construction airtightness are of minor importance. Solar gain is by far the largest peak room heat gain, often exaggerated because apartments have all the allowable glazing merged on to the one solar-exposed facade. Where there is a lack of cross-ventilation, this approximately halves the amount of heat gain that can be removed. In this regard, mechanical ventilation heat recovery is of little help as its ventilation rate is only some 10% of operable window purge ventilation.

Building regulations in the UK have different parts, each dealing with a separate building and performance issue. Asking for a Building Regulations Part L (Appendix P) (www.gov.uk/government/publications/conservation-of-fuel-and-power-approved-document-l) assessment at planning stage can give a good first-order indication of the overheating risk. The assessment is compulsory but, unlike the energy targets in Part L, its incorporation into the design is often overlooked. Beware of assessments done too late in the planning stage, with fragile design solutions subsequently added, such as fully closed reflective blinds that may not actually be provided for the occupant.

In particular, there should be:

• A record of what passive measures were found to be necessary and the assessment assumptions, and confirmation that these measures are integrated into the development.
• A means for occupants to be informed about the measures used and how to keep their home cool.

Typically overlooked is the need for appropriate window sizes and associated ironmongery to make occupants sufficiently confident about security to leave them open. Ceiling fans are also a proven but often overlooked measure of delivering 2–3°C of cooling, and they work well with passive cooling of exposed thermal mass ceilings.

Air-conditioned dwellings are also required to be assessed using Appendix P without their cooling system working. This is to ensure that passive measures of building massing and facade design are provided to minimise the use of energy for air-conditioning.

The planning requirement for district-heating pipework can contribute to summer overheating, particularly in the internal corridors of apartment blocks. Passive methods of reducing this overheating should be harnessed, rather than relying on air-conditioning, which would be counter to the energy savings of district heating.

Urban heat island

Waste heat emitted from buildings and transport, and heat from solar-absorbent surfaces, can create an urban heat island effect. This can make an area 5C or more warmer than the adjacent countryside, and temperatures will rise further in future due to climate change. Urban temperatures can be significantly reduced, however, if city-wide mitigation measures are applied. Indeed, reducing current urban heat island temperatures would mean that cities would have temperature headroom to avoid future climate-change temperature increases. This is something that planning policy can seek to influence, given the absence of any short-term commercial benefits or incentives.

Measures that planners could take to help reduce urban heat island effect include:

• Very low energy building. Virtually all consumed energy is eventually converted to heat emitted into the local environment. A large, air-conditioned building typically emits more than twice the waste heat of a naturally ventilated alternative.
• Electric vehicles. A fully electric direct-drive car can emit as much as 80% less local heat than a conventional car. The reduced street noise and pollution mean that buildings do not require sealed windows and can more easily become naturally ventilated.
• Greatly increased and quantified urban vegetation to boost transpiration moisture cooling, coupled with sustainable drainage to retain rainfall. Doubling current leaf area could more than offset anthropological heat gains.
• Avoiding continuous street canyons. Heat that has been reflected to a low level finds it difficult to radiate to the cooler night sky because its sky view factor is so small.
• Using vegetation canopies to prevent much of the solar heat penetrating down to the pedestrian-level microclimate. This also provides a cooler local environment for natural ventilation.

In support of these important measures, planning policy should give priority to the following built environment features that are difficult to retrofit:

• Building massing and depth suitable for natural cross-ventilation (even if initially the building is air-conditioned behind sealed facades)
• Buildings with large areas of room-exposed thermal mass for passive cooling, such as concrete floor slab soffits (even if initially covered up)
• Planting large, broad-leaf trees (which take 30 years to mature and provide their full shading potential)

Most other elements, such as windows, shading, and mechanical and electrical systems, are changed many times during the life of a building. This will allow them to respond to climate change at a later stage as the issue becomes more pressing.

Flooding

Our changing climate means that flash floods are increasingly frequent, and they extend into areas previously unaffected. Urban rainfall catchment areas, with their extensive hard landscaping, are increasingly vulnerable. Flash flooding happens when rainwater flow exceeds the

Figures 11.11.1 and 11.11.2 Examples of water-sensitive design.

capacity of the local drainage system, and bursts banks or backs up pipes to the weakest point in the system. High tides, stormy conditions and increasing sea levels can compound this where the river meets the sea. Public-realm planting, street trees, permeable surfaces and sustainable drainage systems (SuDS) help to prevent this flooding by providing an alternative to the direct channelling of surface water through networks of pipes and sewers to nearby watercourses.

Imitating natural drainage regimes, SuDS reduces excess surface water runoff, can improve the water quality, and provides the opportunity to enhance the environment’s amenity and biodiversity, as shown in Figure 11.11. SuDS lowers flow rates of water using increasing ground absorption and vegetation, along with other physical measures, to increase temporary water-storage capabilities. The principle is to hold water back and attenuate its discharge when the drainage system is in surcharge. The requirement for this alternative drainage approach is expected to increase with both increasing climate change and future population growth.