by Anthony Lymath
This is the third in an eight-part series of articles, Climate Change Adaptation in Buildings, examining the impact of climate change on the built environment, and the responses that can be made to those changes for both new-build and retro-fitting. Previous articles have covered an overview of the issues and an introduction to excess heat. Forthcoming articles will look at flooding, subsidence, drought and wind.
In the last article, the background and issues concerning the problems of excess heat build-up in buildings were examined. Fortunately, for new construction, there are numerous measures available to militate against the effects of overheating. Just as the degree of overheating is compounded by multiple contributing factors, so a combination of remedial measures will be more effective in offsetting those problems. At the earliest stages of a building project, a detailed site analysis should be undertaken which identifies the optimum positioning and orientation of the building on the site. Features should be noted including:
- The presence of any existing vegetation (particularly deciduous), and if it can be utilised to provide shading
- Shading provided by existing buildings around the site
- The aspect of the site, i.e. the sun-path.
Building orientation is particularly important for windows and window sizes – the peak time of day for solar gain is mid-afternoon to mid-evening (in the summer). South-west to west orientation is consequently worst, as the sun angle is lower as sunset approaches, whereas temperatures are at their peak still; south is better as the sun is still higher (being earlier in the day), so the angle of incidence on the glass is more acute. However, it should be borne in mind that the opposite of this advice is desirable for winter solar gains.
Passive cooling measures can be surprisingly effective, and it is possible to learn valuable lessons from traditional design in hot countries. For example, high thermal mass (utilising adobe), and stack-effect ventilation (drawing air over a pool of water to increase its humidity) have long been features in arid countries such as Egypt. However, whereas thermal mass is a recognised and accepted form of construction, it is not ‘mainstream’ in modern UK building; and successive updates to the building regulations have encouraged designers to increase insulation levels instead. This has led to lightweight, high-insulation, low air-leakage buildings becoming the norm, and it is for this reason that overheating problems have been exacerbated in recent years.
For thermal mass to be utilised effectively in avoiding overheating, the wall and floor elements should be exposed to the interior (but not to direct sunlight). This is for the same reason that underfloor heating is most effective when used under hard floor surfaces, i.e. the heat is able to be radiated into the room, and not absorbed by soft coverings which act as insulators. In the same way, the use of a built-up (or spray-applied) wet plaster finish is better than plasterboard on dabs, which de-couple the heat transfer. Plasterboard drylining has replaced wet plaster in recent years, due to speed and cost advantages. However, the disadvantage of this technique is a poorer level of thermal comfort in winter than is achieved with lightweight, highly-insulated construction, since the thermal mass of the substrate is rendered useless. Also, it needs to be used in conjunction with effective night ventilation to expel the heat absorbed during the day; otherwise heat will build up during prolonged hot periods. According to the NHBC “...when controlled ventilation designed to achieve maximum night cooling and 90% solar shading are implemented, a high thermal mass building fabric performs better than a low thermal mass fabric”1.
Ventilation and cooling strategies can include the use of passive stack-effect ventilation; stairwells and atria can be used as thermal chimneys, with clerestory or roof windows to expel exhaust air in conjunction with low-level inlets on shaded elevations to draw in cooler, fresh air. Openable windows (particularly with top and bottom opening lights in large glazed areas, to generate stack-effect ventilation) are vital, with the focus being on over- rather than under-provision. Where security is an issue, louvred side vents to windows (with a hinged insulated panel behind) can be incorporated. Ventilation positions are also critical; openings on opposing sides of a room promote cross-ventilation, while positioning openable windows on shaded elevations avoids drawing-in the hottest air. Whole-house extract ventilation systems can assist when used in a stack-effect arrangement, although the running costs need to be considered. Ventilation is also most effective – and most necessary – at night, rather than during the day (except for prone spaces, for example south-facing conservatories): only the coolest air is therefore allowed into the building, and daytime heat build-ups are expelled. For this to work successfully however, ventilation rates of up to 5 air changes per hour are necessary2, and this in itself is wholly reliant on the external air being cooler than the internal.
Placement strategies for mechanical and electrical equipment can also assist; locating appliances which generate waste heat outside the insulated envelope (for example, in a domestic garage, or communal heating systems in apartment buildings) – although again, this is obviously at the expense of being able to take advantage of winter gains.
Externally, minimizing hard landscaping materials which absorb heat during the day and re-radiate it at night will assist in counteracting the urban heat island (UHI) effect. The use of reflective colours for hard materials, and the use of more ‘soft’ materials especially vegetation, together with avoiding mechanical cooling plant where possible, are all strategies which can be utilised at a larger scale. Furthermore, reflective surfaces are less likely to cause injury – there have been documented cases of injuries caused by excessive heat absorption due to dark floor materials being specified externally3.
... the ‘no cost – behavioural’ suggestions are for the most part ‘common sense’ and in any event do not make a significant impact on severe and prolonged heat build-up ...
For existing buildings, there are fewer options available. In response to the recent news report on the Green Deal, the ‘Adaptation and Resilience to a Changing Climate Coordination Network’ (ARCC CN) has issued guidance4 to the Department of Energy and Climate Change (DECC) which is being used to advise Green Deal suppliers5, although some of the recommended measures are of debatable merit. For example, the ‘no cost – behavioural’ suggestions are for the most part ‘common sense’ and in any event do not make a significant impact on severe and prolonged heat build-up (curtains are ineffective because, once the heat has passed through the glass, it is already inside the room and the curtain simply acts as a radiator). In the case of ‘low cost’ measures – fans increase electricity usage, internal blinds again have a limited effect for similar reasons to curtains, and applying external wall insulation to high thermal mass buildings would mean that it was less effective in winter due to slower heat transfer, i.e. heating use would increase (this is repeated in the ‘medium to high cost’ category). However, this may be a necessary compromise when refurbishing, for example, Victorian terraced dwellings, where other solutions may be impractical or discouraged by planning legislation.
For lightweight, highly-insulated and air-tight modern construction, reducing solar gains is the best approach. Surface treatments such as solar-reflective paint (for walls), tiles (for roofs) and film (for glass) can offer a low-cost, ‘quick fix’ solution, but their effectiveness will be more limited than bespoke solutions, and tend to lead to on-going maintenance requirements. Low-emissivity, reflective glass has become commonplace in recent years, and insulating glass designed specifically for conservatories is now available which reduces summer gains while minimizing winter heat loss. However, a disadvantage of this type of glass is that it can reduce the effectiveness of internal blinds that are dark-coloured, as it blocks long-wave radiation being re-radiated back outside (although this can be overcome by using reflective-surface blinds which reflect short-wave radiation, which isn’t blocked by the glass). Furthermore, tinted glass can reduce internal winter daylight levels, as well as consequent solar gains.
Solar shading devices can be highly effective in reducing gains particularly through windows, and are available in many forms. Brise-soleil are good for south-facing elevations, while fixed vertical louvres and shutters are more effective in east or west-facing situations. Awnings, canopies and exterior blinds can all prevent direct sunlight incident on the building envelope, thereby giving flexibility and choice in aesthetic and other practical considerations. They can often be the best solution for most house types, typically offering a 50% reduction in overheating exposure6, and don’t restrict views from windows unduly. Retractable or movable devices also have the advantage of being able to avoid inhibiting the desirable winter solar gains and, particularly in the case of awnings, can be withdrawn to avoid damage in the event of high winds, or deterioration during inclement winter weather. Automation and sensors are also available, which enable use while buildings are unoccupied, albeit at increased cost. Other building features can also provide shading, such as extended roof overhangs, balconies and deep window reveals (although weather-protective detailing must be considered more carefully in these instances).
Mechanical and electrical installations can also be designed to reduce internal heat gains. Existing equipment can be upgraded and improved – energy-efficient-electrical appliances, low-energy luminaires, energy-efficient boilers and pipework insulation can all play their part.
... shading and insulation measures need to be considered in tandem with energy efficiency, and care needs to be taken to avoid providing summer cooling at the expense of increasing winter heating costs ...
In summary, shading and insulation measures need to be considered in tandem with energy efficiency, and care needs to be taken to avoid providing summer cooling at the expense of increasing winter heating costs, e.g. solar gain is a benefit in winter. In addition, the relative costs should be weighed against one another (capital costs versus costs in use – how long is the payback period going to be?), and environmental costs should also be considered. For example, active cooling units will not only offset any carbon savings made during the initial building’s construction, but they also dump waste heat back into the external environment and can hence increase the UHI effect.
A new generation of construction materials look as though they could offer a practical solution to the reintroduction of thermal mass in lightweight buildings. ‘Phase change materials’ (PCMs) are substances which can store and release latent heat, when melting and solidifying respectively over a narrow temperature range. Suitable materials for this purpose include waxes (either petrochemical or bio-based) and salt hydrates. These materials can be micro-encapsulated within certain types of building materials, such as plaster or clay, to form either wall-lining boards or ceiling tiles. They can also be macro-encapsulated into, for example, heat exchanger plates for use in cooling and ventilation units, and are being investigated for incorporation within PU foam panels, for applications such as metal-faced composite cladding panels. Wax-based PCMs tend to be more reliable. The other significant advantage is that PCMs can provide significant quantities of thermal mass while being in themselves very thin, i.e. the thermal mass appears disproportionately-large compared to the physical thickness of the material. Although the concept of PCMs has been known for a long time (having first been tested in a construction application in the USA in 19487), their use has not been significant to date, largely due to the significant capital costs and small number of manufacturers and products available. However, a few large manufacturers of insulation and gypsum-based materials are now starting to develop and market products both in the UK and Europe.
Given more development and take-up in popularity, PCM-based lining products could be seen to offer a real advantage in providing thermal mass to lightweight building envelopes, whether during initial construction or as a remedial retrofit solution. As with thermal mass in general however, their effectiveness is reliant on expelling the heat at night in order to re-solidify the PCM.
Regardless of personal opinion on the validity or otherwise of the theory of climate change, the fact remains that, during extreme weather events, buildings have a tendency to fail. This can be due to a number of factors, and under certain circumstances can be due to conflicting requirements on the building envelope. However, as has been identified, there are numerous systems and products available with which to combat weather and climate extremes. In order to assist in producing buildings that are better able to withstand the vagaries of the weather over their design lives, the NBS offers a number of subscription products from which systems and products may be specified to mitigate the effects of climate change.