Theme: insulation and energy efficiency
House building has moved up the political agenda in recent years due to double-digit price inflation and the consequent impact on affordability. The recently published Barker Report identified lack of supply as the main cause, and highlighted the negative social and economic consequences. Kate Barker concluded that if long-term trend price increases were to be limited to 1.8 per cent per annum, a total of close to 200,000 new homes would be required every year.
While there has been much debate about the land-use implications of such a policy, there has been little regarding its impact upon UK carbon emissions. Presumably, it is assumed that if measures such as Building Regulations meet the target of a 60 per cent cut in UK emissions by 2050, the C60 challenge will be achieved.
Currently, the domestic sector accounts for 28 per cent of all UK emissions. With the number of households increasing from 21 million to 25 million during the next 20 years, it is clear the task of reducing emissions by 60 per cent is not going to be easy. There are some easy wins of course; tighter Building Regulations on fabric performance and boiler efficiency will have an impact on new buildings, and on existing buildings once old boilers are replaced.
Reducing carbon emissions from the domestic sector and the solutions used to achieve this will depend on various factors. In new buildings, going beyond the C60 target is relatively easy and cost-effective. Improving the performance of existing buildings is not so easy, although significant savings can be made through a variety of simple measures.
Much can be achieved through good design, the judicious use of sunspaces and optimising orientation and form. What is certain is that the economic resources to improve the carbon performance of buildings are limited, and those resources must be directed to solutions that get the best environmental results for the least investment.
Using data from a number of sources, table 1 compares various strategies to achieve low carbon performance. Two metrics compare the effectiveness of each, cost per tonne of CO 2 saved over the lifetime of the measure and simple economic payback.
It can be seen that considerable savings can be made with very cost-effective measures.
Insulation materials Table 1 shows that insulation is the most effective way to reduce carbon emissions from housing. This is, in part, due to the fact that, once installed, insulation is assumed to last as long as the building.
New build The necessary thickness of common insulation materials in standard wall-type construction is about 75mm. The one principal exception is polyisocyanate from Celotex, which produces the same resistance with just over half the thickness. This is particularly beneficial for renovation of existing buildings but is marginal on new-build, where a small difference in overall wall thickness is easily accommodated at the design stage. While some increase in insulation requirements will be part of future updates to the Building Regulations, there will also be new guidance on the location of the insulation with respect to the thermally massive wall elements.
Retro-fit costs With in excess of 21 million dwellings in the UK, the greatest challenge is improving the performance of existing stock. Applying insulation to an existing building is more costly - particularly if applied to an existing facade. The type of insulation will depend on a number of factors and specialist advice should be sought to ensure a suitable choice is made. The External Wall Insulation Association maintains a database of companies specialising in this type of work. Sto provides external insulation systems with a rendered external skin. A recent development 'StoTherm Solar' selectively allows solar radiation to penetrate an otherwise opaque facade while ensuring insulation levels are maintained. These facade elements can transmit as much as 120W/m 2 on the southern elevation.
Green options Choosing a greener form of insulation is not a clear-cut decision, and cost differences are significant. For instance, sheep's wool, such as that manufactured by Thermafleece, has appeal as a natural product with good performance and potentially beneficial moisture absorption properties, but questions hang over its long-term suitability and organophosphate contamination. It is also 10 times the price of Rockwool and availability is limited. Cellulose fibre from recycled paper is another option, but its treatment with fire inhibitor and biocide, and its microscopic fibre size, have unproven health implications. A pragmatic approach is to exclude options where toxicity is known and hydrofluorocarbons (HFCs) are used in production. This presently excludes urea formaldehyde foam and some polystyrenes and phenolic foams.
A further consideration in the specification of insulation is that materials such as mineral wool can have important dual uses such as sound or fire insulation.
Healthy homes Increasing levels of insulation, and better detailing and build quality are ensuring new buildings are more airtight and energy-efficient. But reduced infiltration can lead to reduced air quality within buildings. Asthma and other allergenic reactions are the highest in Europe and, while there are many causes, the quality of the air we breathe is an important factor.
Opening a window is not always possible in the depths of winter and 'trickle' vents may not be used correctly or provide sufficient outside air to be effective. An alternative is mechanical ventilation with heat recovery (MVHR). The difficulty with MVHR is that the components are inefficient and more carbon is released through the extra electricity used to power the system than is saved by the heat-recovery system. In addition, the heat-recovery devices were so inefficient that not enough heat was transferred to the incoming air. Vent Axia has now developed a whole-house ventilation system designed to overcome this problem. The system, called Air Minder, has a claimed heat-recovery device efficiency of 93 per cent, enough to take the chill off incoming outside air in the middle of winter. The system has been designed with highly efficient DC motors (cutting fan energy by 80 per cent), optional pollen filters, and controls that ensure heat is only recovered when required.
Visible expenditure It is an interesting truism that people are willing to pay for an increase in enjoyment rather than improving their house's energy efficiency. For example, the installation of sun-pipes will reduce carbon emissions, but the cost could never be justified on such grounds.
The value to the homeowner lies in the visual improvement of the space. Conversely, a similar investment in improvements to a heating system may be more difficult to sell because, while the financial benefits are demonstrable, there is no tangible improvement to the house itself. Heating systems need a feature that the homeowner can boast about to their neighbours - perhaps that feature is a boiler that generates electricity.
Micro CHP Micro combined heat and power (micro CHP) is aimed at displacing grid-supplied electricity generated with an overall efficiency of about 45 per cent, with a power source generated with an overall efficiency closer to 90 per cent. The unit, not much bigger than a conventional domestic boiler, uses an external combustion engine (a Sterling engine) to generate a small amount of electricity, enough to satisfy the base electrical load in most houses. The heat generated in the process is supplemented by a conventional burner that can be used to heat the house and the hot water. There are two manufacturers currently developing or offering products: MicroGen and Whisper Tech.
Condensing boilers Micro CHP units are also condensing boilers that extract the energy combined in the water vapour by condensing it. Unfortunately, the UK government incentive programme encouraged the installation of condensing boilers irrespective of the type of system to which they were connected.
Condensing boilers achieve their higher efficiencies only if the return water to the boiler is cool enough to condense the flue gas. This is not the case on a conventional commercial or domestic heating system, which generally operates at 80°C flow and 70°C return temperature to enable radiators to be smaller.
A system incorporating condensing boilers must be designed with much lower mean water temperatures, driven by the fact that optimum condensing requires return temperatures below 25°C. Radiators therefore need to be approximately 25 per cent larger than in a conventional system. Generally, fully condensing boilers are the preserve of the domestic and smaller commercial sector.
Alternatives to radiators include underfloor heating and radiant wall heating. Underfloor heating systems are best suited to applications where continuous heating is required, as they take time to bring a room up to temperature during intermittent operation.
Wind energy Wind energy is the most economic of all the renewable energy sources. It offers a zero-carbon alternative to grid-supplied electricity.
In common with many technologies, wind generation benefits from economies of scale and most wind turbines are unsuitable for location within the built environment. However, a number of companies - spurred on by the potential size of the market and the current raft of government grants - have developed small-scale turbines capable of being mass-produced. One such company is Windsave. Although yet to go into production, the Windsave unit (nominally rated at 800W) is claimed to be capable of producing 1MW/hr per annum at an installed cost of less than ú1,000. The cost is kept to a minimum by designing the system so that it can plug directly into a building ring-main and displace the grid-supplied electricity. While the cost per tonne of CO 2 saved is still high (about ú250), the inclusion of Renewable Obligation Certificates (ROCs) helps the economic case. Key issues, such as safe operation of both the electrical and mechanical elements, need to be evaluated and demonstrated prior to widespread adoption.
Solar energy The economics of solar energy are challenging and, with energy costs at present levels, impossible to justify. From table 1 it can be seen that the cost - as a means of reducing carbon emissions - is an order of magnitude above the demand side measures. However, when a house is well insulated (and therefore has a low heating demand), the largest single heating requirement becomes the hot-water demand. To maximise the benefit of a solar thermal system, the system must be designed carefully, controlled and integrated with other heat sources.
Typically, a domestic system is designed to provide 70 per cent of the annual demand for hot water and requires 3-4m 2 of collector. The control and operation of the solar system, relative to the operation of the boiler, is key to realising the potential of a solar system. While systems can be designed with bespoke equipment, a less risky approach is to use a specialist system supplier who can provide the necessary components and expertise. Viessmann provides systems using both the cheaper and less-efficient flat-plate collectors, and the more expensive and more efficient evacuated-tube collectors. The systems are designed with integration in mind and have appropriate controls to optimise the solar benefit.
Wood energy No matter how well insulated the house or how efficient the boiler, the burning of natural gas, coal or oil will release carbon in the form of carbon dioxide. The sudden release of this carbon, locked in the planet over millions of years, is causing the rise in CO 2 levels, the consequent global warming and resultant climate change. A wood-burning boiler offers an alternative near-zero-carbon heat source. The process of burning wood still releases carbon dioxide, but if a new tree is planted to replace the one burned, the system is balanced. While the costs are high, the whole-life cost per tonne of CO 2 saved is comparable with solar thermal energy. Baxi produces a number of different boilers suitable for the larger house that can burn pellets or logs. The most efficient way to burn logs is at high temperature and quickly. This means that, rather like solar thermal generation, a storage vessel is required to store the heat over a 24/48-hour period between burns.
Although wood-burning boilers are low carbon, they will not suit everyone as they require manual loading and a store of precut logs seasoned over two years, to drive out most of the moisture. The Baxi Innova boiler is accredited and qualifies for grants under the Clear Skies programme.
The challenge It is important that the capital resources available for reducing carbon emissions in housing should be used as effectively as possible without creating any negative impacts.
It is also time for the UK construction industry to start to deliver real CO 2 savings.
The domestic sector has an important role to play, not least because of the size of its estate and its willingness to take a longer view of investment. However, if the best environmental return is to be had for the ever-limited capital resource available, appropriate technologies must be evaluated carefully and prioritised.
Even on new buildings, the route to C60 will often be a journey rather than an event. The time of 'green wash' and environmental 'go faster stripes' is over; it is time for our industry to deliver.
Nick Cullen is a partner in Hoare Lea Consulting Engineers