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Building towards carbon zero
UK domestic buildings are responsible for roughly a quarter of the country’s CO2 emissions and account for more than a third of final energy use. Over three quarters of this energy use is for space and hot water heating, mostly from gas-fired central heating boilers. To meet the EU’s climate change commitments, however, member states including the UK have committed to reducing greenhouse gas emissions by 20% by 2020 – a target that will require the UK not only to take action on transport but also to look again at the UK’s new-build and existing housing stock.
The UK government has set a target date of 2016 for all new homes to be zero carbon: to produce at least as much energy as they consume. But reaching a 100% cut in CO2 emissions will not be easy. To meet the stringent requirements of the UK Code for Sustainable Homes level 5, all emissions from space heating, water heating and lighting must be zero or negative. The first requirement is a highly insulated building fabric, and some form of micro-generation or other on-site energy generation. This means significant reductions in the thermal conductivity, k value, of walls and windows, either by increasing the thickness of the wall or by making the wall from a less conductive material.
Materials such as polyurethane foam or expanded polystyrene are already close to the ultimate in low conductivity, with k values of only 0.03W/mK, which is barely more than air. To achieve the required standards for new-build homes therefore requires not just new materials but different building techniques such as timber-framed construction in place of typical cavity wall insulation, insulated concrete formwork or hybrid blocks.
The majority of current residential development in the UK is based on the cavity wall system. This incorporates an inner skin or frame of block or timber, a layer of insulation, a cavity and an outer protective and weatherproof skin. However, this method of construction is costly, labour intensive, time consuming and subject to delays because of bad weather. Attempts have been made to improve on this by off-site prefabrication – by using timber frame or panels – but these have drawbacks that have prevented more widespread acceptance.
Concrete walls are traditionally constructed from timber formwork, which is taken down after the concrete has set. Timber formwork is fiddly to make and a lot of it cannot be reused. Using polystyrene in place of timber effectively kills two birds with one stone: it acts as formwork to mould the concrete and is then left in place to provide insulation for the walls. This building system is known generically as Insulated Concrete Formwork, or ICF. Most ICF systems give a wall U-value – the amount of heat lost per square metre of material, that is k divided by the thickness of the material in metres – of around 0.2W/m2K, well inside the level demanded by current building regulations.
Insulated concrete formwork is growing in popularity although it is not without problems as frequently highlighted on the UK Channel 4 television series Grand Designs. Typical expanded polystyrene blocks, such as in the Beco Wallform system, are designed to interlock for ease of construction.
The attractiveness of monolithic block construction, meanwhile, has been hampered by the inability of the system to meet current and future insulation requirements without costly layers of additional internal or external insulation. Clay- and cement-based blocks have much better U values because they incorporate more air in the block. However, further reduction of U values is limited as air ingress also reduces the blocks’ strength.
Geopolymer-based materials have inherently much greater strength than cement-based materials and could potentially be used to make blocks with much lower U values but of equivalent strength. Banah UK in Northern Ireland, for example, is developing blocks consisting of a sandwich of external facing, EPS insulation and internal geopolymer load bearing inner leaf. Commercial launch is planned for 2015.
Advantages of the geopolymer blocks, compared with concrete, include:
- lower density – the density of a geopolymer blocks is approximately 60% that of concrete blocks, strength for strength;
- improved thermal resistance – a standard structural geopolymer block is a better insulator than its equivalent concrete block, with a k value of 0.2 to 0.4W/mK against 0.42 to 1.7W/mK; and
- lower CO2 emissions – 10% to 15% that of ordinary Portland cement-based concrete by weight.
On a similar theme, Leeds’ firm Encos has developed so-called ‘carbon-negative bricks’ by using recovered aggregates and vegetable-oil-based binders. The manufacturing process for these bricks consumes no water, and produces no waste. The main glyceride in the vegetable oil, the triglyceride, has three long chains of carbon attached to a central backbone molecule. Applying heat causes the carbon–carbon double bond along these chains to break and form highly reactive molecules, which in turn attach themselves to the remaining triglyceride molecules, building up long, highly branched chains, resulting in the formation of irregular polymer-type structures that are solid, durable and stable. Polyunsaturated oils are used in the process due to their large number of double bonds and hence greater reactivity.
While the government’s ambitions for new homes are laudable, meeting the country’s climate change commitment will also require improvement of the UK’s existing housing stock. Options for reducing heat losses for existing buildings vary according to their age. The existing stock of 8.1m solid wall dwellings – about 34% of the UK’s current total housing stock – is responsible for about 50% of the UK’s total domestic carbon emissions. These older buildings without cavity walls need to be clad either internally or externally. Internal insulation is less popular as it reduces room sizes and involves alterations to plumbing and electrics. External insulation can change the appearance of buildings and cover up classical facades but can also modernise properties and provide an attractive ‘new look’.
External insulation ranges from thin, say 30mm thick, external wall render insulation such as Insowall by Thermilate (U-value 0.51W/m2K) – suitable as a combination with cavity wall insulation for newer properties – to thicker 50–120mm (U-values 0.3–0.15W/m2K) insulation, such as Kingspan Kooltherm K5 external wallboard, suitable for solid blockwork walls.
Useful particularly for older listed or historic buildings, lime renders have been developed by Telling for use in conjunction with Foamglas. The lime renders have the advantages of being more flexible than cement-based renders, reducing cracking, and requiring less energy in their production. Foamglas is foamed glass and consists of millions of closed, gas filled glass cells. Due to the all glass cell geometry, the ‘vapour-control layer’ is already built-in; ingress of moisture into the material, which would lessen its insulation abilities, is prevented and the material has a k value of 0.038W/mK. Cellular glass insulation is also classified as non-combustible to Euro Class A1, meaning that it makes no contribution to fire.
The chemical industry has supported the construction industry with the development of phenolic foam, high performance rigid thermoset polyisocyanurate (PIR) or expanded polystyrene foam (EPS). EPS, for instance, is 98% air trapped in a 2% cellular matrix, which is very light. Densities of between 10 and 35 kg/m2 allow light and safe construction works and make the material easy to transport.
Recent developments by companies such as BASF, Ineos and Sunpor have added carbon to the cellular matrix to produce so-called grey EPS, which has a 25% lower thermal conductivity than the original white EPS. The added carbon acts as an infrared reflector. Since heat radiation is infrared radiation, grey EPS reflects more heat rather than conducting it, compared with ordinary EPS, and hence its lower thermal conductivity. Demand for grey EPS is expected to grow strongly in coming years as the ‘Rebuild’ market for refurbishment of older buildings in Europe recovers and Sunpor, for instance, expects to triple output of grey EPS to 170kt/year by 2013 as output is ramped up in its latest factory opened in 2009 in Radlberg, Austria.
The UK government is promoting a plan to cover up to 8m Victorian and Edwardian homes in 200mm of insulation under its Green Deal scheme.
Manufacture of expandable polystyrene (EPS) involves two stages. During the first stage, polymerisation of styrene is carried out in a stirred batch reactor at temperatures between 80oC and 90oC using water as the dispersed phase. The volume ratio of water to monomer is in the range of 0.4 to 0.6. Monomer styrene is suspended in the form of droplets in the water phase and polymerisation occurs in these droplets until the mixture reaches its glass transition temperature. The mean particle size is determined by the droplet coalescence and the droplet breakage caused by the agitation system. Particle growth is controlled by the aid of suspending agents, which are usually inorganic powders, such as tricalcium phosphate.
In the second or impregnation stage, a blowing agent, such as n-pentane, is loaded into the reactor and the agent diffuses into the beads. The residual monomer is polymerised using an initiator of half-life higher than the one used in the polymerisation process, at higher pressures. Expanded polystyrene typically contains 3–6% of the pentane blowing agent, which is lost during the final expansion process.
Polystyrene granules are expanded by around 50 times by exposure to steam in a pre-expansion step to form larger beads consisting of a series of non-interconnecting cells. These beads are then conditioned allowing air into the pores. After conditioning, the beads are moulded to form boards, blocks or other customised products. The mould serves to shape and retain the pre-foam and steam is again used to promote expansion. During moulding, the steam causes fusion of each bead to its neighbours.
Finally, windows too play an important role in carbon-zero houses; first, they can reduce heat loss despite their large areas of glazing; and secondly, they allow the sunlight to produce additional heat through the glass.
With Internorm windows and heat-conserving glazing, it is possible to achieve U-values of 0.58 W/m2K. These windows have two infrared reflecting coatings and are filled with krypton or argon, which means that the surface temperature of the glass inside the room is comparable to the air temperature of the room itself. When installed in a south-facing position, the heat gains from these window systems are higher than the heat losses even from December to February.
For example, the centre pane U values for efficient double-glazed units is 1.5–1.7W/m2K, while for triple-glazed units it can be 0.8–1W/m2K. The U-value for the frame has to be taken into account, but triple glazing could reduce the heat loss through the window by around a third. Taken all together, the outlook for reducing the climate impact of UK buildings using modern materials and construction technologies would appear to be fairly bright.
Will Dehany is a director of Kemprotec, which provides techno-commercial consultancy services to the chemical and allied industries, and is based in Maltby, Middlesbrough, UK.