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With about 24 million existing units and 180,000 new units being built each year, new-build construction accounts for only a small percentage of the existing building stock. To make a serious dent into carbon emissions from the built environment, the carbon footprint of the existing stock must be reduced.

PART L1B AND PART L2B - REFURBISHMENT The revisions of 2002 and 2006 have changed the way in which building energy use is regulated.

Compliance for new build has moved from a prescriptive approach, focusing on thermal insulation standards, to a 'exible' holistic approach, based on carbon.

The parts dealing with refurbishment, however, are not performance based. These have not progressed away from the prescriptive approach and remain elemental. If greater design exibility is required, carbon calculations can be used to demonstrate the effectiveness of the proposed design solutions.

The 2006 changes enlarge the scope of the Regulations to capture more work by extending the definition of building work to include work on controlled elements and fittings. In addition, and perhaps more significantly, consequential improvements have been introduced for buildings other than dwellings.

In essence, this requires investment in energy efficiency throughout a building, over and above the costs associated with the principal works.

SNUG AS A BUG There are a number of provisions relating to building fabric within the new Approved Documents. In particular, these relate to the renovation of existing thermal elements and work on retained thermal elements.

Retained thermal elements should be upgraded if a building is subject to a material change of use; if an existing element is to become part of the thermal element or, in the case of buildings other than dwellings, consequential improvements of the thermal envelope are required.

The approved document states reasonable provision for renovated or retained thermal elements would be to improve the U-value to levels set out in column B of the table above (table 4 of ADL1B and table 7 in ABL2B). If the retained thermal element is already better than the threshold value in column A or where the area is less than 25 per cent of the surface area, then no enhancement is required.

Improving the U-value of a thermal element such as a wall can be achieved in a number of ways, but one of the most effective is external insulation. Advantages include:

no reduction in internal oor area;

reduction of thermal bridging;

reduction of condensation if caused by cold bridging;

ensuring the building mass can act upon the indoor climate of the building; and greater air tightness, hence reduced infiltration heat loss.

Clearly, adding external insulation requires careful detailing around the eaves and gable ends of buildings. The Energy Saving Trust has produced a guide, available on its website: www. est. org. uk.

There are a number of manufacturers that specialise in external insulation systems, one being Dryvit UK ( www. dryvit.com). Dryvit has a number of different solutions.

The Fedderlite and Conventional panels are prefabricated. They therefore have a significantly higher unit price, but clearly the on-site installation time is reduced and is not weather dependent.

Outsulation and Roxsulation (barrier systems), and Outsulation Plus and Outsulation Rail (moisture drainage systems) are site applied. The Outsulation is an expanded-polystyrene insulation based system.

Roxsulation is a mineral-wool insulation based system. Both are commonly used on solid or framed substrates for refurbishment or new build.

The moisture drainage systems Outsulation Plus or Rail are variations of the Outsulation system. These incorporate a moisture drainage plane, or cavity, to deal with incidental moisture in the event of its entering the system via a failed termination seal. The drainage systems are generally specified over sheathed lightweight-steel or timber-frame walls in new build and are really steered by house building insurers, such as NHBC. The systems can be either mechanically fixed or glued using adhesive; this will depend upon the substrate.

One of the benefits of external insulation is that it enables the use of the mass of the building to provide thermal inertia. This is important if using natural cooling with a night cooling strategy.

The downside of this approach is that during the heating period additional energy is required to heat up the mass. This leads to a modest increase in carbon emissions and an increase in boiler capacity. The significance of this effect depends on the intermittency of operation.

A building that is infrequently used, such as a hall, would incur a significant penalty in terms of heat-capacity requirements and carbon emissions if the mass were exposed. In this case, a better approach would be to have internal insulation. The longer the period of occupancy, the less the intermittency and the less the penalty.

The advantages of external insulation are not simply confined to existing buildings. With the need to avoid thermal bridging in new buildings, this offers an effective approach.

Other companies with experience of external insulation systems include Sto Ltd and Knauf Insulation.

Sto has developed finishes (StoLotusan) that repel dirt, reducing facade staining. There are a number of good practice guides available: Practical Refurbishment of Solid-Walled Houses by the Energy Saving Trust (2006) is the most recent.

The provision of insulation, whether externally or internally applied, is perhaps the most cost-effective means of reducing domestic energy demand as its life expectancy should be in excess of other measures. The next step is to provide a heating system that is energy efficient.

Part L sets out the minimum standards and makes extensive reference to a second-tier document, the Domestic Heating Compliance Guide ( www. dclg. gov. uk).

As we search for ever more efficient and lower-carbon heat sources, we move away from our traditional customs and practices. Although extensively used throughout the world, the heat pump has never been able to compete with the gas-fired boiler in terms of cost. However, with the minimum SEDBUK requirements of Part L requiring condensing boilers, further improvement must come from a change in technology.

HEAT PUMPS Heat pumps transfer heat from a low-grade source to a highergrade sink. They can produce heat more efficiently and with a lower carbon-emission rate than a gas boiler. For example, for every kWhr of electricity used by the heat pump, it can typically provide a heat output of three to four kWhrs. This translates to a carbon emission rate of approximately 0.1 to 0.15kg CO 2/kWhr, compared to a gas boiler with around 0.19kg CO 2/kWhr.

Typically for installations serving dwellings, the system is sized to optimise the economic returns and typically delivers 50 to 80 per cent of the heating and hot water demand, with the rest provided by supplementary electric heating.

Heat pumps qualify for the lower rate of VAT (5 per cent) providing the heat pump is used for heating residences. Results outlined in a recent study, which monitored the performance of a heat-pump system, indicated that an annual carbon reduction of 15 per cent was achievable.

The most common arrangements in the UK are the ground-to-water heat pumps, also known as ground source heat pumps (GSHP), and air-to-water heat pumps.

The use of a heat pump in lieu of a boiler is not a simple substitution; it requires a heating system design to match. The key difference is that conventionally the heat pump will generate hot water at about 45 oC - much less than a typical gas boiler system, which is designed to flow at 82 oC. The average temperature of the heating system is therefore lower, requiring larger heat emitters. This lower temperature lends itself to the use of underfloor heating. It also opens up the possibility of using other low-grade heating sources, such as solar collectors, in future.

There are manufacturers that produce units which can generate higher temperatures, designed to enable retro-fitting of heat pumps for connection to existing radiator systems.

Viessmann manufactures the Vitocal 350, which can generate temperatures up to 65 0C. While this would result in a drop in heat output from the radiator system, other factors such as improved thermal insulation or reduced air infiltration may offset this.

The new planning requirements instigated by PPS22 and being implemented by regional and local government are providing a boost to the use of heat-pump technology within buildings, particularly in London. Heat pumps are deemed to be 'renewable' energy technology and can be considered as part of the strategy to achieve the 10 per cent renewable energy target required by planners.

There are a number of manufacturers and suppliers (see the Heat Pump Association website www. feta. co. uk/hpa) in the UK, many of which offer a range of different heat pumps suitable for both dwellings and other buildings.

GROUND-SOURCE HEAT PUMPS (GSHP) GSHPs are well established globally and becoming more so in the UK. The system consists of a heat pump linked to a large heat exchanger building in the ground. This heat exchanger is typically 40-100m long and can be installed either vertically or horizontally. The ground acts as the heat 'source' and has the advantage of remaining at a fairly constant temperature (10 to 12 oC) throughout the year. This ensures that the heat pump can be configured to operate at a high efficiency.

GSHP is the best solution available to design teams when designing buildings in an urban context, as other alternatives aren't suitable: solar thermal can only provide a fraction of the 10 per cent requirement;

photovoltaics are too expensive;

biomass requires a large plant and has an ongoing significant operation and maintenance requirement; and wind turbines do not operate effectively in the urban environment.

There are two basic types of GSHP. Closed-loop systems need a large heat-exchange area with the ground, which has a significant cost premium.

To a certain extent this can be offset by integrating the pipework with the building's piles, although this may not be sufficient.

An alternative approach is to use an open-loop borehole to abstract water from underground aquifers. Heat is then exchanged with this water before it is returned to the aquifer, or to a river or drain.

Clearly the latter would incur additional sewerage charges and would be difficult to justify given the long-term watersupply concerns in London and the South East.

AIR-TO-WATER HEAT PUMPS Historically, these types have delivered poor performance at low air temperatures, but technology has moved on significantly and they can now provide a satisfactory heat output when air temperatures are as low as -20 oC. With typical average winter temperatures rarely dipping much below zero, the UK climate is perfect for this new generation of air-to-water heat pumps. They can be installed either indoors or outdoors with relatively low installation costs in comparison to groundsource systems, and many of the Dimplex models, for example, are suitable for single-phase electricity supplies. As our winters become, on average, milder, the benefits of air-towater heat pumps will increase.

HEAT PUMPS FOR HOT WATER AND HEAT RECOVERY A product in the Dimplex range is an air-to-water heat pump for hot-water heating only. This compact unit sits on top of a conventional unvented cylinder and is able to extract up to 70 per cent of the energy needed to heat the water from the ambient air. In today's wellinsulated buildings, where water heating is the primary energy user, this type of heat pump, which requires virtually no specialist installation, is a solution to complement highly controllable electric space heating or improve the energy efficiency of existing homes.

Heat pumps can be used effectively in simultaneous heating and cooling (or when dehumidification is required).

This exploits the inherent property of the technology whereby in transferring heat from source to sink it provides cooling at the evaporator and heating at the condenser. This is useful in applications such as swimming pool halls, where incoming fresh air can be heated by extracting heat from the moisture-laden exhaust air.

If the moisture is condensed the latent heat can be recovered, improving overall efficiency.

A company specialising in this approach is Calorex.

LOOKING AHEAD The government has sent very strong signals to the construction industry that minimum standards must continue to improve. The EU Energy Performance of Buildings Directive (EU EPBD), the main driver behind the 2006 Part L changes, requires member states to review their building standards every five years and, during the 2004 consultation, the government suggested that further carbonemission cuts would be implemented. As the targets become tougher, design teams will have to consider new approaches to how buildings are conditioned and operated.

Inevitably improving standards will lead to additional costs, but the economic benefit of more energy-efficient buildings and heating and cooling equipment as seen by the homeowner or building user will increase as the cost of energy increases. The UK has seen a 70 per cent increase in electricity prices and a tripling of wholesale gas prices over the last year or so, and while this has not been passed on to consumers it is clear there is upward pressure on prices. The UK is no longer self-sufficient in terms of natural gas and is therefore more susceptible to the global geo-political world of energy. Using what we have available to us more efficiently is a prudent approach.

It is important to realise that energy efficiency is the most cost-effective means of cutting our carbon emissions.

Whole life CO 2 savings are typically less than ú50 per tonne, whereas 'on-site' renewable generation can cost anything from ú250 to ú1,500 per tonne of CO 2 saved. Since capital is always a scarce resource, surely we should concentrate on the simple things and achieve as much carbon reduction as we can for our investment?

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