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At first sight, delivering zero-carbon homes seems exciting. Think of the kudos, the publicity, the shiny gizmos on the roof. Improving existing homes is dull in comparison - with boring stuff like insulation, boilers, heating controls, draughtproofing and double glazing.

So what if a 1% improvement per year to the existing stock, would give the same carbon dioxide savings as building all new homes to zero-carbon standards? When the government needs headlines, 鈥渮ero carbon鈥� shouts that it is doing something big. But can we really make all homes zero carbon by 2016?

The first question is: what exactly is a zero-carbon home? The definition is simple - a home must generate all its energy, including energy for heating, hot water, lighting and appliances, without adding carbon dioxide to the atmosphere. Depending on need, surplus energy can be exported and energy deficits can be met by importing energy, and the CO2 emitted by burning biomass must be compensated for by exporting zero-carbon electricity to replace grid electricity. Taking these factors into account, the net amount of emissions must be zero over the space of a year.

You could build a zero-carbon home right now if you have enough money or forgo all your energy-using appliances.

The state of play

The scale of the challenge is evident when you look at the costs of making homes zero carbon in terms of heating and lighting 鈥� the equivalent of level 5 in the Code for Sustainable Homes. The feature on page 14 says it would cost between 拢15,000 and 拢30,000 more than Part L standards per dwelling.

To make a home completely zero carbon, the equivalent to level 6 in the code, means roughly 4,000kWh would have to be generated to power appliances, not accounting for losses through the fabric of the house. Depending on technologies used and their scale, this could cost from under 拢5,000, using large-scale offshore wind, to 拢50,000, with photovoltaics.

Money is best spent on reducing energy demand first. Only then should we spend on reducing CO2 output. There are no secrets about how to do this 鈥� all the tools are available, although at high prices.

Germany has been developing its 鈥淧assivHaus鈥� for 10 years, with better insulation, glazing, air-tightness, solar energy and ventilation. But the this is not the only way. Indeed, some feel its insistence on mechanical ventilation is wrong. In addition, when Swedish and German designs are assessed by UK methods, they do not show the same improvements over UK homes as claimed. This doesn鈥檛 suggest anything sinister - different methods give different answers and, as houses don鈥檛 wander across borders, it is not important.

Even with the best materials we are unlikely to cut CO2 emissions by more than half from Part L 2006. A typical 100m2 home built to high fabric standards will still need about 1,500KWh of heat energy, 4,000 to 6,000kWh for hot water and 4,000kWh for fridges, kettles and games consoles. For these, renewable technologies should be examined.

Meeting the challenge

Where power is generated affects costs. Urban wind turbines on individual buildings are useless compare with offshore wind turbines, which generate serious power for the 40% of the time the wind blows. Such wind farms are the most cost effective generator of zero-carbon electricity at present. Land-based wind power is about twice the price of offshore.

Another problem is that some homes cannot fit generation kit on their roofs or inside. Blocks of flats have little roof area compared with their floor areas. Town centre developments overshadowed by trees or buildings may not be suitable for dwelling-level solar or wind kit. These buildings would benefit from shares in grid-connected zero-carbon energy.

PassivHaus has shown how a programme can build up an infrastructure of designers, suppliers and builders who understand the product

Energy can be generated in two ways: the first is with local or individual home-based kit, with private networks for heat and electricity; the second is in large-scale farms with the National Grid as distributor. Both can work, and a mixture of the two is likely to prove best.

On a local power generation scale, biomass CHP is a favourite, assuming there is a ready supply of wood. CHP is good for large sites with more than 200 homes, and commercial users. Pure housing schemes may have too high an electricity demand, compared with their heating requirements, especially if homes are well-insulated. This option is not available for small developments, which account for most new homes, unless the infrastructure is shared.

Biomass is not completely zero carbon owing to the CO2 produced by processing and transport, so extra generation is needed by solar or wind to offset this.

Solar hot-water panels are probably the first fixed-to-house renewable to choose. Costs are falling as larger companies and distributors enter the market, and new designs are being launched. However, even the best system supplies no more than two-thirds of requirements, so a back-up system is needed.

Photovoltaics are generating more power per pound spent, owing to rapid advances in technology. At present, they are the most expensive way to generate electricity but some claim that costs will rival conventional power in 10 years. Panels can easily be incorporated into a building, for example in the roof, glass or cladding systems. Long-term performance depends on the technology and keeping the panels clean.

Looking to the future

The different approaches to zero-carbon homes mean costs vary hugely. But it is certain that it will be expensive initially. Most low-carbon developments in the UK have emerged through the will of individuals rather than regulations, so normal market conditions have not applied. But if new homes are too expensive, buyers will not be able to afford them. The industry is going to have to develop and refine zero-carbon homes so they work and are affordable.

So how are we going to get there? Germany鈥檚 PassivHaus has shown how a programme can build up the infrastructure of designers, suppliers and builders who understand the product. This is the only way to achieve our target.

The Construction Products Association has called for a programme of building a few thousand low and zero homes each year until 2016. These homes will cost a lot and not work exactly how they should, but they have to be built, sold and lived in by customers to gain feedback so succeeding models can be improved.

All the dwelling types, site sizes and locations must be addressed. The government鈥檚 latest Design for Manufacture programme for low and zero-carbon homes calls for sites with over 200 homes. But half of new homes are actually on sites with fewer than 50 homes, so this programme is not relevant.

Getting sites all around the country is important so customers, builders, designers, planners, surveyors, suppliers and tradesmen can visit. Suppliers will need to invest in capacity, and new skills will be needed. The government owns a significant amount of land so it can avail of discounted prices.

Through these programmes problems will be brought to light and solutions developed. Without such a long-term strategy, zero-carbon homes will remain a dream.