In the first of a three-part series on complying with the Code for Sustainable Homes, Arup associate Katherine Holden, an expert on energy efficient and sustainable design, answers questions submitted at last month's Building code webinar

OUr webinar on the Building site last month drew unprecedented interest in complying and understanding the Code for Sustainable Homes. Our three experts on the panel were asked over 160 questions by listeners, the vast majority of which they were unable to answer during the hour broadcast. In the first of a three part series speaker Arup associate Katherine Holden tackles over 30 of the, ranging from wind turbine performance to district heating and solar shading.

Katherine Holden

Question One Peter Caplehorn: What prospect is there of achieving good practical homes with zero carbon performance in the near future, bearing in mind what has been done to date is largely theory?

Answer: One zero carbon home has been achieved recently, the Kingspan Lighthouse, designed by Arup and Sheppard Robson. I am particularly familiar with this project as it was one that I worked on with Chris Twinn and other colleagues in Arup's Building Sustainability Group in London. We developed the energy and water strategies and designed the services engineering. The design team developed concept designs for multiple home developments as well, for Kingspan. Zero carbon homes are technically feasible at any scale. At small scale, the size and shape of the homes are limited by the amount of photovoltaics that can be fit on the roof. At larger scale, greater than 250 homes per development, they are economically feasible too and at even larger scale, there are better economies of scale and there is a greater choice and reliability of technology available. Arup is also working on a private zero carbon home in Richmond, using similar principles to the Kinspan Lighthouse.

Question 2 - Michael Power: What is the biggest stumbling block to achieving zero carbon homes as a matter of course? Is it a lack of technology, a short termism approach (build, demolish and build again within X years) or another factor or number of factors? I am an accountant running a Low to Zero Carbon Solutions plc and cost is always an issue with Renewable Energy Technologies, even Solar Thermal, which is the most cost efficient! Our view is that we should be designing in Zero Carbon at the drawing board and using Renewables such as Solar Thermal to Big Wind to negate lifestyle running costs.

Answer: The cost of renewable technology is a significant stumbling block at the moment. I expect a reduction as the market expands, increased supply and improved efficiencies. The lack of a feed-in tariff for the export of electricity from homes is restricting the economic feasibility of individuals installing renewable technologies. At the moment, the export electricity rate for an energy supply company is about a third of the import price. There are grants available, but they could be more generous. The zero carbon stamp duty exemption rules are quite restrictive, for example at the moment they don't allow exchanging electricity with the grid. These could be revised to be more practical. I agree that we should be designing in zero carbon, for example by making the building envelope and other fixed parts of homes as efficient as possible. On a country-wide basis, renewable technologies should be used in their most efficient forms and the regulations, codes and utility supply rules should be structured in such a way that builders and home owners can invest in these technologies and receive the benefits from reducing their energy use.

Question 3 - Dawn Archambault: How significant an affect would there be if the Government decided that energy coming from off-site renewables would not be allowed to help housebuilders achieve zero-carbon status?

Answer: This would make achieving zero carbon more expensive, particularly for individual homes or small developments. The use of off-site renewables or 'additionality' needs to be developed into a robust scheme so that it is accountable and the carbon saved is not double counted. "Accredited external renewables" were described in the March 2007 edition of the Technical Guide to the Code for Sustainable Homes. The guide said that these accredited renewables, as defined by the Energy Act 2004, would be Renewable Energy Guarantee of Origin (REGO) certified. However, this provision was removed in the subsequent edition. We presume that a scheme like this will be developed.

Question 4 - Declan Hayes: How effective are wind turbines in an urban environment?

Answer: Small wind turbines are not very effective in an urban development. They typically have very low capacity factors, perhaps about 2 to 5%, i.e. if they run for 8,000 hours per year, a 1 kW turbine with a capacity factor of 2% would produce only 160 kWh of energy. This is equivalent to 4 low energy light bulbs on for 2,000 hours a year or 4% of the electrical energy for an electrically efficient 3-bed house. The actual output is highly dependent on the height of the turbine, the location and the relative heights of any obstructions around it. Even a 250 kW turbine could have a capacity factor as low as 10% whereas a large one on a mountain or out at sea could have a capacity factor of 30% to 35%.

Question 5 - Richard Peak: Do you think that ventilation system should be designed-in and not at present fitted around the structure compromising performance? How could this be achieved?

Answer: Generally, all services work better when they are designed in rather than fitted around the structure. This means that it's helpful to have a services design integrated with the rest of the design before the home is built. The ventilation system could be kept simple by putting the bathrooms and kitchens on outside walls. This is not always practical for apartments, so routes need to be agreed with the designers/ builder for extract ducts. If a whole house ventilation system is being used, then obviously the duct routes need to be thought about in more detail early on.

Question 6 - Declan Hayes: How good are Photovoltaics in practice?

Answer: They are generally reliable and easy to maintain. If part of an array is overshadowed, the output of the whole array can be significantly reduced. This can be avoided by connecting the panels that are likely to be shaded separately from the rest, as we did on the Kingspan Lighthouse with the Suntech MSK panels.

Question 7 - Shabha Gasson: Could you please elaborate more on MVHR efficiency in HLP?

Answer: At the moment SAP (the Standard Assessment Procedure), used for Building Regulations calculations for homes, assumes an mechanical ventilation with heat recovery (MVHR) unit efficiency of 66% for the heat loss parameter (HLP) calculation, even if you are using an accredited MVHR unit. For Code Level 6, the BRE did the final calculation for us and used the actual efficiency of the unit times a factor of 75%. We have recommended that SAP be revised to include the actual efficiency of the accredited MVHR unit in the HLP calculation.

Question 8 - Declan Hayes: How effective are solar thermal panels in terms of carbon savings?

Answer: Solar thermal panels can be very effective, even in the UK. For the Kingspan Lighthouse, the supplier, Thermomax, estimated that 4 m² of panels with a collector efficiency of 67% will produce about 2,940 kWh of hot water, which is about 2/3rd of the estimated annual domestic hot water demand. This is equivalent to saving 560 kg CO2/yr compared with gas heating.

Question 9 - Simon Kirton: How much did the Kingspan house cost to construct?

Answer:I don't know, but Kingspan have said that the construction cost was about 40% more than the cost of a typical house. This is likely to be mainly due to the cost of the photovoltaic panels.

Question 10 - Kate Warr: Has the Kingspan Lighthouse energy/water usage been measured post construction and reviewed against the original design specification? If so what is it showing?

Answer: The Kingspan Lighthouse energy and water use is being measured, but it hasn't been in operation long enough to get any meaningful results yet.

Question 11 - John Taylor: Excellent ideas for reducing electricity use from cooking and appliances. But does this reduce the renewable energy requirement for CSH level 6? Isn't the calculation based purely on floor area?

Answer: The calculation for appliances and cooking for Code for Sustainable Homes (CSH) Level 6 is based purely on floor area, as you say. The suggestions I gave were for how anyone could reduce their actual electricity use at home, for example by turning all appliances and lights off when they are not needed, by buying A++ rated, super efficient white goods and entertainment systems with low standby power and by eating more raw food, e.g. salads, rather than cooking. This would help the nation's health too!

Question 12 - Nicholas Cousins: To achieve HLP of 0.8 can you tell me what u values have been used?

Answer: The Kingspan Lighthouse walls, roof and floor have U-values of 0.11 W/m² K with 284mm wide Kingspan Tek panels, glazing U-values of 0.7 W/m² K, which is with triple glazing, very low emissivity coatings and argon filled cavities, and even an insulated door with a U-value of 0.35 W/m² K.

Question 13 - Elisa Yon: What is being done to address the existing housing stock in the UK? As I understand - the amount of carbon emissions from existing housing stock far outweigh the carbon emissions from new housing?

Answer: There are some provisions in the Building Regulations, Part L 2006, that require controlled fittings and services to be similar to standards for new build. However, the provision for "consequential improvement" of existing parts of homes to be upgraded when the home was extended or refurbished was taken out of the draft at the last minute. This measure would be very useful for helping to improve the existing housing stock. There are grants for energy efficiency for poor households and grants for low and renewable technologies. Further regulations could require homes to be upgraded when they are sold or rented.

Question 14 - Dan Asher: Can a building still achieve a code 6 with an air-tightness level of +3m3/h?

Answer: Yes, depending on its shape. It would be difficult for a thin house, but relatively easy for a flat with one or two exposed sides.

Question 15 - Joan McCoy: Do you know how long a photovoltaic panel has to generate electricity in order to offset the carbon used in its manufacture?

Answer: It's about 3.5 years for the energy payback, to offset the embodied energy. As PVs become more efficient, this will reduce.

Question 16 - Brendan Beck: Where do you source your biomass for the Kingspan lighthouse boiler? Is there anything preventing coal being burnt in the boiler?

Answer:The biomass is sourced by the boiler supplier, Solar Thermal. The Windhager boiler is not designed to burn coal, only wood.

Question 17 - Kenneth Jenn: The Bio Mass Boiler - what fuel was used? If wood pellet, is there a supply/cost issue with availability?

Answer: The fuel is wood pellets. The cost of wood pellets is about the same as gas at the moment, per kWh. There are plenty of wood pellet suppliers in the UK.

Question 18 - Nicholas Cousins: Can you indicate the amount of solar panels required to produce electrical generation sufficient to meet code 6?

Answer: An approximate rule of thumb is that you need half the floor area in panels, for an electrically efficient home, i.e. if you had a two-storey house/ block, you would need to use the whole of the roof for the panels, facing south.

Question 19 - Declan Hayes: Is biomass likely to be a key feature of zero carbon homes?

Answer: Yes. At the moment, it's the most cost effective solution for heating and for combined heat and power. There is approximately enough available land in the UK to produce 10% of our building heating from biomass, for buildings with current heat loss standards. There is plenty of biomass available in Scandinavia and Canada that could be shipped to the UK, for a relatively small additional use of energy.

Question 20 - Kate Warr: Could it be possible to receive a breakdown of the products specified in the Lighthouse?

Answer: See the last slide in my presentation, which you can download from the Building website.

Question 21 - Brendan Beck: Does the code require you to prove to sustainability of biomass used in a boiler?

Answer: No. There is a small carbon allowance for biomass, taken from the Building Regulations carbon dioxide emission factors. This allows for the fossil fuel energy used in cultivation, harvesting, processing and transporting the biomass.

Question 22 - Karen Tait-Lane: On the biomass what would you classify as a "small" development?

Answer: For using biomass combined heat and power (CHP), I classify a "small" development as anything less than 250 homes. This is because there aren't small biomass CHP plants available yet for small developments. There are some being developed, such as in Germany and Denmark. For biomass heating, the current smallest size of wood boiler is 10 kW, so this would be appropriate for about 500 m² of well-insulated housing - about 7 2-bed flats or 3 houses.

Question 23 - David Clark: Can you use gas to achieve CSH level 5/6 by providing additional electrical renewables - this would appear to be the only way.

Answer: For Code Level 5, the Building Regulations controlled energy must be zero, i.e. for heating, ventilation and lighting. This does not include cooking, so that could be done by gas. For Code level 6, the energy allowance for appliances and cooking imply that the cooking is electric. However, you could export enough electricity to equal the carbon emissions associated with using gas, as long as you have net annual zero carbon emissions using the Code calculator. Gas use is not allowed for zero carbon stamp duty exemption currently.

Question 24 - Kate Mansfield: How do you suggest to meet CO2 reductions for Levels 5* and 6* in flats where Biomass is not feasible?

Answer: Potentially, you could use a ground source or air source heat pump to provide heating, but you would then need to produce even more renewable electricity to power the heat pump. Alternatively, the flats could be connected to a zero carbon district heating network, where available.

Question 25 - Sue Hitchcock: Why hasn’t the pricing on solar water panels etc become more affordable, hence encouraging wider usage?

Answer: Sorry, I'm not an economist/ business consultant. I expect that the price will reduce. For some renewable technologies, demand is greater than supply at the moment.

Question 26 - Colin Button: What back up measures were taken to compensate if your local biomass supplier goes out of business?

Answer: There are plenty of biomass suppliers in the UK to choose from.

Question 27 - Dan Kantorowich: If heat pumps can achieve 450% efficiency compared to the electrical input, do they not beat gas fired boilers in terms of carbon emissions. Considering likely future moves to improve CO2 output from coal fired power stations such as carbon sequestration should there not be a great push towards heat pumps generally.

Answer:Heat pumps do beat gas fired boilers in terms of carbon dioxide emissions. They are a useful low carbon technology and can produce up to about 20% savings in CO2 emissions. However, they still need electricity to power them, so they are not ideal for a very low or zero carbon design. You may remember, in my talk, I emphasised reducing electrical demand as grid electricity produces 2 1/2 times for CO2 than gas for the same amount of useful energy and it's generally much more expensive to generate from renewable sources than heat is. However, they could be used as an interim means of heating until zero carbon or "waste" heat district heating is available. Air source heat pumps are being used on another Level 6 project that we are involved with and the intention is that these homes will be connected to district heating systems in the future.

Question 28 - Guinn Damask: Should more pressure be made to encourage government to provide renewable energy at source? Is the code not just chipping away in a small way, when more focus could be on large scale sustainable energy generation?

Answer:Yes, it is generally more energy efficient and cost effective to produce renewable energy on a large scale. For example, big wind is much more efficient than small wind. Also, large power stations generally have better electrical efficiencies than small generators, so if they used biomass for part of their fuel, e.g. co-firing in coal plants, and the heat from them was captured and used, that would be a more efficient use of biomass overall than using biomass CHP on a small scale.

Question 29 - Behead Sodagar: Is the whole house mechanical ventilation really necessary for the UK where the climate is milder compared with the rest of Europe?

Answer: MVHR isn't really necessary for the UK unless you need to attain a very low heat loss parameter. In my talk, I showed that passive stack ventilation or simple natural ventilation with trickle vents and intermittent extract fans actually produce less carbon dioxide than MVHR.

Question 30 - Keith Bothwell: Regarding the preference for solar shading over thermal mass - mentioned by Katherine Holden - is this just in relation to thermal comfort, rather than carbon emissions (most homes are not air-conditioned)?

Answer: My preference for solar shading is to improve thermal comfort by reducing summer overheating. However, as the effects of climate change increase and it is likely that UK summers will become hotter, the use of external shading will reduce the likelihood of people buying and installing air-conditioning units in their homes in the future. So, in the medium-term, this will reduce carbon emissions.

Question 31 - Euan Whitmore: Kingspan may have been do-able but at what cost? Is it affordable?

Answer: The additional cost would be significant for single homes at the moment. This is why I recommend concentrating on large scale zero carbon developments at present. However, new smaller scale biomass boilers and biomass CHP plants are being developed and the efficiencies of photovoltaics and solar thermal panels are increasing, so the effective costs will be reduced in the future.

Question 32 - Paul Wright: A code assessor has recommended we use air source heat pumps and then very little else would be needed. Agree?

Answer: Air source heat pumps will reduce the carbon emissions for heating and some of the domestic hot water. However, they don't reduce electricity use at all, they add to it. I see them as making modest carbon emission savings, but they are not ideal for a zero carbon home.

Question 33 - Rob Banes: With reduced hear loss from new homes - is district heating really a viable option? Even biomass CHP?

Answer:Whilst heat loss from new homes is reduced, there is still a significant demand for domestic hot water, plus there are many heat-guzzling existing homes. District heating and biomass CHP are technically viable and very useful. Ideally, new heat networks would be connected to existing homes as well as to new homes, so that all the heat from CHP schemes would be used. This would need the structuring of energy supply to be changed to make this commercially viable.

Question 34 - David Ranson: If there are problems with obtaining small biomass boilers, why not encourage the use of communal heating on larger scale developments with one boiler plant for all?

Answer: Where you have larger developments, communal heating with a large boiler plant is a good solution. This also means reduced maintenance, a possibility for having multiple boilers for standby/ redundancy and a single delivery point for the biomass.