330 Construction Technology

The Code for Sustainable Homes was launched in May 2008 by the Government as part of a package of measures to drive down CO2 emissions in new dwellings. The scheme has been developed by BRE Global under contract to the Department of Communities and Local Government. It replaces the earlier scheme known as the EcoHomes scheme.

The Code gives new homebuyers better information about the environmental impact of their new home and its potential running costs. It also offers house-builders a tool with which to differentiate themselves in sustainability terms within the housing market.

The Code uses a 1 to 6 star rating system to communicate the overall sustainability performance of a new home, setting minimum standards for energy and water use at each level within England, and rating the dwelling as a complete package. Each level includes mandatory requirements for energy performance and water usage, together with tradable requirements for other aspects of sustainability to be assessed at the design and post-construction stages.

In September 2010, the Code became mandatory for all new-build dwellings, with a minimum code level 3 specified, which is currently enshrined in AD part L (2010).

Table 7.5.1 sets out the code levels for energy and water use, which account for nearly half of the final award.

In addition to meeting mandatory standards, achievement of the requirements in each design category scores a number of percentage points (see Table 7.5.1). This establishes the code level or rating for the dwelling. The code certificate illustrates the rating achieved with a row of stars, with a blue star

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Code for Sustainable Homes 331

awarded for each level achieved. Table 7.5.2 summarises what is examined and the relative weightings of each issue.

Each issue is a source of environmental impact that can be assessed against a performance target and awarded one or more credits. Performance targets are more demanding than the minimum standard needed to satisfy Building Regulations. They represent best practice, are technically feasible and can be delivered by the construction industry today.

The code level is assessed on the credits awarded for each category (with weighting applied), which is then expressed as a percentage of the total available in each category. These are then summated and rounded to two decimal places. The final code assessment is then identified from the minimum threshold values shown in Table 7.5.3.

Fig. 7.5.1 (page 334) offers some examples of the build-up of credits under the Code for Sustainable Homes for a two-bed end-terrace house based on Part L 2010. Consultation on the next revisions for 2013 are currently underway, with the aim of achieving the zero carbon emission requirements for 2016. This will mean even tighter controls on emissions and air permeability, involving more detailed calculations and the use of evolving low-energy technologies.

PASSIVhAuS STANdARdSOne voluntary scheme that has become influential in driving forward the zero carbon agenda is the Passivhaus standards scheme for domestic housing. The Passivhaus methodology originated from joint research done by Scandinavian and German researchers. The first dwelling to be built to these early standards was in 1990 in Germany. In Europe’s temperate climate, the Passivhaus has been very popular, due to its super-insulation, low air leakage

Table 7.5.1 code for Sustainable homes levels for energy and water use.

Code level Minimum percentage improvement in Dwelling Emission Rate over Target Emission Rate (set by AD Part L: 2006)

Maximum water consumption per person per day (litres)

1 star 0% (compliance with Part L 2010 only is required)

120

2 star 0% (compliance with Part L 2010 only is required)

120

3 star 0% (compliance with Part L 2010 only is required)

105

4 star 44% improvement on Part L 105

5 star 100% improvement on Part L 80

6 star More than 100%, effectively ‘zero carbon’ where the dwelling generates more energy than it needs, and does not lose any

80

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332 Construction Technology

Table 7.5.2 Requirements for star rating under the code for Sustainable homes.

Categories Issue (those in italics are mandatory)

Total credits available

Weighted value of each credit

Net weighting (% point contribution)

Energy and CO2 emissions

Dwelling emission rate – DERFabric energy efficiencyEnergy display devicesDrying spaceEnergy-labelled white goodsExternal lightingLow- and zero-carbon technologiesCycle storageHome office

31 1.17 36.4

Water Indoor water useExternal water use

6 1.5 9

Materials Environmental impact of materialsResponsible sourcing of materials – basic building elementsResponsible sourcing of materials – finishing elements

24 0.3 7.2

Surface water run-off

Management of surface water run-off from developmentsFlood risk

4 0.55 2.2

Waste Storage of non-recyclable waste and recyclable household wasteConstruction site waste managementComposting

8 0.8 6.4

Pollution Global warming potential of insulantsNOX (nitrogen oxides) emissions

4 0.7 2.8

Health and well-being

DaylightingSound insulationPrivate spaceLifetime Homes (mandatory for level 6 only)

12 1.17 14

Management Home user guideConsiderate Constructors SchemeConstruction site impactsSecurity

9 1.11 10

Ecology Ecological value of siteEcological enhancementProtection of ecological featuresChange in ecological value of siteBuilding footprint

9 1.33 12

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Code for Sustainable Homes 333

Table 7.5.3 Minimum scores to achieve code levels.

Code level Min % score to achieve code level

1 star 36

2 star 48

3 star 57

4 star 68

5 star 84

6 star 90

and use of mechanical ventilation heat recovery. The main factor is that the heating demand is very low – in the order of 15kWh per square metre per year – which reduces fuel costs to a negligible amount. It has been shown that a Passivhaus will not fall below 16 °C, even without heating during the coldest winter months.

Here are the main components of a Passivhaus.

■■ Compact form and good insulation All components of the exterior shell of the house are insulated to achieve a U-value that does not exceed 0.15 W/m² K and 0.8 W/m² K for external windows and doors. By comparison, the current Building Regulation requirements to Part L 2010 only require 0.3 W/m² K for fabric and 1.6-1.8 W/m² K for windows.

■■ Passive use of solar energy Careful design of southern orientation and shading methods maximise solar gain in winter but reduce overheating in summer.

■■ Building envelope air-tightness Air leakage through unsealed joints must be less than 0.6 times the house volume per hour. A typical small, detached house under current Building Regulations is allowed to leak 1,600 m3 of air, while a similar Passivhaus must only leak 200 m3 in the same time.

■■ An efficient air-to-air heat exchanger Here around 80 per cent of the heat in the exhaust air is transferred to the incoming fresh air.

■■ Energy-saving household appliances As well as using modern energy-saving household appliances, Passivhaus buildings use hot water supplied from solar collectors or heat pumps.

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334 Construction Technology

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Low- or zero-carbon energy sources 335

Most householders receive energy by being connected up to the national energy distribution grid. The Office for National Statistics reports that, in 2008, the energy supplied to households was in excess of 83M tonnes of oil-equivalent energy: 68M tonnes were supplied by gas, oil and coal-fired electricity power stations, and around an additional 15M tonnes were needed to cover generation and distribution losses. The total energy supplied for all sectors was 227M tonnes, of which only 5M tonnes were generated from renewable sources (around 2 per cent).

Low- or zero-carbon (LZC) technologies aim to cut down on our reliance on these fossil fuels with no, or limited, carbon by-products. Another advantage to LZC technologies is that they can involve small, local generation installations with reduced distribution losses and with the ability to meet local environmental conditions. The UK is aiming to achieve 20 per cent of its energy from renewable sources by 2020, so there is a considerable push to bring these technologies into both new and existing dwellings. To assist this process the Government has introduced the ‘feed-in tariff’ (FiT), under which all installations of LZC installed under the Microgeneration Certification Scheme (MCS) will be able to receive payment for any energy generation, whether it is used or sold back to the energy supply companies.

The most popular LZC technologies currently being applied to small-scale domestic dwellings are:

■■ solar thermal hot water;

■■ solar photovoltaic energy;

■■ heat pumps;

■■ wind turbines;

■■ greywater recycling.

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336 Construction Technology

SoLAR ThERMAL hoT WATER

This is one of the most popular LZC technologies used in both new-build and refit dwellings. In simple terms they collect thermal energy direct from the sun, which is then used to heat up the domestic hot water for washing and bathing. There are a number of systems on the market that can be open-loop or closed-loop systems. The open-loop is the cheapest as it warms up the water that is supplied to the end user. A main disadvantage is that the system may become quickly furred up, particularly in hard-water areas where limescale is deposited on the inside of the piping when the water is heated. Also the installation needs to be robust enough not to corrode and allow potable water to be contaminated by external agents. Closed-loop systems are preferred as the above problems can be avoided and the water in the closed loop only has to give up its contained limescale once, so furring is not a major issue. The components of a typical solar collector closed-loop system are shown in Fig. 7.6.1.

It is important to monitor the temperatures at the collector and at the tank as these can inform the central controller on the circulating pump operation. The heat-exchange liquid can be just water; however, in these systems, a drain-down tank is included in the circuit so that during freezing conditions, when the pump is switched off, water drains out of the exposed roof collectors to an insulated tank, thus avoiding expensive water leaks through freeze-thaw action within the exposed roof collector. If a drain-down tank is not used, the water needs a suitable concentration of glycol-based non-toxic antifreeze present. An expansion tank and pressure relief valve are also required, as in any sealed system, to prevent dangerous expansion and possible rupture of the piping. It is also necessary to include a secondary heating source to supplement the solar heat exchange, which can be by electric immersion heater or via secondary heat coil from a conventional gas-fired condensing boiler.

Two main types of solar panel are used in the UK: the flat plate collector and the vacuum tube collector.

■■ The flat plate collector is the traditional solar heat collector. It consists of a network of black painted pipes bedded in insulation within a black box with a glazed top, constructed in accordance with ENV 12977-1 and 2. A typical flat plate collector is shown in Fig. 7.6.2. These collectors heat the circulating fluid to a temperature considerably less than that of the boiling point of water. They are best suited to applications where the demand temperature is 30–70 °C, or for applications that require heat during the winter months, such as low-heat under-floor space heating.

■■ The vacuum tube collector is about 30–40 per cent more thermally efficient than a flat plate collector and achieves higher temperatures. They are also easier to maintain and repair without having to decommission the whole system. Like flat plates they are fitted at an angle, usually to the sloping roof of a dwelling. They operate by the solar energy heating up liquid contained in a sealed tube within the vacuum tube. A typical vacuum tube collector element is shown in Fig. 7.6.3.

M07_CONS_SB_6828_P07.indd 336 13/10/2011 12:27

Low- or zero-carbon energy sources 337

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M07_CONS_SB_6828_P07.indd 337 13/10/2011 12:27

338 Construction Technology

Insulated black enclosure box

Black-coated copper pipes

Black-painted copper absorption plate

Insulation

Inlet connectionOutlet connection

Glazing frame

Glazing

Figure 7.6.2 flat plate collector.

Heated water

Heat transfer

Heat transfer

Solar energy

Vapour rises to top

Condensed liquid returns to bottom

Manifold header into which 15–20 other vacuum tubes are connected

Curved reflector fin to focus diffused rays onto heat pipe

Evacuated glass tube

Sealed copper heat pipe

Closed-loop heated water to solar storage heat exchange tank

Copper sleeve in manifold

Aluminium header casing

Insulation

Copper manifold

Figure 7.6.3 Vacuum tube collector.

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Low- or zero-carbon energy sources 339

SoLAR PhoToVoLTAIc ENERGY

Photovoltaic (PV) cells convert sunlight directly into direct current (DC) electricity. They are not to be confused with thermal solar panels, which essentially use the sun’s thermal energy to heat water. The PV cells comprise a thin piece of silicon semiconductor material that has been engineered into two different layers by a process known as doping, where impurities are added to the semiconductor to form a negative layer (n-layer) and a positive layer (p-type). When sunlight hits these two layers, a flow of electrons is released between them that causes an electric current to flow.

A standard PV cell will produce a voltage of around 0.5 V. To generate a higher voltage, a number of cells are connected in series to form typically a 150 x 150 mm module; these are then further connected together and embedded in glass, to protect and electrically insulate them. The final configuration of embedded modules is known as a photovoltaic array. To be useful in domestic situations, the DC power has to be converted to alternating current (AC) power using an in-line device known as an inverter. Most systems currently being installed are standalone systems that supplement public power-grid supplies.

Traditionally PV arrays have been installed on existing buildings as a retrofitted addition, usually fixed on top of an existing sloping roof. However, they are increasingly being used within the main fabric of new buildings, thanks to the development of new products such as integrated roof tiles, glass curtain walling, or ‘in-roof’ glass panels (which often serve a dual purpose as a solar thermal shade).

Advantages of using PV solar panels■■ Few moving parts to undergo wear and tear.

■■ PV modules can be integrated into the building fabric, which can help to reduce costs.

■■ Can be grid connected and earn money by supplying electricity via a ‘feed-in tariff’.

■■ Can produce around 100 kWh/m2 power per year throughout the whole year.

Disadvantages of using PV solar panels■■ Relatively high capital and installation cost and long payback time, where

no grants available.

■■ Requires specialist fitting and careful positioning to obtain optimum performance.

■■ Must be sited away from shade created by adjacent buildings and trees, and will require regular cleaning.

■■ Solar panels only generate at full capacity 10–30 per cent of the time.

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340 Construction Technology

hEAT PuMPS

There are two main types of system that use highly efficient refrigerant technologies to maximise heat gain (or cooling):

■■ ground-source heat pumps;

■■ air-source heat pumps.

The schematic elements of a heat pump system are shown in Fig. 7.6.4. The heat pump contains an evaporator – a heat exchanger that causes a circulating refrigerant, such as propane, to turn to vapour as it absorbs the air or ground source heat energy. This warm propane vapour is compressed by the pump and its temperature is further boosted. It then enters the condenser and releases the heat to water via the second heat exchanger within the pump system. This heat is then distributed to space or hot water outlets. The vapour, which has now lost its heat, condenses back into liquid propane, which is then allowed to expand via the expansion valve, ready for the next cycle of heat transference.

The temperature of water for distribution released at the condenser typically ranges from 35 to 60 oC, depending on the season. It needs additional heating in winter, which can be via an integrated solar thermal or traditional gas- or electric-fired heating; even so, the contribution made by heat pumps alone can make very useful savings for the owner.

Systems are rated on their coefficient of performance (CoP), which means the ratio of heat delivered to the amount of electricity consumed (the amount of energy needed to drive the compressor and controls as well as any losses in the system). Table 7.6.1 gives typical CoP values for a water-to-water heat pump operation with various heat distribution systems.

Higher grade heat for distribution for space or water heating within dwelling

Refrigerant vaporised and under pressure

Low-grade heat supply

from air or ground

source

Compressor

Expansion valve

Evaporator Condensor

Refrigerant in liquid form circulating back

Heat pump

Figure 7.6.4 Elements of a heat pump system.

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