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How green is your house?

CONCRETE ENGINEERING - How homes are built today will decide how much energy we use tomorrow. A new report compares building materials on the grounds of long-term energy efficiency. Andrew Minson reports.

Around 50% of the UK's carbon emissions are due to the energy used to heat, cool and light buildings. So it may come as no surprise that, in the long term, the operational carbon emissions of an average home will have far more of an environmental impact than the embodied carbon dioxide of the materials used to build it.

Research recently carried out by consultant Arup compares different house building materials against the backdrop of an ever warming planet to nd out which type of construction really is the greenest.

Andrew Minson is the head of framed buildings at The Concrete Centre.

The scenario A typical two-bedroom semi-detached house in the south east of England - similar to the 'starter home' envisaged by the Government - was considered using four 'weights' of construction: light, medium, medium-heavy and heavy.

Occupancy was assumed to be continuous, with a family of two adults, one of whom was at home during the day with a pre-school age child. Gas-red central heating with radiators was assumed with a set point of 19C for the bedrooms, 21C for the living room and 22C for the bathroom. In the summer, three operating modes were considered: natural ventilation; conventional air conditioning; and mixed-mode, where the two are combined.

When it came to cooling, in line with guidance from the Chartered Institution of Building Services Engineers, a house was judged to have overheated if 1% of the occupied hours were over 28C in the living room or 26C in a bedroom. The research also included the caveat that this must occur in at least three years in ve before it was assumed that air conditioning would be installed. From the outset, all the houses had solar shading and an appropriate ventilation strategy to help mitigate the effects of climate change.

The model took weather data from a London suburb during the 20 year period from 1 January 1976 to 31 December 1995. The data was repeated to cover the 104 year period between 1996 and 2100. Temperatures were modied in line with the UK Climate Impacts Programme's predictions for climate change, for a medium-high emission scenario where global temperature rises by 3.3C.

The results The research found that the heavyweight masonry home experienced the lowest total energy consumption and CO 2 emissions over its life.

The medium-weight masonry home was calculated to have around 1.25t more embodied CO 2 than the equivalent timber house, yet over a 60 year period, the timber framed home was found to emit 9 to 15t more CO 2. This was because the heavyweight home could stay cool in the summer, whereas the timber frame house would have to become more reliant on air conditioning.

The research also highlighted additional savings that could be achieved by using thermal mass to capture solar energy and reduce fuel consumption during the winter (see 'Other things to consider').

These savings offset the slightly higher level of embodied CO 2 in a masonry house in as little as 11 years.

Other things to consider l Passive solar design can help reduce heating bills in the winter.

Thermal mass can be used to capture solar and internal gains during the heating season and re-radiate the heat into the room as the temperature begins to fall in the late afternoon. Otherwise known as passive solar design, this energy-saving technique is very simple and basically requires little more than glazing that is orientated to the south, and adequate thermal mass in the floors and walls to capture and store heat from the low winter sun. It is applicable to standard house designs and its ability to reduce the load on conventional heating systems enables worthwhile savings in heating fuel and CO 2 emissions to be realised over the life of a house. Straightforward guidance on passive solar design is available to download from the Carbon Trust website, www.thecarbontrust. co. uk

The need for air conditioning can be exacerbated by the drive to reduce heat loss in winter. Energy saving measures such as increased air tightness and insulation can lead to a greater risk of overheating during the summer, as heat cannot escape so readily. The application of thermal mass can help alleviate this problem, and it is therefore important that energy efficiency and thermal characteristics are not evaluated simply in terms of winter performance. A more holistic approach is required, considering how the building functions on a year-round basis.

The conclusion A building's environmental impact does not stop once it has been built, the report proved.

The claim sometimes made that lightweight construction is the most energy efficient is too simplistic and reflects past standards of design.

To evaluate energy consumption properly, we must consider new and improved construction standards, the impact of climate change, the potential for passive heating and cooling techniques and yearround analysis.

How different types of houses fared in the tests Lightweight house:

Timber-frame home with timber oors, exterior brick and internal plasterboard nish.

CO 2 produced by 2006: 41t Air conditioning installed: 2021 CO 2 produced by 2023: 65t CO 2 produced by 2100: 216t Medium weight house:

Timber-frame home with timber oors, with brick and block cavity walls.

CO 2 produced by 2006: 42t CO 2 produced by 2023: 63t Air conditioning installed: 2041 CO 2 produced by 2100: 194t Medium heavyweight house:

Precast concrete rst oor and ground oor partitions of mediumweight concrete blocks with a plasterboard nish.

CO 2 produced by 2006: 43t CO 2 produced by 2023: 64t Air conditioning installed: 2061 CO 2 produced by 2100: 184t Heavyweight house:

Heavyweight blocks used for external walls and internal partitions, with a pre-cast concrete rst oor and loft oor.

CO 2 produced by 2006: 45t CO 2 produced by 2023: 65t Air conditioning installed: 2061 CO 2 produced by 2100: 180t

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