Lifecycle analysis sheds light on whether retrofitting or rebuilding will best help the government to deliver its 2050 carbon reduction targets
01 / INTRODUCTION
Over the last year, we have explored the potential implications of whole-life carbon analysis for office, educational and retail schemes (18 May 2012, 14 September 2012
and 28 March 2013). In this piece, we compare three residential scenarios: a typical 19th-century terraced house, its upgraded refurbishment and its replacement with an exemplar Passivhaus.
Forthcoming UK legislation, such as Zero Carbon Homes and minimum energy performance certificates for rented properties, deals with regulated operational emissions from the energy used to heat, cool and light a building. From this perspective, traditionally constructed pre-1919 houses appear to perform badly, with the English Housing Survey (2012) calculating average emissions to be twice those of dwellings built after 1990.
What the legislation does not cover are the embodied emissions - those from the extraction, processing and transport of materials, to construction, refurbishment and maintenance.
As in our previous studies, we have carried out carbon lifecycle analysis of several options, in order to explore the relationship between operational and embodied emissions over the long term. This allows us to begin to answer questions such as:
- Are traditional buildings really performing so badly?
- Can they be improved?
- Is it best to rebuild poorly performing properties?
- Is demolition really the sustainable option?
- What policy would achieve an 80% cut in emissions by 2050?
WHOLE-LIFE CARBON ASSESSMENT: A BRIEF RECAP
The European standard CEN/TC 350 identifies four stages in the life of a building - product manufacture, construction, in-use and end-of-life - with more detailed sub-categories used to pinpoint specific sources of emissions.
The first stage encompasses the extraction of raw materials, their transport to a point of manufacture and the process of transforming them into construction products.
The second involves transporting construction products to site and the on-site processes involved in assembling them into a building.
The third stage includes maintenance, repair, replacement and refurbishment of the building, as well as the use of energy and water during its occupation.
In the final stage, the building is deconstructed and its redundant components transported off site, processed and disposed of.
At each stage, whole-life carbon analysis can identify solutions with lower carbon impacts, delivering considerable savings overall.
The first part of the standard, EN 15804, was published early in 2012 and establishes product category rules for environmental product declarations.
Last year, CEN TC 350 was incorporated into a British standard, BS EN 15978. Therefore, although not legally required, the framework for whole-life carbon now exists in the UK.
02 / SCENARIOS
- Scenario A The baseline is a two-storey 19th-century, brick terraced house with a slate pitched roof and a flat roof extension to the rear - typical of much urban housing in Britain. We have assumed that no work has been carried out beyond basic maintenance and decoration.
- Scenario B Retrofit of the baseline building is the second option. This takes a comprehensive “fabric first” approach to improving building envelope and the operational performance, while maintaining the vapour permeability of the original construction. The house is in a conservation area, ruling out measures such as external wall insulation, but solar photovoltaic panels (PVs) and mechanical ventilation heat recovery are proposed to the rear of the property.
- Scenario C The last option is the replacement of the existing building with an exemplar new building. The house is insulated to the Passivhaus standard and is “zero carbon”, as defined by the UK government’s 2016 target for new homes (all regulated operational carbon is offset by 25m2 solar PVs). The substructure is concrete. The frame, wall cladding and roof are all timber.
|SCENARIO A: Unimproved 19th century terraced house||SCENARIO B: Retrofitted 19th century terraced house||SCENARIO C: New build Passivhaus|
|Construction||Facade||Two skin brick and plaster||150mm wood fibre internal wall insulation. General draught proofing||Timber frame, larch cladding and 250mm cellulose wall insulation|
|Ground||Suspended timber floors||150mm hemp insulation laid between floor joists||Concrete foundations and 240mm polystyrene insulation|
|Windows||Leaky sash and casement windows||Slimline double glazed sash and case windows||Triple glazed timber windows|
|Roof||Pitched slate roof. Flat roof extension to garden||300mm mineral wool insulation between rafters||540mm cellulose roof insulation. Larch tiles|
|Services||Wet central heating system run off old gas boiler||Combi boiler, LED lighting and MVHR||Combi boiler and MVHR|
|Renewables||None||3m2 solar PVs||25m2 solar PVs|
|Airtightness||Very poor||Improved||Very high|
|Other||Minor decoration||Major redecoration associated with works||Demolition of existing and waste transport|
|Construction costs (£/m2)at practical completion*||50||500||1,750|
|Dwelling emission rate (kg CO2/m2)**||72.8||21.6||0|
* Average cost based on previous projects in the London area ** FSAP 2009. Base for scenarios A and B produced by Furness Partnership
03 / ASSUMPTIONS
- Sequestration - the absorption of carbon dioxide by plants and trees - is outside the scope of CEN TC 350. The benefit may have already been claimed by producers, and varies depending on factors such as tree type, growth rate, processing and treatment of waste, as explored in our earlier article on schools. The whole-life carbon of scenario C would be reduced by about 11% over 60 years if sequestration was included for the timber frame.
- It was assumed that 25% of construction waste would be sent to landfill, increasing end-of-life carbon emissions for organic materials such as timber and cellulose insulation. This figure is attributed to the timber in the product stage for the reasons outlined above, but if separated would increase the end-of-life stage, particularly for scenario C.
- Allowance made for grid decarbonisation of 92% by 2045*. Plant efficiency is assumed to improve by 20% per replacement.
- Unregulated operational emissions from items such as cookers and televisions were kept constant across all three scenarios. These depend on occupant behaviour and efficiency of appliances, so would likely be smaller for scenario C as all of the fittings would be new.
- The impact of projected climate change on demand for heating and cooling has not been included. For some of the issues involved, refer to Sturgis Carbon Profiling’s 2011 real estate climate change model for the RICS.
* Embodied Carbon: A Look Forward by C Jones (Sustain 2011)
04 / FINDINGS
|Stage||Scenario A||Scenario B||Scenario C|
|Product (materials, transport, manufacturing)||12||94||588|
|Construction (transport and site works including demolition of existing)||1||6||109|
|In use (maintenance, refurbishment)||129||172||397|
|End of life (deconstruction, transport etc)||50||50||35|
|Operational (regulated and unregulated)||2,913||1,010||313|
|Whole-life carbon over 60 years (kgCO2e/m2)||3,106||1,331||1,422|
As can be seen in the graph and table above, there are considerable differences between the whole-life carbon of the three options.
- The baseline’s whole-life carbon at 60 years is double that of the other scenarios. With very little capital expenditure, almost all of the emissions are operational.
- Scenario B shows that it is possible to make substantial reductions in emissions through retrofit - though the operational performance is obviously still well below Passivhaus standards.
- The additional embodied carbon required for installing and maintaining the retrofit measures is less than 150kg CO2e/m2. This carbon expenditure effectively halves the whole-life emissions through operational savings. This supports retrofit investment as an “allowable solution” for offsetting emissions.
- The lifecycle analysis suggests scenarios B and C to be comparable at 60 years.
- The key difference between the retrofit and rebuild whole-life carbon is the ratio of operational to embodied emissions, with the 80/20% split reversed in scenarios B and C.
- Most emissions beyond practical completion of the Passivhaus are due to replacement of elements such as solar panels and cladding. This is partly because renewal of items would likely bring forward the replacement of connected elements - for example, timber cladding would probably be replaced at the same time as cellulose insulation.
- Over the long term, the emissions of the unimproved house continue to increase more rapidly than the other options due to its larger operational demand.
- This is also due to heavier reliance on gas, which is more efficient in the short term, but less so as the grid decarbonises. It may therefore be appropriate to change the heating and hot water systems to electricity in the medium term.
- As shown in the diagram above, scenario B has the lowest whole-life carbon at 2050. This suggests that retrofit is the best strategy for reducing emissions by this date in order to mitigate anthropogenic climate change.
- As it does not benefit from retained fabric, the whole-life carbon footprint of scenario C is significantly higher at practical completion, reflecting the embodied carbon in demolition, materials and construction.
- The gap narrows between scenarios B and C, with the Passivhaus performing better than the retrofit after 100 years.
- However, the materials in the new build, while “green”, may be less durable than traditional construction - the emissions from which have already been expended. The lifespan of new build may also be shorter as the existing building is protected by historic environment legislation.
- Low housebuilding rates make it likely that most houses in 2050 already exist today. Therefore the biggest gains are to be made in improving the energy efficiency of our existing building stock. According to the Office for National Statistics’ English Housing Survey - Homes 2010, less than 5% of solid walls have some form of insulation, in contrast to 61% of cavity walls.
05 / CONCLUSIONS
This research highlights the pressing need to improve our existing building stock - both to cut emissions and to future-proof traditional buildings, which have enormous heritage and economic value at the centre of many communities.
The findings favour retrofit where possible. The National Trust for England and Wales have committed to reducing their dependence on fossil fuels by 50% by 2020, and Historic Scotland likewise 25% by 2015. The action of these leading heritage organisations suggests that there is scope to improve the operational performance of even the most sensitive historic buildings.
There are of course economic and social arguments for building new housing. The Passivhaus will undoubtedly perform far better than most new buildings, and such exemplar development should be encouraged.
In our experience, the whole-life carbon of the new build could be reduced through careful specification of items such as concrete. Further reductions can be achievedthrough design, for example timber foundations or a more durable timber for cladding, such as chestnut or oak.
This may be a more cost-effective way of offsetting regulated operational emissions to achieve “zero carbon” than solar PVs (as explored in our On-Site Renewables appraisal for the British Council for Offices).
As well as embodied carbon reductions, another proposed allowable solution for new build is investment in the energy efficiency of existing buildings - a solution that these findings suggest would be very carbon-efficient.
We would like to thank Victoria Herring and Michael Levey of Grosvenor for their assistance and advice in preparing this article, as well as Matthew Shepherd of Sturgis Associates for his diagrams.