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Retrofitting of buildings: a cost-efficiency analysis

In 2010, the French Ministry of Research funded a three-year study to analyse possible solutions leading to a reduction in CO2 emissions that could be feasibly implemented across a large city. This study sought to construct cost-effectiveness indicators in various sectors (building, transport, non-carbon energy production). Cost-effectiveness ratios measure the effort required to implement a solution and the impact in terms of CO2 savings. The results for the buildings are summarised below.

Diagnosis of the housing stock in Grenoble

The housing stock in Grenoble can be depicted in 40 segments; 5 construction ages x 4 technologies used in heating apartments and identical for single houses. The energy consumption and CO2 emissions for each segment were estimated for 2010 and summarised in the table below.

Energy consumption and CO2 emissions for space heating in Grenoble in 2010

Source: Enerdata

A great deal of heterogeneity exists within the building stock. For example, individual houses consume significantly more energy than apartments. Similarly, housing built before 1974 accounts for 65% of the total area but 82% of CO2 emissions.

Three strategies to reduce CO2 emissions in space heating

There are three ways to reduce CO2 emissions in space heating: insulation, upgrading heating equipment and the reduction of CO2 content of electricity and district heating.

Insulating the building is the solution that comes first to mind. This work includes the insulation of facades, roofs, floors and the replacement of old windows with new less emitting ones. The impact of renovation operations differs from one situation to another. The insulation of old buildings obviously generates more CO2 savings than an operation undertaken on a more recent construction submitted to insulation standards when built.

The second solution is to consider upgrading the heating systems. The installation of a modern boiler can save about 25% in energy consumption - and therefore CO2 - against the initial boiler. When considering this replacement, it may be appropriate to plan an energy switch. For example, it could make sense to replace a collective gas boiler with district heating or to replace individual gas boilers with heat pumps.

The third solution consists of a CO2 content reduction of electricity and district heating. In terms of district heating, the heat can be produced from coal, gas, municipal waste, biomass, etc. Increasing the share of biomass aids in the reducing CO2 content of this energy carrier. For electricity, it is also possible to influence the relative share of energy inputs and thus promote non-carbon electricity. In most developed countries, the CO2 content of electricity is between 400 and 500 grams per kWh. France this figure is much less (80 gCO2/kWh) because of the importance of nuclear. In Switzerland and Finland, it is even lower and electricity is almost carbon free. For the last 30 years, the CO2 content of electricity has declined in many developed countries. But past trends do not necessarily reflect those of the future and making accurate predictions on this subject is a notoriously difficult exercise.

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