4.11 Embodied Energy in Retrofit

Embodied Energy – the “Carbon Burp”

Going back to the 3 CLR modelled house types introduced in Lessons 4.8 to 4.10, and the different energy efficiency retrofit scenarios assessed (e.g. IWI and EWI), the embodied energy of the main materials used has also been considered.

To keep the exercise simple and manageable, whilst retaining a good degree of realism, we have focused on the manufacture and installation of the key energy efficiency measures: insulations, windows and doors.

This is intended to represent the majority of the materials used for achieving low energy performance. The exercise doesn’t include repair, maintenance or the various items of ‘consequential’ work. We use embodied carbon data from the University of Bath ICE database (see http://www.circularecology.com/embodied-energy-and-carbon-footprint-database.html accessed 04/01/16).

The 3 CLR modelled house types use a variety of insulation types in this exercise.

For example:

• with less vulnerable and higher thermal performance (with higher embodied energy) materials used in some situations e.g. insulations used in cavity walls and in solid floors and
• with lower embodied energy materials such as woodfibre insulation for example used for IWI.

The choice of different materials for different applications is ultimately made by the retrofitter, and the scenarios illustrated below are simply for illustration as part of this exercise. A minimum of underlying detail used in these calculations is shown here – for example the embodied energy of different sorts of window upgrades is not detailed.

More research – and consistent, standardised accounting and reporting – is generally needed in this area.

The carbon pollution produced as a result of these measures occurs in the period leading up to and during the retrofit work and we have, perhaps inelegantly, described this as a ‘burp’ of CO₂ emitted in order to achieve on-going CO₂ reductions from space heating – the ‘carbon burp’.

2. How much material is used?

The graph below shows the relative amounts of each insulation material (or window/door assembly) used, expressed as the relative amount of carbon dioxide emitted by each in tonnes of CO₂:

PU: polyurethane
EPS: expanded polystyrene (in this case of the ‘graphitised’ or ‘grey’ type)

Above The bungalow in this illustration uses a lot of PU for the medium and deep retrofits as the poor form factor requires a low wall U-value and the cavity is filled with PU foam as well as additional layers of insulation.

3. The “Carbon Payback”

The graphs below show the CO₂ produced by each house’s space heating as dark blue bars.

• The first column (year 1) is before/during retrofit (we assume in this exercise that around the retrofit period the house remains continuously heated during the heating season).
• The CO₂ emissions from space heating after retrofit are shown in columns 2- 5 and are assumed to represent the on-going annual post-retrofit emissions.
• Added to year 1 is the carbon dioxide ‘burp’ related to the key retrofit materials – shown as pale blue patterns (keyed).

Obviously it is possible – guided by cost benefit analysis and the client’s brief – to choose ‘carbon sequestering’ EWI materials or those that have lower embodied energy. This exercise simply illustrates the principle of ‘carbon payback’ and suggests orders of magnitude when compared to lifetime CO₂ pollution.

In our model the carbon pollution emitted in year 1 is ‘paid back’ in around 4-5 years for EWI deep retrofits and 1-2 years for IWI retrofits – as described in the following set of graphs, for each house type with each type subdivided into EWI and IWI retrofits:

Above In year 2 (the first year after retrofit) the house starts to deliver CO₂ savings from reduced space heating fuel consumption (with the 3°C comfort take). In the example above, it takes 3.4 years for the CO₂ saved by the retrofit to equal the CO₂ emissions related to the materials and products used to achieve the retrofit. Only after this period does the atmosphere ‘see’ on-going reduced CO₂ pollution from the home.

4. Carbon Savings in the longer term

Although reduced, these residual emissions of course accumulate over time in the atmosphere as indicated in the graphs below.

Starting from the initial carbon burp and year 1 heating emissions, the graphs below show cumulative CO₂ emissions over 60 years following each retrofit compared to emissions from the unimproved ‘basecase’ houses.

While the deepest retrofits have the highest embodied carbon in these modelled scenarios, they also save more carbon with each decade (to the end of the life span of the retrofit).

Obviously all emissions would fall if – hopefully when – the UK heat supply becomes progressively decarbonised.

As well as considering greenhouse gas pollution resulting from the products and materials used to achieve low energy performance, it is also important to also think about carbon sequestration related to our gardens, streetscapes and public or private greenspaces around buildings (see further reading below).