There are many reasons a building may no longer be fit for purpose and need replacing. More often than not, the argument for improved energy efficiency and reduced carbon emission of a new build gets thrown into the mix as a reason to demolish. At Greengauge we’ve had a look at the numbers to try and estimate how long it would take for the energy savings from a more efficient building to break even with the carbon released in the construction of a new build.

We will compare the estimated embodied emissions released from construction of a new house and heating it to Passivhaus levels, against the operational carbon emissions from heating an existing house built to building regulations and a Victorian aged house.

Embodied Carbon

To estimate the carbon released in the construction of new build we have used the RIBA 2030 challenge targets for embodied carbon. You can read more about the challenge here.

For 2020, the target for embodied carbon is <600 kgCO2e/m2 of floor area as calculated by the RICS methodology over the full lifecycle of the building, Stages A-C. (Current benchmarks are 1000 kgCO2e/m2 and by 2030 the target is 300 kgCO2e/m2)

Using a nominal dwelling with 400m2 of floor area this results in total embodied emissions of:  240000 kgCO2e. *

*(Emissions equivalent to: the manufacture of 840m2 of monocrystalline PV panels, 340000 miles driven at 33mpg, 40000 pairs of jeans, 4.4million Bananas bought in the UK, 20000 kg of cheese, 52 return flights from London to Hong Kong, 16 years of average annual emissions from the lifestyle of a British person. Figures calculated from: How Bad Are Bananas? The Carbon Footprint of Everything, Mike Berners-Lee.)

Operational Carbon

The table below shows the difference in performance between a Passivhaus; a building built to building regulations (including the likely estimated performance gap from poor design/construction); and a Victorian-age house. Multiplying by our nominal floor area of 400m2 we get the annual heating demand:

Standard  Heating Demand (kWh/m2.yr)Heating Demand (kWh/yr)
Passivhaus (Classic)156000
Building Regulations House (in use*) – Passivhaus Trust 20198634400
Victorian House (source: BRE)16465600

Using proposed carbon factors for SAP 10.1 it is possible to calculate the carbon emissions of heating the building for a year from different fuel sources. Then by dividing the total embodied emissions by the operational emissions from heating we will be able to see how many years it will take for the savings to equal the released embodied emissions.

Carbon factors

Unfortunately, this is where it gets tricky, because whilst a kWh is always a kWh. How much carbon is released to produce the kWh of energy varies depending on the fuel source and changes day to day and hour by hour depending which way the wind is blowing. (As can be seen on the Real Time British Electricity Fuel Mix.)

The table below lists the carbon factors for different energy sources, note how electricity is decreasing over time due to decarbonisation of the grid:

grams CO2e / kWhSAP 2012
October 2013
SAP 10.0
July 2018
SAP 10.1
October 2019
Heating oil298298298
Mains gas216210210

One of the many benefits of Passivhaus building is that their low heating demand makes them suitable to heating with a heat pump using lower temperature hot water than you would find in a normal radiator. Heat pumps can have a coefficient of performance COP of around 3, meaning every 1 kWh of electricity put into it can be used to generate 3 kWh of heat. This is significant as it effectively divides the grams CO2e / kWh for electricity in the table by 3. It is worth noting that although electricity is now preferred for environmental reasons, the cost differential between electricity and oil/gas means that electricity is not affordable for heating old and inefficient properties.  The lower heating demand of a Passivhaus building means the heat pump can run with high efficiency reducing this penalty.

A possible scenario of heating used for the calculation would be:

  • Passivhaus – Electric Heat Pump
  • New Build House – Mains Gas
  • Victorian House – Mains Gas

Using the above scenario and SAP 10.1 proposed carbon figures the results have been calculated below.

Standard  Carbon emissions from heating (kgCO2e.yr)Savings in Carbon emissions (kgCO2e.yr)‘Break Even Period’ (years)
Passivhaus (Classic)272
Building Regulations House (in use*) – Passivhaus Trust 20197224695235
Victorian House (source: BRE)137761350418

The final column is labelled ‘break even period’ this is the number of years before the carbon emissions from constructing a new building and heating it to Passivhaus levels is equal to the emissions of leaving the old building and heating it at its current heating demand. The RICS methodology estimates a new building will have a lifetime of approximately 60 years therefore, this calculation shows demolishing and rebuilding to Passivhaus could result in less carbon emitted over the building lifetime.

However, these numbers are very sensitive to the carbon factors and assumed fuel source! Assuming the Building regulations house was also heated with a heat pump, (with underfloor heating and oversized radiators) the ‘break even’ time would be 186 years.

Another fundamental flaw with this calculation is that is heavily reliant on new buildings being heated with an electric heat pump, the UK grid currently does not have the capacity to support a mass switch over to electric heating. Read our blog, Treading Lightly on the Grid, which goes into this issue in more depth.

Do not demolish buildings!

If there is nothing structurally wrong with a building the last thing we should be doing is demolishing buildings. The previous calculations can be misinterpreted to believing ‘breaking even’ within the lifetime of a building is a good thing. It is not. A huge amount of carbon has still been released into the atmosphere and there are alternative options which release a fraction of the carbon of a new build.

Our previous blog on retrofitting, demonstrated that external insulation could have a carbon emission ’break even’ point of less than a year and go a long way to improving  the performance and comfort of a building. So, by all means, add insulation wherever possible, replace single glazing with triple, even add extensions if necessary, especially if it improves form factor! This still allows the main structure of the building to remain in-situ, preventing the need for more concrete and steel required for a new build, greatly reducing overall carbon emissions.

Repurposing, this is the challenge for the fantastic architects we work with; to use their ingenuity and creativity to match our existing building stock to the right need. To uncover opportunities to repurpose buildings and marry them with new retrofit solutions.

A key factor to consider in weighing up embodied and operational carbon is timing of emissions. We are in a climate crisis now, how much carbon we emit into the atmosphere in the next couple of years is of paramount importance. Embodied carbon released today is up there and, regardless of the ‘break even’ period, it is not coming back down. With any luck the national electricity grid will continue to grow and decarbonise, and eventually we can all switch over to heat pumps, reducing the operational carbon.

In the mean time to accelerate that transition we need to do everything we can to make our existing buildings as energy efficient as possible.

Written by Jack Preece

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