There is a long standing near consensus that ‘fabric first’ is the best approach to decarbonise heat for buildings.  After all, surely it is best to reduce, or eliminate, the need for energy through good insulation before installing other expensive measures?

I would argue it is not so simple.  Perhaps ‘fabric sixth’ should be the new mantra!

Fabric fifth?

An article by Nigel Banks, Technical Director at Octopus Energy, on LinkedIn got me thinking.  He argued for ‘fabric fifth’.  Given limited funds, and the climate emergency, his proposed priorities are to install:

1. An air sourced heat pump
2. Smart meter and smart heating controls
3. Simple measures such as measure and monitor your energy use; a timer and controls; and low to medium cost measures such as loft and cavity wall insulation
4. Solar pv, combined with a battery to increase in-house use
5. Major fabric improvements such as internal or external wall insulation (and triple glazing and insulated doors).

Of course, Nigel also argued that a combination of the above is ideal if the resources are available.

This article invoked a heated debate.  I have picked out a few clear messages:

• Agreement that an ASHP is the quickest way to dramatically cut carbon emissions
• Criticism that ASHP are expensive to install, and no cheaper than gas to operate (although the use of smart tariffs, solar pv and battery will reduce the running cost)
• Concern that an ASHP in a poorly insulated house will be expensive to operate, and will become oversized if insulation is installed later
• Good ventilation is also important to avoid damp
• Agreement that every house is different, so you should start with a retrofit advisor’s assessment of your house.

Discussion

Life is complicated. Buildings are complicated.  Money, expertise, resources, time, and willingness to adopt change are all in short supply.  Perhaps we need a menu of options. 

I’d start with 3 new options, all ignored in the debate above:

1. Downsize – move to a smaller house, perhaps more suited to your needs.
2. Share fabric – move to a semi-detached, terrace or best of all a tenement, flat or apartment with shared walls, floor and roof.
3. Share living space – cut your carbon footprint by 75% if you share with 3 others.

Ground source heat pumps should be considered as well as air sourced.  They are more expensive to install but work more efficiently which reduces running costs.  They also reduce peak demand on the electricity grid. This may reduce the need for costly electricity grid upgrades.

Deep Fabric Eighth?

Here is my tentative menu of options to cut your carbon footprint and reduce your heating costs:

1. Downsize
2. Share fabric
3. Share living space
4. Commission a retrofit advisor’s report
5. Simple measures such as smart meter, timer, and controls; and ‘shallow’ fabric retrofit measures such as draught reduction, loft and cavity wall insulation
6. Install a heat pump
7. Install solar pv and a battery
8. Deep fabric retrofit (to passivhaus standards)

To help you decide on your priorities within your available budget, I have graded each option against the following criteria:

• Installation cost (capital)
• Impact on running costs and financial payback
• Embodied carbon and carbon payback
• Carbon impact (operational)

Installation Cost

Options 1 and 2 will save you money.  Option 3 at no cost.  Option 4 and 5 are relatively low-cost measures.  Option 6, 7 and 8 are all expensive, in the order of £10,000+ each.

Running Cost and Payback

Option 1, 2 and particularly 3 will cut your energy costs significantly.  Option 5 should have a short to medium payback, whilst 6, 7 and 8 will all have long financial payback periods.

Embodied Carbon

This refers to the carbon emissions from mining raw materials, manufacture, transport and installation.

Option 1,2 and 3 will reduce the embodied carbon per person.   5 has a relatively low impact.  In fact, the photograph is of insulation made of hemp, manufactured by IndiNature in Scotland – a natural insulation material with a negative carbon footprint.

6,7 and 8 all have a medium to high impact.  A battery and deep retrofit will both have a high impact – it will take many years before the operational carbon savings outweigh the carbon impact of the materials and labour required to manufacture and install a battery or external wall insulation.  Interestingly, although a heat pump is relatively carbon intensive to install, the emission savings are so significant that the carbon payback is just over one year.

Carbon Impact 

1,2 and 3 will all result in an immediate and significant carbon reduction per person.  5 could have a small to significant impact depending on circumstances.  6 will have a major impact – my air source heat pump cut my carbon emissions by 80%.  7 has a surprisingly low carbon benefit because our UK electricity grid is trending towards a very low carbon footprint. 8 can cut your carbon impact significantly.

Conclusions

Rather than fabric first, fifth or even eighth, there is a menu of options.

The no cost way to reduce your carbon footprint is to ‘rightsize’ to a smaller house, preferably a flat and, depending on its size, share it with family or friends. Very few people seem to advocate this.

Assuming you can’t or won’t downsize, then install simple measures which are low cost and will have a quick carbon and financial payback.

Before undertaking other measures, commission an independent retrofit advisor’s report.  This may avoid you making an expensive mistake.  There is a strong argument for the Government to subsidise public access to independent retrofit advisors.

If you can afford it, then installing a heat pump will dramatically reduce your carbon emissions.  Unfortunately, the financial payback is minimal given current high electricity prices versus gas prices.  

It feels good to produce your own power from solar pv.  It will cut your household’s total electricity bill, especially if combined with a battery.  A (cheaper) alternative is to divert any surplus electricity to heat your hot water.

All new houses should be built with deep fabric insulation, but it is expensive, and potentially disruptive to retrofit houses to this standard.  Again, it is a nice thing to do if you can afford it but will not provide a fast payback.  It will substantially reduce, but not eliminate your carbon emissions.   

So, to cut your carbon footprint the slogan should be “rightsize first”, “heat pump third”; “shallow fabric fourth” and “deep fabric sixth”.  Not very catchy, but real life is complicated!

1. Rightsize first
2. Retrofit report second
3. Heat pump third
4. Shallow fabric fourth [in reality, do this alongside no. 3]
5. Solar pv fifth
6. Deep retrofit sixth

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Carbon Choices

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