Retrofitting the future: how can we make ‘Zero carbon ready’ homes?

by Blog

What does it mean to have a home fit for a ‘climate safe’ future? An 80% cut in carbon emissions? A ‘net zero’ home? And why bother at all if we transition away from gas to electric only homes and the grid is completely decarbonised?  Marianne Heaslip of URBED and Jonathan Atkinson of Carbon Co-op use this blog post to outline the key issues and propose a new ambition for housing retrofit: ‘Zero Carbon Ready’ homes.

This article forms a primer for our forthcoming Carbon Co-op AGM 2020 debate on Friday 17th July 2020 at which Marianne and Julie Godefroy present and invite contributions from members and attendees.

For many years Carbon Co-op’s ambition for retrofitting homes was expressed in terms of ‘2050 Homes’ – achieving today the kind of domestic carbon emissions we at that time needed to meet by 2050. But since we coined the term much has changed, the consensus now is 2050 will be far too late. 

In this post we explore some of these terms and concepts and assess their relevance for our work. We reflect on the performance targets we should be encouraging members to aim for and the benchmarks we should be using in our My Home Energy Planner assessment reports.

We think about the term ‘zero carbon ready’ and conclude that we need to start to think in absolute terms about reducing our energy demand for heating, about reducing peak electricity loads and ultimately see our individual homes in the context of wider energy system decarbonisation and collective action. 

The bigger picture

Home retrofit needs to be viewed within the wider decarbonisation strategy – ‘the plan’ for the UK is to decarbonise electricity generation whilst moving domestic heating and transportation away from gas and oil and on to electricity, heat pumps rather than gas boilers, and electric cars in place of petrol and diesel (we are very sceptical about the idea of hydrogen as a viable option for domestic heating). 

This means that over the next few years, as more heat and transport is electrified there will be more and more demand for electricity putting more pressure on decarbonisation. The answer? At least in terms of heat, greatly reducing our need for energy in our homes through reducing demand by retrofitting homes with insulation and other energy efficiency measures.   

Over the next few decades energy efficiency is key to the energy transition and tackling climate change. 

Net Zero homes?

A lot of people are talking about ‘Net Zero’ homes, but it is a problematic concept, not necessarily focussing on reducing carbon emissions but lowering them to a point they can be balanced against so called ‘negative emissions’ or offset against ‘zero carbon’ renewable energy generation. It’s comparable to a set of scales, on one side the ‘costs’ of carbon used, on the other the ‘benefits’ of clean energy generated, ‘Net Zero’ is conceptualised as the scales balancing, but what goes in the scales?

On a system level negative emissions might controversially include carbon capture and storage or tree planting but on a domestic scale measures are limited to renewables like solar panels. However, though a home might generate enough  to meet its total energy demand measured over a whole year  this would not take account of the peaks and troughs of day/night and summer/winter. A battery could balance generation to an extent, but the amount of embodied carbon required to manufacture a battery is huge and not ‘repaid’ through its use. 

Likewise, a house might be retrofitted to bring down its energy requirements, but use high energy, synthetic materials and taken over its lifetime, incorporating embodied energy costs the result may not be ‘net zero carbon’. 

In many ways the idea of individualised ‘net zero’ homes is incredibly problematic and not really viable. 

Carbon emissions and primary energy 

Domestic primary energy use and associated carbon emissions are poor metrics – they tell you as much about the wider energy system as they do about the performance of the building. 

Primary energy is the measure of the total amount of raw energy required to meet the energy needs of a home. In the case of electricity, this is the energy required to generate and transmit electricity from the power station to your home. In the case of gas, it isn’t just the energy from burning the gas, but other activities such as extraction, transmission, storage etc. 

Primary energy and carbon factors are the way we calculate how the energy used in your home equates to primary energy demand and carbon dioxide emissions. We simply multiply the energy measured at your meter by an agreed figure, the ‘factor’ in the case of carbon this is an assumed average of kilogrammes of carbon dioxide equivalent per kilowatt hour used (kgCO2e per kWh). It is important to understand that this number is an approximate figure, which for grid electricity is based on an assumed average. In reality these factors change all the time as different forms of generation contribute more or less to the grid and demand fluctuates. They can vary significantly between summer and winter, day and night, and also over years as the transition to renewable energy accelerates.  More recently for example they have swung wildly between periods of lockdown and opening. 

Carbon and primary energy factors are therefore largely beyond the control of the householder – you could do absolutely nothing to a house and its carbon emissions reduce significantly over time. Worse still, for people interested in buildings and energy, articulating targets in terms of carbon means trying to hit a moving target that wobbles around every year, month, and even day, making it incredibly difficult  to track progress over time. 

Less is more – energy performance is key

Instead we should be concerned with the energy performance of a home because that is something we can actually influence.

Our Carbon Co-op ‘Retrofit End’ refers to the adoption of a ‘fabric first’ approach, this means focussing on the energy performance of a home through the components that make up the building envelope i.e. the walls, floors, ceilings, windows, doors etc, before thinking about heating systems or renewable technologies like solar panels. 

Carbon Co-op are committed to ‘fabric first’ for lots of reasons:

  • We know it works! Avoiding the ‘performance gap’.
  • Improving building fabric can address issues such as mould and damp, and in turn the health and comfort. 
  • It offers energy bill reductions and long term, predictable, bill security for householders.  
  • It helps with wider energy system management by reducing peak demand.

Not carbon but space heating and peak demand

Space heating demand is a measure of how much energy a home requires to heat it and is a better metric for home energy performance – especially in a climate like the UK’s, where most energy used in our homes currently goes into keeping them warm in winter. Usually measured in terms of the energy needed per meter squared of floor space per year (kWh/m2.a) it is independent of carbon grid intensity and offers a static metric that can be used on any home to benchmark energy performance against other homes and against climate change targets. 

In the context of the grid, more renewable generation means more plentiful, cheap generation at some times and constraints at others when generation is low and demand high. What becomes more important at the domestic level isn’t the total amount of energy used but the peak requirement over the year, known as peak load. If a home’s total energy requirements at any one time can be kept within a reasonable limit, then that home will be able to utilise more renewable energy and help reduce our reliance on fossil fuels generation. 


This can be done using heat pumps, as these can be run at a constant but low level. In addition, the use of automated load shifting of very high loads via the use of ‘smart-enabled’ components such as electric vehicle chargers, unlock the potential to shift loads to different times of the day when the grid is less constrained. A well insulated home has the potential to shift demand because the whole house acts like a ‘battery’ retaining heat for long periods.

All of our suggested benchmarks are absolute rather than relative targets. This provides a measurable number for designers and engineers to aim for, rather than a % reduction which is another moving target. This also encourages fairness, following the principles of ‘contraction and convergence’ – with the aim that in the end every home has a fair share of the energy that can be produced by a decarbonised grid.   

Zero Carbon Ready Homes?

One of the unique things about Carbon Co-op is that our members are keen to demonstrate what is possible in terms of energy efficiency and lead the way in inspiring others. 

With that in mind we want to help people be ambitious, and to do as much as is reasonable and technically feasible. We don’t think carbon targets should be met by doing nothing and waiting for the grid to decarbonise or covering homes in PV panels or installing expensive and high carbon batteries. Ultimately in any case, that will not result in the dramatic reduction in real carbon emissions that we desperately need. 

Instead, we propose focusing on making homes ‘zero carbon ready’. Working on the assumption that over time most energy uses will be electrified, and that in turn electricity generation will be wholly decarbonised – and we should be doing all we can at a household level to enable this – but recognising that it doesn’t all happen at the level of the house or flat.

Breaking down the targets

To help householders understand what the concept of Zero Carbon Ready Homes’ means in practice we’re proposing to make some changes to how we present information within My Home Energy Planner reports – but these metrics can be used by householders, irrespective of whether they’ve had an assessment from us or not. 

We will renew our focus on ‘fabric first’ by placing Space Heating Demand (kWh/m2.a) as our headline metric. The targets for this will be set with reference to the work of the Zero Carbon Hub and the AECB and we will explore further sophistication by setting more bespoke targets with reference to building form factor.

We will support Space Heating Demand with a supplementary metric of peak heating load (W/m2) – similar to that used in Passive House. This also encourages efficiency by helping to avoid oversizing heating systems. 

Furthermore, we propose making clear the ambition to remove fossil fuels from space heating supply. We will always present a ‘fossil fuel free’ heating scenario as part of My Home Energy Planner report recommendations. This will be based on technology that is currently available and that we know works – which will often mean a heat pump or other form of electrified heating. 

We will introduce a new metric of ‘Energy Use Intensity’ as suggested by LETI (London Energy Transformation Initiative) in their zero carbon standard. This is measurable at the meter and therefore a very useful performance metric. What this is set at needs further research – LETI suggest 35kWh/m2.a for new buildings though we may relax this for retrofit.

Instead of focusing on carbon emissions now, we will look at projected carbon emissions over time, comparing each scenario we recommend. This could be a more powerful motivator, with graphs showing a ‘do nothing’ option against the savings created by each scenario. 

Recommendations on domestic renewable generation options will be carefully considered. We will encourage people who have good sites to add PVs etc but avoid recommending other, less optimal technologies in the wrong circumstances. We will also take a wider system and embodied carbon view, so for example PV diverters could be encouraged, but batteries less so. 

Put together we believe Zero Carbon Ready Homes will provide a powerful benchmark for changes and a set of metrics for all householders aiming to retrofit their homes to high standards. 

What do you think about our proposals? Do they make sense? Is the badge of Zero Carbon Ready Homes an effective one? Drop us an email info@carbon.coop, leave a comment or come along to our AGM on 17th July. 

Further reading

The key to tackling climate change: electrify everything, Dave Roberts, Vox

Net Zero One Pager, LETI

Deploying Heat Pumps at scale, lessons from Ireland

Passivhaus – the path to net zero, Passivhaus Trust

How lower carbon factors in SAP will change heating design, CIBSE Journal

The meaning of life: Comparing whole life carbon for gas and electric heating, CIBSE Journal

8 thoughts on “Retrofitting the future: how can we make ‘Zero carbon ready’ homes?”

  1. I think that while the move to the Space Heating Demand is certainly an improvement, and makes sense in architectural terms, it is still a misguided metric that builds in an incentive for perverse behaviour.

    It suggests that a single person living in a 17-bedroom mansion can use more energy simply because they can afford an excessively large house.

    It does not include any incentive for zoning within the property: for heating say just one of the bedrooms and just one day room.

    Even for larger families an incentive is needed to return to the practice of the majority of people back in the 1950’s. With fuel being expensive and with coal fired creating serious amounts of daily work, most households would heat only the living room during the winter, and occasionally light a fire in a second day room to entertain guests in the “best” room (parlour, front room, etc). I grew up under that regime, and in the colder months we expected to sit together as a family in the one heated room. The bedroom was cold, mitigated by the use of hot water bottles which were put into the beds half an hour before bedtime.

    The introduction of central heating from the sixties to the eighties changed our expectations: as a society we now expect to have ever room in our home at a comfortable temperature. I never noticed there was a problem with having a cold house, providing the living room was kept warm

    I would like to start a discussion about how far that is necessary, and how far it is worth going back to the post war expectations.

    My suggestion would be to adopt a metric that covers two things:

    (1) the energy use when the house is occupied at a level given by the rules on overcrowding for tenancies (whether or not the house is let or owner occupied). This figure would be final for a system that did not allow zoning, but would be reduced for a system that did.

    The final figure where zoning is possible would be calculated by working out for each combination of zones the energy use divided by the number of ppl intended to occupy that combination. The worst case would be the final answer.

    I see this as similar to the ratings of computer power supply units: a PSU is rated on its worst efficiency between 20% and 100% of capacity. You can’t get a hold or platinum rating for a PSU just on its efficiency at 100%.

    You should not, in my opinion, use a rating system for home heating that makes a mansion look good when a single occupier uses more heat in a day than a family in a small flat uses in a month: and your metric would lead to that distortion.

    • We should possibly have said – within the MHEP report and benchmarking at the moment we also include ‘per person’ carbon emissions, based on actual occupancy, which goes some way to answering this point.

      Including a total ‘peak demand’ metric, as Andy points out below, also helps.

      Zoning can certainly be useful in bigger houses – but in smaller properties that are even just reasonably well insulated the overall temperature tends to even out, so it becomes less useful.

      Thanks for contributing – looking forward to the debate at the AGM. Should be interesting.

      • Hi Marianne,
        True-river echoes my thought patterns.
        Having marveled and wondered at the technology available, I have also come full circle in a short time of thinking about ‘our human needs v carbon consumption’ and the shelf life of the methods consumers currently employ.
        eg. External insulation on retrofits – How long does this last?
        How easily can it fail?
        If we create a market for external insulation, will that be from fossil fuels/plastics?
        Can external insulation show a carbon offset return on retrofit?
        Can we train installers and home-owners well enough to ensure the correct installations and care?
        Not to mention the huge expense of external insulation – £25, 000 given as a ball park figure to do one side of my house!!
        Being at the lower end on the earnings table, this is not obtainable for my household or the larger general population.

        It is internal insulation, ventilation and draught proofing, which I feel are the most viable fabric first approaches. Leading us to take what I visualise as a ‘mini retrofit approach’ and would be what True calls ‘zoning’.
        Having insulated the loft with sheepwool, next we have selected the more temperate rooms at the back of the house (where we spend most of our family time) to start creating internalthermal enveloping with insulation to retain heat and reduce loss.
        Coupled with educating ourselves to do the work and keep costs down. We feel optimistic and enabled via CC workshops, to make our home comfortable, reduce our reliance on whole house heating and be zero carbon ready on a budget. While spending less, choosing wisely and changing our behaviour…put a jumper on!! We know we are already reducing our carbon footprint and reliance.
        Moreover ,perhaps the biggest realization in all of this is how our human desires and expectations are without argument driving up carbon consumption.
        The bigger the spender – the bigger the footprint!!
        It is clear that we all have to make life changes – which feels like a lot of pressure and sacrifice!!
        Yet on the flip side of this, is the liberating realization, that we don’t have to sacrifice our comfort, creativity or style. There are just as new, as well as old and exciting ways of achieving these ideals, as we support the natural world we all love.

  2. Great to see this understanding that energy use doesn’t, in reality, net to zero over a year (say by offsetting a gas boiler with PV), and I fully agree that we shouldn’t be focusing on individual buildings as net zero islands, but thinking about the whole national system. Peak energy use makes a lot of sense for electrifying heat too. Lately I’ve been thinking that providing peak winter evening heating is what we need to design for, and things which don’t contribute to that (such as PV) are of questionable value.

    I’d question whether using energy rather than carbon as the primary metric could reduce the apparent benefit of electrifying heat (it’s huge), though at least you would still see a big drop in energy demand thanks to the COP of the heat pump. I’d be interested to see where you got the numbers for batteries not paying back their carbon – I’ve wondered this myself but haven’t seen a good study on it.

  3. I agree with many of the points made by Trueriver. Watching Grand Design’s you can see houses with sitting rooms the size of airport lounges being described as low carbon because they are “well insulated”. I cannot recollect any mention of the amount of power any of the Grand Design houses uses in a year.

    My simple simple test of a low carbon house is the amount of electricity it uses on a cold day, in a post fossil fuel future. If a house’s electricity usage exceeds 1 kW on a winter’s day, it fails the test. A way to pass the test is to insulate your house (moderate insulation if the house is less than 100 sq metres, more serious insulation if above) and instal a 5 kW or lower output Heat Pump. The Heat Pump may take more than a Kilowatt at start up but will settle down to 1 kW input. As most Heat Pumps work inefficiently because they are oversized, limiting the size to 5 kW will save money and electricity. The installation of an effective Heat Pump is not easy in the UK, but the issues involved will be discussed in the forthcoming Manchester Carbon Co-op Heat Pump course starting 15th July.

    Over a year a 5kW Heat Pump will consume 2 to 2.5 megawatt hours of electricity. This electricity can be supplied by a 3 kW PV panel roof array. To be truly carbon neutral a 4kW PV array is needed to account for electricity lost in transmission to and from the grid.

    However there are many ways of achieving Near Zero Energy Buildings (NZEB) and assessing their performance. This has been a serious concern of the European Commission in the 21st Century. I recommend study of the EC directives and reports in this area. Please see:-

    https://ec.europa.eu/energy/topics/energy-efficiency/energy-efficient-buildings/nearly-zero-energy-buildings_en

    UK legislation for new buildings to be NZEB, in 2016, in line with the rest of the EU, was cancelled. The UK has failed to roll out an effective retrofit programme, (remember Green Deal?) and thus was ranked 27th out of 28 EU countries in Building Energy Efficiency in 2015. We have a lot to learn from our continental neighbours. But great that the Carbon Co-op has started the debate on how we make up for lost time.

    • Hi Andy, I like to challenge this idea that PV meaningfully offsets electric heating. For obvious reasons, heat demand is inversely correlated with solar generation, with some overlap in the shoulder seasons. The net balance works so long as you have gas power stations (already paid for) which turn down on summer days when your panels generate, and turn up on winter nights when your heat pump draws power. As long as we still have gas stations generating on summer days, we haven’t got very far in decarbonising.

      So the true test of the idea of PV powering heat pumps would be whether we can afford to build some kind of interseasonal storage on the grid (orders of magnitude larger than any hydro dam we have today, yet used for far fewer hours in the year). How would the cost of a system like that stack up against building enough wind generation to cover the heat pump demand? Wind, fortunately, increases in the winter, similar to our home energy demands.

      Point taken about the large new Grand Design homes, but for existing homes is it really reasonable to allocate everyone the same power regardless of home size? The reality is we live in a very diverse housing stock, from dense flats built to modern building code, to detached stone buildings over a century old.

      Interested to hear your thoughts.

  4. Hi Lewis, you have raised some interesting points.

    By itself, PV will not offset a Heat Pump over a season, without a substantial energy store. However there are many low carbon sources of power and, as you say, some produce more power in the winter – Hydro, Wind, etc. The combination of energy sources over a season is discussed in the NZEB reference I have given above.

    For more detail, the major Kombikraftwerk projects (2007-2013) in Germany show that Germany could rely on low carbon energy sources all year without significant increase in backup storage. See:-

    https://www.unendlich-viel-energie.de/english/kombikraftwerk-2-english.html

    The application of Kombikraftwerk principles in the UK is discussed in Zero Carbon Britain 2030. See:-

    https://www.cat.org.uk/info-resources/zero-carbon-britain/

    Massive backup power storage, by pumping water from Loch Ness (at times of excessive electricity generation) to a new large lake 400 metres above the Loch, is feasible, and would probably cost less than Hinkley Point, now under construction. But Kombikraftwerk indicates that it is not needed.

    I think it is reasonable to allocate an annual energy quota to everybody, and to tax people who exceed their quota, if that is what is needed to avoid Catastrophic Climate Change. Those people who live in detached stone buildings over a century old should pay a premium for excessive energy use – or, better still, convert their house into a 1 kW home. My 1750, stone built detached home, 250 metres up in the Pennines, is a 1 kW home, and if you join the Heat Pump course on Zoom, on 15th July, I will tell you how it is done.

  5. Having only just started on a fabric first approach- I have to say that when I read about the technology involved for heating and decarbonisation my mind boggles.
    Where I have a basic understanding and will seek to understand these things thoroughly when I get to a point of an electrical heating installation in my home – I know I will be seeking to understand a whole new level of education, when at school I would have most likely failed!!
    So when I hear and read these conversations, I am concerned by how many people would be put off or fall foul via misunderstanding and also extremely grateful to you technical minded people searching for the best approaches to decarbonise our grids.
    Please keep blazing the trail for us!!!

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