How can we make our homes dramatically more energy efficient?

On 1st September 2021 Tim Martel - an experienced Retrofit Coordinator and Chartered Architectural Specialist based in Stroud - set out to answer this question, in conversation with Richard Erskine, Secretary of NailsworthCAN.

In case you missed it, here is the video of Matt’s talk, the conversation, and questions and answers:

We’ll try to summarise the key points from the event (some of the more technical points are pushed into Notes at the end, so as not to break up the flow of this write-up). This write-up also aims to address some of the questions raised at the event.

Tim’s background

Tim came from a background in architectural technology and Passivhaus design. About 10 years ago he realised that the biggest challenge we face as a society in dealing with the climate crisis in the context of homes is retrofitting existing homes. Tim has played a significant role in the development of tools to support retrofit at both the AECB (Association of Environmentally Conscious Builders) and The Retrofit Academy.

What is retrofit?

Tim defined retrofit as adding insulation to an existing building without changing the structure and improving the energy standard of the building. Although of course this work is often done in conjunction with some other change - such as renovating a kitchen [2].

The term retrofit has now broadened to encompass other measures to improve the performance of buildings, including replacing windows, mechanical ventilation and heat recovery and using heat pumps (to replace gas boilers) to provide a heat source.

Keeping heat in

As the loss of heat through surfaces is reduced through added insulation, the other type of heat loss - due to the leakage of air - becomes increasingly important and can easily account for 50% of the heat lost from a building. This is because our homes are often very leaky.

As less air leaks from a building, the greater the need to consider measures to control the flow of air through ‘mechanical extract ventilation’ (MEV) [3]. This replaces the uncontrolled flow of air with a controlled flow. 

In addition to ensuring sufficient (but not too much!) fresh air in a property, controlling air flow can also be important in dealing with condensation issues. It is possible to remove moisture from the flow of extracted moist air and harvest the heat from this. This heat is then given it back as dry warm air to the building (this is termed Mechanical Ventilation and Heat Recovery (MVHR)) [4].

Tim’s aims for the talk were to address the following three topics (with the main focus on the second topic):

• Heat Pumps versus Fabric approaches

• Fabric approaches for retrofit

• Qualified people and targets

Heat Pumps versus Fabric approaches

Tim showed a graph. It showed that for a typical home over 60 years a cumulative amount 160 tonnes of CO2 would be emitted from a gas boiler providing heating for the home.

The key message from the description of the graph was that while retrofit alone (dashed line) might halve the cumulative emissions, to massively reduce this a heat pump will be essential. And even if we look at different levels of retrofit combined with a heat pump, the savings in CO2 emissions are broadly similar.

When replacing an old gas boiler, running costs do not need to rise when installing an air-source heat pump, as we previously explored in How to heat your home with the minimum amount of energy, and save the planet, and this is also the single biggest thing a householder can do to reduce the carbon footprint of heating.

So why don’t we all just install a heat pump without adding insulation?

Tim’s answer to this was that there are other reasons - often called ‘co-benefits’ - from doing retrofit, which a heat pump alone won’t deliver, with ‘comfort’ being a key one.

For example, if your favourite spot in the house is a chair by a bay window that is poorly insulated and single glazed, then even if the house as a whole is being heated perfectly satisfactorily by a heat pump, you will feel that coldness from the cold surface next to you (this will be true however you heat the home). To fix this, you need to fix the bay window.

Another point Tim raised was that a heat pump will be a bit more expensive to run without any insulation than with (although the cut over point can be quite modest - if replacing an old gas boiler and after doing modest insulation - the running costs may be roughly equivalent [8]). Heat pumps are currently seen as being expensive [5], but there has been help in the capital costs through the RHI grant [6]. Something may come to replace the RHI when it is no longer offered after March 2022 [7].

Tim said it is not an open and shut case that you’d go for a heat pump as a first measure, but it is an option.

He mentioned the importance of understanding the efficiency of the heat pump averaged over the whole year (this is called the Seasonal Coefficient Of Performance (SCOP)). Currently, the relative cost of a unit of energy (in kilowatthours, kWh) for electricity is greater than gas. But the much greater efficiency of a heat pump compared to a gas boiler mitigates this considerably (It is worth noting that the SCOP used in creating an Energy Performance Certificate (EPC) has hitherto been quite low compared to the latest technology, around 2.5; whereas the actual SCOP for a modern Air Source Heat Pump (ASHP) is now 3.5 or more).

The question raised is, will the SCOP be good enough to bring the cost of heating with a heat pump into line with the gas boiler it replaces, after modest fabric measures? Actually, the answer is surprising yes in many cases [8] [9].

We did an in-depth discussion on heat pumps in a previous In Conversation that you can catch up with here.

As gas boilers start being phased out for new buildings, there will be pressure for everyone to start to consider the switch to a heat pump. Changes to the relatives unit prices of electricity and gas being discussed in policy circles could help nudge homeowners in this direction.

Fabric Approaches to Retrofit

Tim showed pictures of great retrofit examples in Stroud. These have in the past been made available for visits under the Transition Stroud ‘Openhomes’ event held periodically over the years.

Examples include:

• A home with thick wood-fibre external wall insulation

• An old stone cottage insulated through and with lots of Solar PV (Photovoltaic)

• An example where sills were recreated after external wall insulation

Tim noted that many old stone cottages in Stroud were originally rendered, but people have taken the render off because they think it looks nicer. But the render was there to serve a purpose; it was for waterproofing, so that rain came off the house and didn’t soak into the wall. Removal of the render can lead to damp issues because the water can seep into the wall (a damp wall will also increase heat loss).

One can see many different types of retrofit on old barns and blocks of flats.

Portsmouth City Council did external wall insulation on a block of flats using Rockwool (which has very high fire resistance, an important consideration).

Tim showed some work being done on his own home (a bungalow in Stroud), including external wall insulation and triple glazed windows [10]. He showed images of the insulation detailing around windows, prior to them being rendered.

Measuring the performance of insulation, walls and windows

Tim then discussed the insulation performance of walls and windows. The insulation performance of a given thickness of a given material is expressed as a ‘U value’ which is a measure of the rate of flow of heat through this material per unit area and per the temperature difference between the inside and outside of the building [11].

For a given type of material (brick, stone, wood, glass, Rockwool, etc.) if you double the thickness of the material you will halve the U value [12]. If you use a material that is a better insulator, you don’t need it to be as thick to achieve the same U value.

Walls, floors and windows will all have U values. Tim showed a figure to illustrate the scale of U values, first for walls:

Cavity wall, filled with mineral fibre 0.6 is ok but not brilliant (half as good as a modern house)

For windows [13]:

Single glazed 5.8 which is 10x worse (than the insulated cavity wall)

Double glazing (prior to 1970) 2.8 which  is 5x  worse

Newer double glazing (low E) 1.1 which is 2x worse

The lower E (Emissivity) means that the glass does not radiate as much thermal radiation to the outside which keeps more heat inside the building.

Tim showed a range of solutions for windows: curtains, secondary glazing, double glazing and triple glazing. Costs vary considerable (see www.theecoexperts.co.uk ).

For a typical bungalow or semi, secondary glazing throughout might total £800, but using good quality triple glazing as a long term solution can cost £10,000 or more.

Tim then considered the question of the thickness of insulation to use. He cautioned that if you go to a builder - say for external wall insulation - they may well want to quote for the minimum standard possible (under building regulations), in order not to be undercut. It is worth having an understanding of the level of insulation you are aiming for. Tim believes we should not be satisfied with 50mm, or even 100mm thickness, but aim much higher (200-300mm). As was noted by David Austen, there may be issues with some structures related to overhangs and other features that constrain what is practically achievable. Professional advice is always advised. 

But there is a range of applications, with different materials suited to these, and while adding insulation it is worth going beyond the minimum [14]. For example, he showed mineral wood which is often used in loft insulation or in a suspended floor and ‘Graphitised EPS’ which is fire retardant and used for external wall insulation. There is a huge range of synthetic and natural materials. One question related to sourcing of materials [15].

Qualified people

There is a retrofit standard PAS2035 that has been around for a while now and The Retrofit Academy offers courses on becoming a:

• Retrofit Coordinator

• Retrofit Designer

• Retrofit Assessor

PAS2035 input is required for ECO and PAS2030 installers.

A PAS2030 retrofit installers must work with a PAS2035 coordinator (e.g. follow their recommendations). 

There are a variety of standards for how much insulation etc. is done.

U values are useful but not enough to make a decision on a plan of action. What is needed is an understanding of the total heat energy (expressed in kilowatt-hours, kWh) that needs to be delivered to a property to meet the needs of the occupants over a full year. To be able to have a standard that can be used as a target for all sizes of building, it is useful to divide this target total energy by the floor area of the home. This gives a measure - which can be termed the ‘operational energy’ - that retrofit coordinators can target [16], and use to compare different levels of interventions.

The value of this quantity that AECB aims for is 50, which is about the level that a new house should reach; but for special cases (e.g. a listed building) this can be relaxed to 100. An old leaky stone cottage might have a figure of 400; so 8 times better than the AECB target of 50.

The AECB does not have a target for the CO₂ emissions (either for the running of the building or the ‘embodied carbon’ [17]), but their advisors must calculate this and do a comparison with at least two other options (for treatments to the home).

RIBA (the Royal Institute of British Architects) has ‘2030 Carbon Challenge’ target metrics for operational energy and embodied carbon. For 2025 the target is 60 and 2030 it is 35. Business as usual (current compliance) is 120.

Others such as LETI also have targets.

Tim showed the output from AECB software he has been involved in developing to calculate cumulative energy use and carbon emissions for buildings (pre and post retrofit), used in guiding advisors in making recommendations.

Co-Benefits

Often the focus is either on running costs or on carbon reductions, but there are other benefits.

Providing a more comfortable living space is one of the most important co-benefits, both in terms of cold weather and during hot summers (triple glazing can help prevent overheating, for example).

Air quality and moisture issues will also be significant co-benefits, in terms of living in the building but also in terms of the conservation of it.

Wider benefits to society are the contribution retrofit makes to decarbonising heating, and providing a basis for developing a new industry that is needed for the UK to achieve net zero.

Other points raised

Retrofit needs to consider the whole house and to make a judgment on priorities. For example, one householder may decide to put a lot of effort into the living room where they spend a lot of time, and relatively little in the bedroom.

A great deal will depend on the overall budget.

There was a question on the benefits of natural materials versus synthetic materials. From a sustainability perspective natural materials are often much better, although they can be more costly and sourcing them can be a challenge, and in some applications, finding an equivalent substitute natural material is problematic.

However, Tim stressed that the operational CO₂ emissions far exceed the embodied CO₂ over the lifetime of the building/ materials (except for very well insulated buildings such as Passivhaus). So it is hardly worth worrying about embodied carbon because the main priority is limiting emissions from running the heating system.

Tim recommended the TrustMark quality mark for retrofit tradespeople (and this is linked to the PAS2035 initiative). If people are concerned about quotes, get a few quotes to compare them.

There was some discussion of modelling [18]. Advisors use a range of tools to determine, but EPCs are quite crude (they utilise a cut down quite approximate model) [19].

The question was raised as to whether to install a heat pump before all other retrofit work was complete (in order to take advantage of the RHI before it is withdrawn). Tim said “I think it is ok, but…”. If the insulation work is expected to greatly reduce heat loss, then the risk is that the heat pump will be sized for the current situation, and possibly oversized for the future state of the fabric [20].

Concern was expressed as to whether ‘comfort’ is an incentive for people buying a home, and people only stay for seven years on average. Replacement windows and heat pumps are significant investments. Tim answered this by showing a survey done by the Government of 300,000 homes. They found that in the Midlands and South West, and in the North, there were significant increases in home valuation following retrofit.

Finally, the issue of moisture management was discussed. For traditional buildings, specialists are required. Historic England is concerned about this, and has published a number of reports and guidelines. Materials such as wood fibre-based ones are often preferred (because of their ability to breath naturally through capillary action).

Here as in every aspect covered, it is always worth seeking advice.

The meeting closed with a thank you to Tim for a very interesting and informative talk and discussion.

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Notes

1. Some estimate that 80% of the homes that will be around in 2050 have already been built. That is not to say that we should not make new homes much better, and more energy efficient, and avoid adding to the burden of retrofitting them in the future.

2. This raises an important point. Often retrofit is described as disruptive, but when seen in the context of a positive effort to improve a home, the insulation work can be seen as part of a wider effort. It is very much easier, and less disruptive, to insulated the back wall of a cold kitchen before new units are installed than afterwards.

3. This is analogous to the extraction fans in showers, for example, but taking a ‘whole house’ view of the problem.

4. This is true but relative. A heat pump can be equivalent to or even less than the cost of replacing all the windows in a home, or installing a kitchen.

5. In modern homes built to Passivhaus standards or similar, MVHR is an integral part of the ‘whole house’ design. In a retrofit context, it is not always possible to implement a ‘whole house MVHR’ system at a reasonable cost, but it can still be applied in a more localised fashion (e.g. to the bathroom or kitchen), to deal with the biggest ‘moisture creating’ offenders.

6. Tim mentioned the RHI (Renewable Heat Incentive), which expires at the end of March 2022. This is quite a generous grant, that is paid in quarterly instalments over 7 years. It requires that an EPC has been conducted. While the EPC may recommend many measures to the home, only 2 are preconditions for getting the RHI: adequate loft insulation (which for Rockwoll would mean at least 270mm thickness); and, if the house has a cavity wall, that it has cavity wall insulation (unless there is an architectural letter giving a reason why this is not possible).

7. The word on the street is that it is likely to be a much smaller grant (e.g. £4K), but paid in full up front. This is yet to be confirmed.

8. Just to clarify Tim’s point, here is a simple calculation. If the SCOP of a new Air Source Heat Pump (ASHP) is say 3 then the average efficiency of the heat pump is 300% (this is conservative because modern ASHPs can achieve 350% or better). The old gas boiler might be only 70% efficient. Two ratios are important. A: The efficiency ratio equals 300%/70% equals 4.3 (call this number A). B: The unit cost ratio. Now the ratio of the unit price of electricity to gas currently is 15p/3p = 5 (call this B).

   • So in this case, B is greater than A, so the cost of running the heat pump would be slightly more than the old boiler. But, by doing modest retrofit (e.g. getting the loft insulation up to standard), and saving just 20% on the heat loss, the running costs would then be about the same!

   • Replacing the old boiler is therefore not a difficult decision on running costs ground, alongside modest insulation and draft proofing measures.

   • If it is a newer gas boiler that is nominally 90% efficient, then a heat pump would be a bit more expensive to run at current gas prices even with modest insulation. So maybe now is not the time to replace it.

   • However, according to energy analysts, consumer gas prices are likely to rise because of a combination of wholesale/ market price changes and Government policy changes aimed at nudging people off gas. Note: the old department DECC is now replaced by BEIS.

   • Heat pumps are also getting more and more efficient. It is therefore very likely that in the near term, even for newer gas boilers, it will be no more expensive to run a ASHP in place of it. For example, a SCOP of 3.5 would mean parity with the old gas boiler, without any additional insulation.

9. Insulation reduces the cost to run a gas boiler, and also will reduce the cost to run a heat pump. Tim’s main point is that insulation is a good idea to reduce energy bills, including those for a heat pump. With today’s skewed priced for electricity and gas, it is prudent to reduce the heat loss of the building when moving to a heat pump. The relative cost of insulation measures and a heat pump depends a great deal on the nature of the building you start with. A very leaky school may not be a great place to start with a heat pump, but for a typical home, it can be a different decision.

10. The triple glazed windows Tim is having fitted are about 100mm front to back, and of high quality, and were sourced from the Green Buildings Store https://www.greenbuildingstore.co.uk

11. A practical example. A U value of 0.5 for a wall, that is 4 metres wide and 2 metres high (i.e. 8 square metres), when the temperature inside is 21°C and outside is 10°C would means that the rate of heat loss through that wall would be equal to: 0.5 x (21-10) x 8 = 44 W (44 Watts), i.e. similar to the rate of heat loss from an old incandescent light bulb.

12. The data sheet for a material will often quote a number called the thermal conductivity, which is independent of the thickness (it is a measure of the inherent insulation properties of the material). To get an expected U value for a given thickness of that material you divide the thermal conductivity (in metric units) by the thickness that will be applied (in metres). So, for example, glass fibre roll has a thermal conductivity of 0.044, so a thickness of 0.3m (300mm) would give a U-value of 0.044/0.3 = 0.15. Whenever you try this calculation, see if the number is as good as you need (i.e. ‘low enough’). If not, then to get a better U value, either you need to use a different material with a lower thermal conductivity AND/OR the thickness applied needs to be greater (usually suppliers offer varying thicknesses for the materials they sell).

    • There are some examples of materials with thermal conductivity figures shown here https://www.greenspec.co.uk/building-design/insulation-materials-thermal-properties/

13. Here, just looking at the glazing. The quality of the frame and fitting of the window also has an impact, but for the example Tim showed, he was keeping it simple to make a point about multiple glazing.

14. There will be a point of diminishing returns from adding grater depth of insulation. It is always worth getting professional advice. In a loft which is not going to be used, there is freedom to use a mineral wool for example with thickness of 300mm or more, but in other applications space may be limited and a higher performance material may be required (e.g. 150mm of Cellotex is roughly equivalent to 300mm of mineral wool).

15. Worth repeating the Green Buildings Store https://www.greenbuildingstore.co.uk and, Ty-Mawr Lime Ltd https://www.lime.org.uk which specialises in traditional and natural materials such as wool based and wood based insulation.

16. The units here are kilowatt-hours per square metre per annum or kWh/m².a - the average for the UK is about 130 kWh/m².a according to Ovo Energy, but a Passivhaus would aim for 15 kWh/m².a

17. Embodies Carbon represents the emissions in the full supply chain involved in the production and delivery of a material. This is something manufacturers increasingly are expected to calculate and provide evidence for. It can depend a lot on where materials are manufactured. India has a high ‘carbon intensity’ in its electricity grid because they burn so much coal; the UK’s is much better as more and more wind is deployed; and Iceland have huge reservoirs of geothermal energy. So Rockwool made inn India, UK and Iceland will have progressively lowe embodied carbon (the shipping is relatively small contribution).

18. The industry standard is SAP (Standard Assessment Procedure) from BRE (the Building Research Establishment. There is a ‘reduced’ simpler version RDSAP, and that is embedded in the tool used by surveyors who create EPCs (Energy Saving Certificates).

19. The issues with EPCs have been further explored by Jan Rosenow (European Director of Regulatory Assistance Project (RAP)) in an article: https://www.raponline.org/blog/stuck-in-the-past-energy-performance-certificates-hold-back-heat-decarbonisation/ - On twitter he has shared a graphic of his home in Oxford and his immediate neighbour, with identical terrace house original constructions. He has undertaken considerable insulation retrofit work and installed a heat pump, but once again, EPCs are penalising doing the right thing, if a heat pump is specified. It seems that BRE arbitrarily added on at least 30% for the heat demand of a house if a heat pump is added. Neither logic, nor Jan Rosenow’s bills and energy monitoring backs up this assertion. Many others are critical of EPCs and one hope in due course the bugs will be fixed.

20. The RHI requires that the loft insulation is up to standard, and that if there is a cavity wall then it should be insulated (although there can be exceptions for cavities that are problematic). The MCS (Microgeneration Certification Scheme) standard that professional heat pump installers follow means that they have to do a full heat loss assessment (room by room) and analyse the size of heat pump, and the required radiators (including any upgrades) and design flow temperature to meet the MCS target temperatures (21°C in living room, 18°C in bedroom, etc.) on the nominal coldest night at the homes location. The system can be audited on paper or through a visit, so the installers are obliged to ensure that retrofit measures are in place before issuing a certificate.