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Low-carbon retrofitting

Two workers install solid wall insulation to the exterior of a building. They are spreading mortar onto polystyrene blocks.

Retrofit refers to renovations to an existing home or building, particularly with a view to energy efficiency improvements. This page looks at the benefits of taking a whole-house approach to your upgrade.

Last updated, January 2023

While any home can benefit from retrofitting, older homes usually have more problems, and older homes are the focus of this page. Common issues include damp, condensation, draughts and a lack of insulation that makes them harder to heat.

If we retrofitted the majority of homes across the UK, it would make a huge difference in reducing our collective carbon footprint.

Making homes more self-sufficient, through energy-efficiency refits like insulation and renewable energy, helps to protect against rising energy costs.

Where do I start with energy efficient improvements to my home?

Retrofit work is most successful when a ‘whole-house approach’ is taken to identifying the most impactful and cost-effective steps. This will help you make the best choices for your home and get better results for your money. It also means avoiding problems which can occur if we look at measures in isolation from one another.

We can help you decide the best measures to choose when retrofitting your home and work out the best approach to take. And it’s relevant if you want to install only one measure, or if you’re considering refurbishing your whole house.

The first step is to understand your home …

A home is made up of many elements, including how it was built, how it has been adapted, what materials were used and how the building is currently being used now.

The ways in which these elements are connected and how any new measures may impact them needs to be considered in a comprehensive and logical way.

Buildings function as a whole structure, sensitive to changes that happen within them. So planning measures in isolation rather than as part of a joined-up process risks setting off a unintended chain reaction. This will likely cost you more money – or mean that you don’t improve energy efficiency much in the long run.

We recommend looking at the Responsible Retrofit Guidance Wheel. The Sustainable Traditional Buildings Alliance designed this, and it is a useful tool to refer to at the planning stage.The guidance wheel allows you to input your building’s key characteristics and then select the different works you are planning to do. The guidance will list any technical, heritage or energy concerns you need to think about, as well as other related measures you might want to consider installing at the same time.

In which order should I upgrade my home?

Maybe you’ve heard lots about solid wall insulation and want to apply it to your house. Or you want to save on energy bills by installing solar panels…

By considering the whole house, you can determine if these are the best options or if another measure is more suitable.

These five themes will help you work through the best approach.

  1. Maintenance first
  2. Reduce risk
  3. Order of measures
  4. Energy hierarchy
  5. Heritage impact

1 Maintenance first

Simply maintaining your home is the main way you can increase its energy efficiency and your own comfort. This is because this allows the building to work as effectively as possible.

For instance, rotten windows will be letting draughts in and heat out. And cracks in the guttering or render may contribute to damp, making the home harder to heat. Maintenance makes your home ‘retrofit ready’ and should be done before undertaking larger measures.

Regular maintenance gives you opportunities to think about energy efficiency. If wooden window frames need repairing or replacing, you could consider installing shutters or double-glazing at the same time. Carrying out maintenance can also be a chance to think about the alterations that have been made to your home over the years, and whether any of them might now be detrimental.

There might be opportunities to ‘make good’ again through more sensitive refurbishment, which perhaps restores part of the building’s original appearance or uses more sensitive materials for the age and construction of the building.

2 Reduce risk

Think about any risk new measures could pose which might negatively affect the building. The risk is higher in older, traditional construction, homes because how they were originally designed to be lived in is very different to how we tend to use them now.

Our information on individual energy efficiency measures (below) explore the associated risks in more detail – but in general, the more complicated the measure the more you need to think about designing out any risk.

Most measures will have an impact, and if you don’t think about them in advance, they may cause issues further down the line. But if you plan how to minimise these potential issues you’ll be less likely to experience problems.

A good approach is to start with installing lower risk measures, such as draught-proofing, before considering higher risk measures, such as solid wall insulation. Lower-risk measures are usually cheaper to install and are usually easier to reverse if needed. One option is to install a low-risk measure, see if it improves your comfort and the cost of your bills to the level you want, and if not you can then consider the next measure.

Of course, it is not always as straight forward as this, especially if you are planning a larger scale retrofit programme or if, in practice, the measures should be done in a certain order, but it is a good principle to follow.

3 Order of measures

You should not think about measures in isolation. If you’ve got several measures you want to install, you need to decide on a logical order. Sometimes it won’t make much difference, but in many cases it probably will.

You need to think about whether installing one measure will make another harder or will undo some of the work involved. One example is external wall insulation and replacing the windows. If you want to do both, you should do the windows first. If you do the external wall insulation first, there’s a risk it will be damaged when the old windows are taken out and replaced. Similarly, if you’re considering a new heating system and significant new insulation measures, do the latter first. Otherwise, your heating system is likely to be over-sized once your insulation measures have been installed.

The other situation where measures impact each other is when two or more measures are more cost effective when installed together. For example, scaffolding is expensive to erect and if you have some for solid wall insulation then you could think about solar panels at the same time.

4 Energy hierarchy

When undertaking a low-carbon retrofit of your home, we we recommend using the ‘energy hierarchy’ as a way of deciding which measures to prioritise.

The energy hierarchy starts with reducing the need for energy by looking at how you use your home and the appliances within it. Examples include simple things like closing curtains to retain heat, using the heating controls efficiently, or running the washing machine on a low temperature. Many small actions around the home can make a significant impact on your annual fuel bill. Regular maintenance fits in here too.

The next stage is installing types of energy efficiency measures, keeping in mind the stages and approaches we’ve covered so far.

The final stage is installing renewable energy measures. By reducing energy demand through the first two stages, you should have cut down on how much energy you use and therefore need to produce. This means the size of energy system you require to meet your needs may be smaller and cheaper than if you’d installed it without considering the other two factors.

5 Heritage impact

Where a building is considered to have historic significance (whether through its design, location or previous occupants) the dwelling may then be protected through classification. This can restrict what you are able to do, and you will need to apply for Listed Building Consent if you want to make any changes.

The table below gives an indication of when you might need consent, but it could vary by local authority. If you are unsure, it is always best to check with your council’s planning department.

Planning officers will base their decisions on local policies and plans, and these can vary significantly between different local authorities. This is another reason why it’s a good idea to discuss your plans with a planning officer before applying for consent. Sometimes you can remove potential objections to the works you want to undertake by implementing the measures in a particular way or by choosing different materials.

Even if the planning department does not impose any restrictions on your proposed works, you should still consider the impact of these measures on the heritage value of your home and how to minimize it. One way to do so is consider whether the measure is reversible, should that become necessary in the future. Examples include installing window shutters or laying loft insulation.

Consent needed for internal workConsent needed for external work
Listed buildings (Grades I and II)YesYes
Conservation areaNoYes
World Heritage SiteNoYes
Area of outstanding natural beautyNoYes
Schedule of MonumentsYesYes
Undesignated heritage buildingsNoNo
A curving row of traditional brick-built terraced townhouse properties in Newcastle, England.
A curving row of traditional brick-built terraced townhouse properties in Newcastle, England.

What do I need to consider when retrofitting my home?

There’s obviously loads to think about when planning home upgrades. Five particularly important and interrelated considerations stand out. Professionals can help with the finer details, but it is worth being aware of them yourself.

  1. Moisture
  2. Ventilation
  3. Materials
  4. Damp
  5. Cold bridging

1 Moisture

The management of moisture, or water vapour, is integral to maintaining a healthy home and avoiding damp and mould problems. Understanding where moisture comes from is the starting point. Cooking and bathing produce most moisture in the home, and we call kitchens and bathrooms ‘high moisture’ areas. Appropriate ventilation is needed to remove moist air because moisture that stays within the home is attracted to cold spots. Here it condenses to become water and risks causing damp.

There are different approaches to managing moisture depending on the age and build of your home.

Most homes built before 1920 are likely to be of traditional construction. These were built with more breathable materials, and tend to be well ventilated through open fires and poorly fitting windows. In contrast, modern buildings are generally designed to keep moisture out. They are constructed with non-breathable materials and have less draughty windows and doors. Instead they employ mechanical ventilation (e.g. extractor fans) to remove moisture laden air.

Over time we’ve improved our homes, installing measures such as insulation or double-glazing. These affect the way these buildings work and manage the moisture we create. When planning future improvements it is important to understand the impact on moisture movement of any previous measures (including mitigating measures that have been taken) and any future measures.

It means ventilation and building materials need to be thought about carefully, because if a building cannot work as designed, or has had no mitigating measures included, then damp or condensation problems could occur.

2 Ventilation

Ventilation can be thought about in terms of airtightness. Low airtightness means a lot of uncontrolled (passive) ventilation occurs, through things like draughts around windows, chimneys and floors. In a more airtight building this is minimised, and instead ventilation needs to be consciously thought about, such as using extractor fans.

Traditional construction homes were not designed to be airtight, partly because a home with open fires needs good ventilation. Most energy efficiency measures will improve airtightness, impacting the movement of moisture, which means appropriate ventilation needs to be considered. The more airtightness is increased, the more important ventilation becomes.

Properties with increased levels of airtightness need planned ventilation that should be provided by mechanical ventilation systems. The most common mechanical ventilation systems pull fresh air into the property through background vents and expel moist and stale internal air out of the property through extractor fans.

If you are considering a very deep retrofit, by stripping the house back to the basic structure before improving its thermal performance, it may be worth considering a mechanical ventilation heat recovery system (MVHR). These are a system of ducts connected to a heat exchanger and an air intake and outlet. Moist air from kitchens and bathrooms is extracted, but the heat it contains is first captured by the heat exchanger and used to warm fresh incoming air.

3 Materials

In broad terms, traditional buildings and materials are ‘breathable while modern materials are not. Modern materials are designed to prevent any movement of moisture, while traditional materials are ‘moisture-open’. Materials can be moisture-open in different ways; for example, by allowing water vapour to pass through them, or by absorbing the moisture and releasing it again when the humidity drops in the surrounding air.

In well maintained buildings, moisture-open materials can be an energy efficient system to use, as they provide both insulation and a comfortable internal environment (e.g. helping to balance humidity), whilst also causing less risk to the building fabric than moisture-closed materials. The very fact that our stock of traditional homes has lasted so long visibly demonstrates this.

However, while traditional homes were designed to be moisture-open, over time alterations may have included moisture-closed materials such as replacing lime render with cement or the use of impermeable paint. Moisture-open and closed materials can be used alongside each other, but the impact on moisture movement needs to be carefully considered.

For example, some internal wall insulation products are moisture-closed but this can lead to a build-up of moisture trapped behind the insulation as the wall is colder than it was before it was insulated and takes longer to dry out.

Breathable or vapour-permeable insulation products are recommended to be used with traditional building materials such as solid brick or stone. However, if non-breathable insulation products have already been installed, issues with condensation may be resolved by mechanical ventilation, though this won’t help any damp from outside. If problems persist, it may be best to remove the non-breathable building elements before it causes further damage to the original, traditional structure.

The choice of material for each measure needs to be carefully tailored to the context of the individual building. When the balance of moisture movement is upset, moisture can be retained in materials for longer than intended. This can cause permanent damage to the building fabric.

Homes in an early 20th century suburban street in West London
Understanding how your home was built and how it has changed over the years is an important step in a whole-house approach to home upgrade.

4 Damp

The main sign of unbalanced moisture movement is damp, which comes in various forms:

Appropriate maintenance can usual correct issues with penetrating damp. But rising damp may need more work, for example the removal of the plaster and rotten wood and injecting or inserting a damp-proof course.

Some treatments for rising damp may make the problem worse, such as tanking a wall with cement render rather than using lime plaster which moisture can pass through and then evaporate. A wet wall should be left to thoroughly dry before plastering, even with lime.

Often what looks like rising or penetrating damp may actually be condensation caused by cold spots on walls due to poorly installed insulation or inappropriate use of other materials. For instance, repairing a traditional lime render with cement can create unintended cold spots where moisture forms damp patches of condensation.

Once correctly identified, condensation can normally be rectified. There are normally three factors to consider: under-heating, a lack of insulation, and a lack of ventilation to remove moisture. Some interventions are cheap and easy, but others more invasive and costly.

An example is if you make an old home more open plan, warm moist air from the kitchen can condense on the ceilings of other rooms. The most important measures are ensuring you have an extractor fan in your kitchen (as well as your bathrooms and utility room) and having adequate continuous insulation in your loft. Even a very small space of uninsulated wall or ceiling can be the cause of water condensing on the surface).

This is why you need to think about the whole house, and how moisture moves around the building, in conjunction with how you use the house.

Interstitial condensation is another danger. This occurs when water vapour condenses within the building structure, risking a build-up of moisture. This can lead to mould growth and damage to the structure itself, especially wooden joists and lintels. It is a particular risk with internal wall insulation as the original wall becomes much colder, making condensation more likely which can then become trapped.

This is why detailing and material choice are so important for internal wall insulation. Airtightness on the warm side of the insulation is critical regardless of the material. And if you use moisture-closed materials you should have a vented cavity to the outside. If the original wall is moisture-open, however, it is better to use moisture-open insulation. This will allow moisture movement through the entire wall and reduce the risk of interstitial condensation.

5 Cold bridging

Where a material within the building structure transfers heat at a significantly higher rate than the material(s) surrounding it, this creates a cold, or thermal, bridge. Some materials, like metal, transfer heat quickly, meaning they have a high level of ‘thermal conductivity’ compared to others. Wood fibre board, on the other hand, has a low thermal conductivity and therefore transfers heat more slowly. So if you attache wood fibre board insulation to a wall with metal screws, the heat will flow more quickly through the metal, making the screws an example of a cold bridge.

Imagine it like a breech in a dammed river, with a high volume of water passing through the breech. It is impossible to eliminate cold bridging entirely. But it is better to spend time and money on fixing these areas rather than dealing with the potentially serious consequences later.

The problem with cold bridges is two-fold. Firstly, they let out more heat than the surrounding building fabric, reducing your property’s thermal energy efficiency. The second is that they create cold spots, which increases the risk of condensation forming.

The surface area of a cold bridge is typically smaller. But as you increase the insulation levels, cold bridges make up a higher percentage of total heat loss. In extreme cases, some well insulated buildings lose 30% of their total heat through cold bridges.

For this reason it’s usually better to have less insulation everywhere and get the cold bridge areas correct rather than a lot of insulation in some places and none in others. You are likely to lose less heat by doing this, and will also reduce the risk of condensation and mould.

An example might be the internal and external reveals of windows and doors. These tend to be tricky places to insulate, due to depth of the frame. But it’s still best to add a thin layer of insulation around the reveal itself (for external wall insulation this may involve removing some render first). While this won’t stop the heat loss from this area, it will slow it down. This will reduce the risk of moisture condensing on the cooler surfaces. In summary, continuity of insulation can be more important than its depth.

Can we help?

CSE offers retrofit support to homeowners through a team of trained retrofit advisors.

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