Retrofit design guide - 2. How to retrofit

Retrofit works may seem complex and expensive.

You can always start with low-cost measures to save energy. For example, you can only heat the rooms that you use and lower the thermostat temperature if your health allows. Other measures include doing repairs and installing basic draught proofing by filling the unplanned gaps in the existing building. These gaps, often occur around door and window frames, lofts doors, where cables and services come in, even the electricity entry points in ceilings. Closing these gaps and holes can be an easy DIY task, sometimes you might just need some mastics and sealants. However, it is important to avoid covering up planned air vents and window trickle vents, as they allow good ventilation to the building. Along with closing out the cracks and gaps of the building, using only energy efficient white goods and light fittings will also reduce electricity use.

Retrofit should focus on “fabric first” approach. This means you should improve the building fabric first before installing low carbon technologies. This is because your heating system will be designed based on how much heat you need and lose.

The guide below gives you an overview of the works you can do to your existing homes.

Improving the fabric

Building fabric means the wall, roof, floor, windows and doors which forms your property.

The first step is to identify what your property already has, what materials are used, the current conditions and to see if there is already some insulation within the existing building fabric. This will then help you to decide the best retrofit approach.

There are different methods of fabric improvement: see some useful guidance and videos by the Energy Saving Trust.

It is also important to note historic buildings were built differently to modern buildings, so they will need specialist retrofit methods as well as planning consents. Historic England has provided a useful technical guide on retrofit works to historic buildings. 

Wall insulation

The most suitable type of installation will depend on whether your property has solid or cavity walls. Find out the type of wall your property has.

See short explanations of different types of wall insulation.

Internal wall insulation (IWI)

This adds insulation to the internal sides of the walls. This is suitable for properties where EWI cannot be used for aesthetic or heritage reasons, but this will affect the interior of your home and could affect the size of your rooms.

We recommend using breathable insulation such as wood fibre. A hygrothermal analysis should be carried out by a competent person to check there is no risk of moisture building up between the internal insulation and the existing wall.

If you have an existing problem with damp and mould, you will need to speak to specialist for advice and fix the issue before carrying out any retrofitting works. 

Roof insulation

Pitched roof and flat roof are common roof types, but there are other roof types and conditions too. You need to understand what you have currently to understand how best you can insulate your roof. See guidance from the Energy Saving Trust here.

Flat roof

This is to add insulation on top of the plywood deck above the existing roof joists, and you will need to remove and replace the existing roof covering to carry out the works. You might be able to add the insulation between the joists from below, but you will need a competent person to check for condensation risk.

Floor insulation

You can see an explanation of different floor insulations here. 

Suspended timber floor

If you have air bricks or ventilation bricks on the outside wall(s) of your house that are below floor level, you probably have a suspended timber floor. This is a ground floor build-up with a ventilated space below the timber floor joists, this is typical in Victorian and Edwardian properties. Insulation can be fitted in between the timber joists or spraying from below if there is a large void.

Solid concrete slab

Insulation such as phenolic or high-performing aerogel insulation can be added to the top on the slab. Insulation with suitable compressive strength should be chosen for this application. The floor finishes will need to be replaced, and the floor level will be raised. If you are thinking of replacing the floor finishes of your home, this would be a good opportunity to add insulation as well. It can insulate in both the winter and summer.

Windows

Secondary glazing

If you cannot change your existing windows as you have been advised there are heritage constraints, you can install a separate window behind the existing window to the inside face of the wall.

Click here for further information from the Energy Saving Trust.

Air tightness

Heat can leak through the building fabric and the junctions of building elements such as windows and walls. To stop the heat loss through the ‘leaks’ in your building, you can improve the air tightness of the building. Different methods can be used including applying air-tightness membrane or wet plaster. All junctions and pipework penetrations should be taped or sealed properly to achieve a continuous surface with no air leaking.

To help closing out the ‘leaks’ in your building, you can see information by the Energy Saving Trust for some useful tips on draught proofing yourself or by professionals.

You can watch a video to learn more about air tightness test.

If you are improving the air tightness of the building, it is essential to improve your ventilation to ensure good internal air quality for occupants and to avoid damp and mould growth. More help with damp and mould.

Low-carbon energy systems

A heat pump is a reverse fridge in principle. It extracts energy from a source to provide heating for the buildings. Usually there are two types of heat pumps – air source heat pump (ASHP) and ground source heat pump (GSHP).

Ground source heat pump (GSHP)

Heat is extracted from the ground through installing pipework in the ground with a shallow or deep system. Pipes can still run if there is not enough space, instead deep boreholes can be drilled into the ground to run the pipework. GSHP is generally less common as it needs large space or drilling.

More about ground source heat pumps

You will need to check if you need planning permission for installation of an ASHP, GSHP or solar panel system.

Overheating

Homes can sometimes overheat in summer. You can feel uncomfortable with high temperature, but everyone’s health can be at risk, especially for vulnerable groups and people with existing health conditions.

Certain homes are more likely to overheat. They can be buildings with poor insulation, have large openings facing east, west and south and with little shading from sun. They can also be buildings located in a densely built-up urban area with little green space around.

You can lower the risks of overheating by reducing heat from getting into your home, by closing curtains or blinds during the day. It also helps to make sure there is ventilation to purge heat by opening windows at night and lastly you can use mechanical systems to reduce the indoor temperature.

Retrofit should also include works to reduce overheating risks, not only to reduce heat loss in winter. Your focus should be to reduce heat from getting into your home, but there are also other measures you can take.

• To insulate the building fabric of your home to reduce heat gain.
• Adding external shutters or blinds outside the windows to provide shading. You might need to check if you require planning application as this affects the external appearance of your home.
• Trees and greenery can also provide external shading.
• You can install internal shutters, blinds or curtains to windows.
• If you are replacing your windows as part of the retrofit works, make sure the new windows are openable to provide good ventilation across your home.
• Finally using active cooling such as air-conditioning units should be the last solution, as air-conditioners use a lot of electricity and deliver waste heat to neighbouring properties.

More guidance from the Government on how to combat heat.

Local Green Homes Events

In Haringey there are fantastic examples of retrofitted homes. You can find out more by visiting them in Muswell Hill Sustainability Group’s local Green Homes Events that happen in October/November every year.

The event weblink also includes some useful local case studies and YouTube channels with videos showing some of the previous Green Homes visits.