Insulating your solid walls could cut your heating costs considerably and make your home more comfortable.

If your home was built before the 1920s, its external walls are probably solid walls rather than cavity walls.

• solid walls have no gap, so they can’t be filled with cavity wall insulation
• cavity walls are made of two layers with a small gap or ‘cavity’ between them

Solid walls can be insulated though – either from the inside or the outside. This will cost more than insulating a standard cavity wall, but the savings on your heating bills will be bigger too.

Internal Wall Insulation

Internal wall insulation is done by fitting rigid insulation boards to the wall, or by building a stud wall filled in with insulation material such as mineral wool fibre.

• is generally cheaper to install than external wall insulation
• will slightly reduce the floor area of any rooms in which it is applied (the thickness of the insulation is around 100mm)
• can be quite disruptive, but can be done room by room
• requires skirting boards, door frames and external fittings to be removed and reattached
• can make it hard to fix heavy items to inside walls – although special fixings are available
• Cannot be done before fixing any problems with penetrating or rising damp

External wall insulation

This involves fixing a layer of insulation material to the wall, then covering it with a special type of render (plasterwork) or cladding.

The finish can be smooth, textured, painted, tiled, panelled, pebble-dashed, or finished with brick slips.

External insulation:

• can be applied without disruption to the household does not reduce the floor area of your home
• will renew the appearance of outer walls
• will improve weatherproofing and sound resistance
• fills cracks and gaps in the brickwork, which will reduce draughts
• increases the lifespan of your walls by protecting the brickwork
• reduces condensation on internal walls and can help prevent damp (but will not solve rising or penetration damp)
• best installed at the same time as external refurbishment work to reduce the cost
• may need planning permission – check with your local council
• requires good access to the outer walls
• not recommended if the outer walls are structurally unsound and cannot be repaired

Moisture Movement and Ventilation

In traditionally built properties with solid walls, water vapour can usually move quite freely through the building. This is partly because of the high levels of ventilation and draughts, but also because water vapour can travel through the bricks and stones that the walls are made of. When you insulate an older building, you will change the way that water vapour behaves in several ways:

• adding wall insulation will usually cut down on draughts through the walls and round the windows
• the insulation may create a barrier to vapour movement, depending on what materials are used
• adding insulation to the inside of a wall will make the wall colder. This means that any water vapour entering the wall from inside will get a lot colder, and may condense inside the wall

Whenever you fit solid wall insulation to a building you need to take account of water vapour to make sure that you don’t create new damp problems in the future. This may involve using “breathable” insulation materials that will allow the vapour to carry on permeating the walls, or it could involve creating a continuous vapour barrier to make sure no vapour can get into the walls from the inside. You will need an experienced specialist installer to develop a moisture control strategy that is specific to your building.

You should check with your installer that the installation is covered by an appropriate 25 year guarantee. They may be members of the SWIGA guarantee scheme, or they may offer an independent insurance-backed guarantee. You can find a list of Ofgem approved guarantee schemes.

Typical installation costs* of solid wall insulation can vary as follows:

external wall insulation: around £10,000

internal wall insulation: around £8,200

*Based on a typical semi-detached house in Great Britain

Over 20 years payback period for a gas heated UK detached home. However, off gas grid is more expensive to heat and therefore annual savings will be greater.

The costs we suggest for installing solid wall insulation are for paying a company to come in, insulate your whole house in one go, fully redecorate and replace everything just as it was.

If you’re looking to spend less, it is advisable to insulate a wall when you are having other building or decorating work done. Internal insulation can be fitted when you’re planning to redecorate anyway, or to fit a new kitchen or bathroom. You can also spread the cost by tackling one room at a time.

External insulation will also cost less if you fit it when you’re having other work done to the outside. If you’re having a new roof, or painting the windows, or even having solar PV panels fitted, then you will probably have scaffolding up already, which can save a bit on the costs. If your walls need repointing or other repair work, it’s worth getting a quote for a complete refurbishment including insulation – it will probably work out cheaper than doing the two things separately.

Sometimes there is financial support available to help with the cost of solid wall insulation.

Certified Installers:

We recommend you use a Solid Wall Insulation Guarantee Agency (SWIGA) certified installer for internal and external wall insulation. This may also be important for securing funding. See more about SWIGA and the up to date list of certified installers on their website here.

Financial help:

HEEPS ABS – Interior insulation grants for eligible households

Warmwowrks – all insulation types for eligible households

Home Energy Scotland – all insulation types loans and grants

Energy Company Obligation (ECO3) – all insulations for eligible households