Skip to main content
Ecofrenzy logo

Cavity wall insulation





cavity wall insulation

Savings dependent on receiving a government grant’

Modern homes built after 1924, usually have a space (called a cavity) between the internal and external walls. This was originally designed to stop rain water reaching the inside of the property, however today this gap can be filled with insulation to minimise heat loss and make your property more energy efficient. After 1982, building regulations changed and required insulation to be fitted in the cavity at the time of build. Recent research, carried out by the Department of Energy and Climate Change (DECC), has shown that 70% of all properties with cavity walls in the UK have been insulated. People in the remaining 30% are allowing heat and money to pour out of their homes.

Solid Wall

Homes built before 1924 were often constructed with solid walls and lose even more heat than cavity walls! This can be reduced by installing insulation on either the inside or outside surface and is one of the most impactful changes that can be made to a home. This method of insulating walls is significantly more expensive and disruptive than cavity insulation, as it can sometimes require substantial building and decorating work.


Savings following cavity wall insulation depend on a number of factors, such as:

  1. How much insulation is already present
  2. The size of the building
  3. How many external walls there are (i.e. if the property is detached, semi-detached or a flat)
  4. The fuel used to heat (and/or cool) the property

Average Cavity Wall Insulation Cost (according to Glasswells)

By finding quotes from 6 cavity wall insulation installers and 8 secondary sources, Glasswells discovered the following all-in average costs for cavity wall insulation.

Whilst some materials are slightly more expensive than others (e.g. Polyurethane PUR), the difference is pretty small overall and falls within the following average ranges.

Type of Property      Installation cost         Energy savings
Detached house (4 bedrooms+)£1000 – £1,250           £200 – £250
Semi-detached house (3 bedrooms)£900 – £1,150              £180 – £230
Mid-terrace house (3 bedrooms)£600 – £800                 £120 – £160
Detached bungalow (2 bedrooms)£600 – £900                 £120 – £180
Flat (1-2 bedrooms)£600 – £900                 £120 – £180

Maximum fuel savings are around £250 per year but there is an average 5 year payback before you start to save money, however you may be eligible for a government grant (see below), in which case you will start saving money in the first year.

Also, we indicate the CO₂e savings when you install cavity wall insulation, but it’s important to understand that the environmental benefit of a reduced CO₂e footprint due to insulation only starts once the embedded CO₂e in an insulation material as well as the CO₂e generated during its transport and installation is removed from the equation.

Government Grants

Many of us may qualify for FREE Cavity Wall Insulation. To see if you are eligible for a grant have a look at
However a contribution towards the cost may be required.

Types of cavity wall insulation

The three core insulation materials that are used in the UK are.

1. Glass fibre / mineral wool rolls or slabs

2. Polystyrene bead insulation.

3. Cavity foam insulation.

Their efficiency is measured as a ƛ value (lambda value) ie the thermal conductivity of the insulation material. The better the insulation, the lower the λ-value and the less heat will be lost through insulation.

Note – costs per sq. metre shown below do not include costs of the installer.

Glass fibre or mineral wool batts – new builds and extensions only.

During construction of new buildings, cavities are often filled with:

a) Rolls of glass fibre wool or mineral wool

b) Glass fibres or mineral wool compressed into slabs of varying thickness from 50mm to 150mm and covered with a water resistant film.

Prices range from £6 per sq. metre of the 50mm thickness to £16 per sq. metre for a batt of 150mm thickness.

The likelihood of damp problems is lower with slabs than rolls.

The only downside compared to the other insulation materials is a slightly higher λ-value of 0.037 W/mk meaning the insulation efficiency is slightly lower, but not enough to worry.

Note: Blown mineral fibre is one of the most environmentally friendly insulation options. Fibreglass flakes have little impact on the environment, and the raw materials used in the production of glass wool can be sourced from recycled glass, sand, and other minerals. Better still, the insulation can also be recycled once you need it replaced.

Polystyrene balls – can be retrofitted

For existing buildings without cavity wall insulation, polystyrene balls are pumped into the cavity through suitably drilled holes until it fills the entire wall space.

Expanded polystyrene beads (EPS) are made of a carbon polymer and are installed into your walls with a bonding agent.

EPS beads are easy to install in your home and the bonding agent sticks the beads together which leaves next to no mess after installation. They are a good low cost option for cavity wall insulation at £10 to £15 per sq. metre.

If water or moisture enters the cavity , the droplets won’t penetrate or cling to the beads, allowing the material to dry out before causing any damp in your home.

EPS beads have a good insulation value of 0.03 to 0.034 W/mk making them a good option. Another great benefit of EPS beads is their recyclability. The material can be broken down into its original beads and made into other products such as coffee cups or refrigerator trays.

All good so far but it is important to make sure the cavity is cleaned prior to installation, this can be costly depending on the state of the cavity.

As mentioned previously make sure your installer is competent, if EPS beads are installed with insufficient bonding agent it will cause problems, especially if you carry out home renovations that involve exposing the cavity inside your walls. Beads will fall out and cause a mess. In addition without the bonding agent, the beads will lose some of their water resistant properties, meaning the possibility of dampness in your home.

Cavity foam insulation – can be retrofitted

Foam can also be used to fill the gap in the cavity wall. Although some foams used in the past, such as urea-formaldehyde, are no longer suitable (some people are allergic to this material), others, such as polyurethane (PUR) have taken their place.

PUR is essentially a plastic material.

With a λ-value of just 0.03 W/mk, PUR is the best insulator of all the materials shown here. This means less warm air escapes from your home and gives the highest energy savings, however PUR is more expensive to install than the other products costing around £20 to £25 per sq. metre. In the long run it will save you more money and CO₂

PUR can shrink over time, this means water can creep into uninsulated parts of the cavity wall. This pulling away from the walls overtime can also reduce the insulation properties of the foam. To reduce the chances of this happening make sure you use a recommended installer.

Paradoxically if the PUR fills the entire width of the cavity, then the cavity cannot breathe. If damp enters the cavity there is no ventilation and it can’t dry out. Eventually this will cause dampness in your home.

Spotting cavity walls

There are three simple checks which should tell you if your house has cavity walls. If in doubt a professional survey will tell you for certain

Check 1: How old is your house?

Cavity walls didn’t start to be used in general building construction until 1924. Even an old house without cavity walls may have had a more recent extension which could have cavity walls so check the brick pattern on more than one wall.

When was your house built?Does it have cavity walls?
Before 1924Unlikely
Between 1924 – 1982Highly likely
After 1982Almost certain

Check 2: Check the brick pattern on your outside walls

There are three common types of brick wall: Stretcher bond, Flemish bond and English bond. Only Stretcher bond walls are likely to have cavity walls.

Stretcher bond – all the bricks are laid on the long side and usually indicates a cavity wall.

Flemish bond, where the bricks alternate between a full length brick and a half-length brick, are unlikely to have cavities.

English bond, where there are alternating rows of full and half length bricks, are also unlikely to have cavities.

Check 3: Measure the thickness of your outside walls

Cavity walls tend to be thicker than solid walls because they have an air gap between the external and internal wall. To measure the thickness of your outside walls, simply open your front or back door (or a window in the front or back walls) and measure from the outside face of the wall, through the door/window opening to the inside face of the wall.

A thickness of less than 30cm (11.5 inches) is NOT likely to be a cavity wall.
More than 30cm (11.5 inches) probably IS a cavity wall.

The above is only a rough guide. For advice contact your local Energy Advice Centre, which is run by the Energy Saving Trust (EST)

Some homes are not eligible for cavity wall insulation

There are a number of reasons your home may not be eligible for cavity wall insulation:

1. Your property was built before 1924 or after 1982.

2. The walls have already been filled with blown Insulation –  you will see refilled holes as highlighted by the red circle in the image.

3. You live in a flat and do not have the agreement of neighbours above and below.

4. There are any signs of damp on the inside or outside of the walls. This can be rectified before applying for cavity wall insulation.

The property is timber/steel framed, concrete or stone, however you may be able to insulate them in the same way as a solid wall – contact the National Insulation Association (NIA).

Ventilation is inadequate and a vent cannot be fitted.

Damp problems

Cavity wall insulation can cause damp, but it’s worth assessing whether your home is at risk by using the checklist below. Also ask any potential installer about these factors before committing to the work.

Damp could occur in properties as a result of cavity wall insulation if there is a combination of these factors:

Your home is exposed to severe levels of wind-driven rain.

Your home is located in an unsheltered position, e.g. not protected by trees or other buildings.

The external walls are poorly built or maintained with, for example, cracks in the brickwork or rendering.

Published guidance by the Building Research Establishment says that in these cases there is ‘an increased risk of rain penetration if a cavity is fully filled with insulation’. Rain could penetrate the outer wall, bridge the cavity via the insulation material and transfer moisture to internal walls, causing damp.

Frighteningly recent research is suggesting that there are problems with up to 25% of houses with cavity wall insulation. Most of these problems are found with retrofit insulation to existing properties where poor installation, poor materials and installation to unsuitable properties has been carried out. Remedial action can be expensive but is essential if the problems esculate.