Cavity Wall Insulation Calculator

How to Use the Cavity Wall Insulation Calculator

1. Answer the suitability questions: when your house was built, the brick pattern, the cavity width if you know it, whether the walls are damp or damaged, and how exposed your home is to wind-driven rain.
2. Choose your house type: mid-terrace, end-terrace, semi-detached, detached, bungalow or flat. Or switch to "Enter wall area" for a per-m² estimate.
3. Choose the insulation material (optional): polystyrene beads, mineral wool or foam.
4. Read the result: a suitability rating, the cost range, typical yearly savings and the payback period.

How Much Does Cavity Wall Insulation Cost?

Typical 2026 supply-and-install prices:

Measured another way, cavity wall insulation usually costs about £15–£30 per m² of external wall, with London and the South East at the higher end.

The installer drills small holes (usually about 22–25mm) in the mortar joints at regular intervals, blows the insulation into the cavity, then fills the holes with matching mortar. There's no mess inside your home.

How Much Will Cavity Wall Insulation Save?

The Energy Saving Trust estimates these yearly savings on heating bills for a gas-heated home (based on July 2026 energy prices):

The Energy Saving Trust says cavity wall insulation should pay for itself within about five years for most homes. Detached homes save the most because they have the most exposed external wall.

Do I Have Cavity Walls?

Most UK homes built from around the 1920s onwards have cavity walls: two layers (leaves) of brick or block with a gap between them. Homes built before that usually have solid walls.

You can often tell from the brick pattern:

• Cavity wall: all the bricks are laid lengthways, so you see only the long sides. This is called stretcher bond.
• Solid wall: you'll see some bricks laid end-on, showing their short ends, often in alternating rows. These are the headers that tie the solid wall together.

You can also measure the wall's thickness at a window or door opening. A brick cavity wall is usually more than about 260mm thick; a solid brick wall is usually around 225mm. If your walls are rendered or clad, you may need a surveyor to check.

Many homes built since the 1990s already had insulation put in the cavity when they were built. An installer can check with a borescope through a small hole.

Is My Home Suitable?

Cavity wall insulation is generally suitable if:

• The house was built after the 1920s with unfilled cavity walls
• The cavity is at least 50mm wide and clear of rubble and debris
• The brickwork or render is in good condition, without major cracks
• The walls show no signs of damp
• The home isn't heavily exposed to wind-driven rain, or a material suitable for high exposure is used

It may not be suitable for homes in very exposed coastal or upland locations, timber-framed or steel-framed houses, walls with existing damp problems, or homes with unrepaired defects such as cracked render, failed pointing or blocked air bricks. Poorly installed insulation in the wrong house can bridge the cavity and let damp through, and removing failed insulation typically costs £1,500–£3,000. That's why a proper survey matters.

How Much Heat Do Cavity Walls Lose?

Heat loss through a wall is measured by its U-value: the lower the number, the less heat escapes. An empty brick cavity wall typically has a U-value of around 1.5 W/m²K, which means walls are often the biggest source of heat loss in an uninsulated house, often around a third of the total.

Filling a typical 50–75mm cavity usually brings the U-value down to roughly 0.5–0.6 W/m²K, cutting heat loss through the walls by more than half. That's why Building Regulations treat filling the cavity as the expected improvement when cavity walls are renovated, with a target of 0.55 W/m²K.

The benefits go beyond your bills:

• Warmer walls feel more comfortable and reduce cold spots near external walls.
• Less condensation on the inside of external walls, when the wall is suitable and correctly filled.
• A better EPC rating, which matters to buyers, mortgage lenders and landlords.
• A smaller heat pump, if you switch heating later, because the house needs less heat.

What Happens on Installation Day

1. The installer checks the walls, air bricks and any vents, and protects flues and ventilation paths.
2. Holes are drilled in a set pattern, usually about a metre apart, through the mortar joints.
3. Insulation is blown or injected into the cavity, working from the bottom up.
4. The holes are filled with mortar coloured to match.
5. You receive the guarantee paperwork, normally within a few weeks.

Guarantees and Choosing an Installer

Choose an installer who:

• Carries out a pre-installation survey, including checking the cavity with a borescope and assessing exposure
• Is a member of a recognised trade scheme and can provide an independent, insurance-backed 25-year guarantee, often through CIGA (the Cavity Insulation Guarantee Agency)
• Gives a written quote showing the material, the drilling pattern and how ventilation (air bricks) will be protected
• Will make good the drill holes to match your mortar

Keep the guarantee paperwork with your house documents. Buyers' surveyors and solicitors will ask for it.

Grants and Funding

Some households can get cavity wall insulation free or discounted:

• ECO4 runs until the end of 2026 for eligible households, such as those on certain benefits or in homes with low EPC ratings, though funding is limited.
• Warm Homes: Local Grant (England) funds energy efficiency upgrades for lower-income households in homes rated EPC D to G through participating councils.
• Wales, Scotland and Northern Ireland run their own schemes.

Check the government's grants checker or contact your council and energy supplier. Avoid cold callers offering "free" insulation; check that any installer is registered and that the guarantee is genuine.

What About Solid Walls?

If your home has solid walls, there's no cavity to fill. The options are internal wall insulation (insulated boards fixed inside) or external wall insulation (insulation fixed outside and finished with render). Both cost far more than cavity fill but can cut heat loss through walls substantially. The Render Cost Calculator includes external wall insulation costs.

While you're improving your home's efficiency, check the loft too: the Insulation Calculator works out loft rolls, and topping up an empty loft is usually the cheapest saving of all.

Frequently Asked Questions