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London & South-East England

Cavity Wall Insulation

The cheapest wall measure there is, but only for the right walls: a borescope survey first, then a 25-year guaranteed fill.

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Accredited, certified installersOne point of contactFixed written quote

Is cavity wall insulation right for your home?

  • Houses built between the 1930s and the 1990s with unfilled cavity walls
  • Sheltered, sound brickwork in good repair with a cavity of 50 mm or more
  • Owners who want the cheapest wall measure with the least disruption
  • Semis and terraces where external wall insulation is more than the budget allows
  • Homes with the loft already insulated, where the walls are now the biggest heat loss

Cavity Wall Insulation: key facts

  • Cavity wall insulation fills the gap between the two leaves of a cavity wall with bonded EPS bead or blown mineral wool, injected through small drilled holes.
  • Most houses built after the 1930s have cavity walls; a wall around 270 to 300 mm thick with only the long faces of the bricks showing is the usual sign.
  • A 3-bed semi typically costs £1,500 to £3,500, a mid-terrace £1,000 to £2,200, about £18 to £30 per m² of wall.
  • Installed in a day, with the drill holes made good in mortar matched to the existing joints.
  • Charged at 0% VAT as an energy-saving material until 31 March 2027.
  • Arranged and project-managed by Stay Warm, installed by a PAS 2030 certified, TrustMark registered accredited installer with a 25-year CIGA guarantee.

What is cavity wall insulation?

Cavity wall insulation fills the air gap between the outer and inner leaf of a cavity wall. The installer drills a pattern of small holes in the mortar joints, injects bonded polystyrene bead or blown mineral wool until the cavity is full, then makes the holes good. Nothing changes inside the house and the outside looks the same once the mortar has weathered.

Houses built from the 1930s onwards across the London suburbs, Surrey, Kent and Hertfordshire mostly have cavity walls, and a large number of them are still unfilled. An empty cavity does very little; the walls lose heat almost as fast as a solid wall. Filling the cavity is the cheapest wall measure there is, but only if the house suits it. Exposed walls, narrow cavities, rubble in the cavity and existing damp all need to be found before anything is injected, which is why the borescope survey matters more than the price.

Stay Warm surveys the house, confirms the wall type and cavity width with a borescope, and tells you plainly whether the walls should be filled. If they should, we arrange and project-manage the work and an accredited, PAS 2030 certified installer carries it out under a 25-year CIGA guarantee. If they should not, we say so and quote for external or internal wall insulation instead, which our own team installs.

Who does the work

Cavity wall insulation is arranged and project-managed by Stay Warm and installed by an accredited installer who holds PAS 2030 for cavity wall insulation and is TrustMark registered, with a 25-year CIGA (Cavity Insulation Guarantee Agency) guarantee.

Cavity Wall Insulation cost guide

Indicative 2026 market ranges for London and the South-East. Your fixed quote follows a free borescope survey and depends on wall area, access and the fill material.

Job typeIndicative cost
Mid-terrace (front and back walls)£1,000 – £2,200
3-bed semi-detached£1,500 – £3,500
Detached house£2,500 – £5,000
Per m² of wall£18 – £30 / m²
Borescope surveyFree

VAT: 0%. Cavity wall insulation is an energy-saving material, so the installation is charged at 0% VAT until 31 March 2027 (HMRC VAT Notice 708/6), reverting to 5% after that date.

What a cavity wall installation includes

Borescope inspection of the cavity before any work is quoted
Bonded EPS bead or blown mineral wool, matched to the cavity width and exposure
Drilling pattern set out to the system approval, then filled from the bottom up
Cavity brushes and sleeves at party walls, vents, flues and air bricks
Holes made good with mortar matched to the existing joints
25-year CIGA guarantee and Building Regulations notification through the installer's scheme

How cavity wall insulation works

1

Free survey

We check the wall thickness, brick pattern, pointing and damp proof course, and put a borescope into the cavity to measure its width and see what is in it.

2

Fixed quote

A written quote for the fill material, wall area and making good, with a plain statement of whether the walls are suitable. If they are not, we say so and quote for the alternative.

3

Installation

The accredited installer drills the hole pattern, injects the bead or wool from the bottom up, checks the fill, then makes the holes good. Usually one day.

4

Guarantee and paperwork

You receive the 25-year CIGA guarantee certificate and the Building Regulations paperwork, and Stay Warm remains your point of contact.

Free survey & fixed quote

Book a free, no-obligation survey and we will send a clear, fixed written quote. One point of contact from survey to sign-off, and a written guarantee on the work.

Get a free quote

Cavity Wall Insulation FAQs

How do I know if my house has cavity walls?
Look at the brick pattern. Cavity walls usually show only the long faces of the bricks (stretchers) in every course. Solid walls show bricks end on (headers) in a regular pattern. A cavity wall is about 270 to 300 mm thick measured at a door or window, a solid brick wall about 230 mm. A borescope check on the survey settles it.
How much does cavity wall insulation cost in London?
A mid-terrace is typically £1,000 to £2,200, a 3-bed semi £1,500 to £3,500 and a detached house £2,500 to £5,000, or about £18 to £30 per m² of wall. VAT is 0% until 31 March 2027. Quotes are free and fixed.
Bead or mineral wool?
Bonded EPS bead flows around obstructions, does not hold water and is easier to extract if that is ever needed, so it is our usual recommendation. Blown mineral wool costs a little less and suits wide, clean cavities in sheltered positions. The survey decides.
When should a cavity not be filled?
When the wall faces driving rain on an exposed site, the cavity is under 50 mm, the brickwork or pointing is in poor condition, there is existing damp, or the cavity is bridged by rubble or mortar droppings. Filling in those cases can carry water across to the inner leaf. We tell you on the survey and suggest external wall insulation instead.
My house already has cavity wall insulation that has failed. What now?
Failed fill, whether wet or slumped wool or old foam, should come out before anything else is done. We arrange cavity wall extraction through the same kind of accredited installer, and once the cavity is dry it can be refilled with bead or the wall can be insulated externally. See our cavity wall extraction page.
Who does the work and what guarantee do I get?
An accredited installer who holds PAS 2030 for cavity wall insulation and is TrustMark registered. The work carries a 25-year CIGA guarantee. Stay Warm arranges the survey, quote and project management.
Can I get a grant for cavity wall insulation?
Sometimes. The Great British Insulation Scheme closed in March 2026 and ECO4 ends on 31 December 2026, so the live route in England is the Warm Homes: Local Grant, run by councils for households on a low income or in an EPC D to G home. Your council chooses the installer under that scheme. For everyone else the installation is 0% VAT until 31 March 2027, and our fixed quote already reflects that. See our [insulation grants guide](/guides/insulation-grants-2026).

Areas we cover

We arrange cavity wall insulation across Greater London, Surrey, Kent and Hertfordshire. See all areas we cover →

Related services

Cavity Wall Insulation ExtractionExternal Wall InsulationLoft & Roof InsulationFree Heat Loss SurveyFlat Roof InsulationRoom in Roof InsulationSoundproofing & Acoustic InsulationExternal Render & CladdingDraught Proofing

Related guides

In-depth, independent guidance to help you decide.

Cavity wall insulation costInsulation grants 2026Cavity wall or solid wall: how to tellCavity wall insulation problemsInsulating a 1930s semiInsulating a 1960s house

Last reviewed September 2026 by Stay Warm Insulation.

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