Cavity Wall Insulation

What is cavity wall insulation?

Most UK homes built after the 1920s have cavity walls: two separate leaves of brick or block with a gap, the cavity, running between them. That gap was originally designed to keep rain and damp out, but left empty it also lets a lot of heat escape. In fact, around 33% of the heat lost from an uninsulated home goes straight through the walls.

Cavity wall insulation fills or lines that gap. It works by trapping air within the insulation material and slowing the movement of heat from the warm inside of your home to the cold outside - a bit like putting a lid on a hot cup of tea. The result is a warmer, more comfortable home, lower heating bills and a lower risk of condensation and damp on your internal walls.

The benefits of insulating your cavity walls

• Lower energy bills - with far less heat escaping through the walls, your home stays warmer for longer and your heating can work more efficiently.
• A more comfortable home - fewer cold spots and draughts and more even temperatures from room to room.
• Damp and condensation control - the right insulation, correctly fitted, helps keep moisture out of the inner wall.
• Better sound insulation - mineral wool in particular adds useful acoustic performance as well as warmth.
• A long-life upgrade - properly installed cavity insulation is designed to last the lifetime of the building with very little maintenance.

Types of cavity insulation

There are two main approaches, and this collection covers the boards, batts and slabs for both.

Full fill cavity insulation fills the whole cavity, sitting tight against both leaves to form a continuous layer of warmth with no gaps. It’s widely used for new builds and extensions. Mineral wool cavity batts are the most popular full fill option - non-combustible, budget-friendly, and flexible enough to fit snugly around wall ties, while rigid PIR cavity boards give you a better U-value from a thinner board where cavity space is tight.

Partial fill cavity insulation uses a rigid board fixed to the inner leaf, leaving a clear air gap to the outer wall so moisture can drain away and the wall can breathe. It's the preferred choice for more exposed homes that catch a lot of wind-driven rain. Partial fill boards are usually PIR or phenolic, with phenolic offering some of the lowest thermal conductivity values available.

Not sure which you need? Our dedicated full fill cavity insulation and partial fill cavity insulation pages break down each option in more detail.

Cavity insulation materials

Mineral wool - the most widely used and most cost-effective material, available as batts and slabs. Non-combustible, with strong thermal and acoustic performance.

PIR - a rigid board that delivers high thermal performance from a slim profile, often with tongue-and-groove edges that interlock to reduce heat loss at the joints.

Phenolic - a premium rigid board with some of the lowest thermal conductivity on the market, ideal where you need the best possible U-value from the thinnest board.

How cavity insulation is fitted

If you're building a new wall, extension or self-build, fitting cavity insulation yourself is very achievable. Boards and batts are simply placed into the cavity as the wall goes up, held in place against the inner leaf with wall-tie clips. For full fill, the insulation fills the whole cavity tight to both leaves. For partial fill, you leave a residual air gap of at least 50mm, rising to 75mm in areas of very severe exposure, between the board and the outer leaf so any moisture can drain away.

A few tips to get it right:

• Keep boards butted tightly together with no gaps, and stagger the joints where possible to avoid heat escaping.
• Keep the cavity clean as you go, brush mortar off wall ties and the tops of boards so nothing bridges the gap.
• Fit cavity closers around door and window reveals to seal the cavity and stop heat, draughts and damp getting in at the edges.

Fitting boards or batts during a build is a job many competent DIYers and self-builders take on themselves. Injecting or blowing insulation into an existing, already-built cavity is a different process that's best left to a registered installer, if that's what you need, this range isn't the right product for the job.

Cavity insulation and building regulations

New builds and extensions have to meet the thermal targets in Part L of the Building Regulations, a U-value of 0.18 W/m²K or lower for new builds, and around 0.26 W/m²K for renovations and extensions. Since the 2022 Part L update, 150mm cavities have become common practice for full fill, higher-performance PIR and phenolic boards have become the popular way to hit lower U-values without making walls any thicker. For partial fill, remember to keep that 50mm air gap, or 75mm in very severe exposure to the outer leaf.

Available thicknesses and brands

We supply cavity insulation in all the common thicknesses, from 40mm to 150mm, as boards, batts and slabs to suit your cavity width and target U-value. The range includes trusted products such as Knauf DriTherm and Superglass Superwall cavity batts in mineral wool, Kingspan Kooltherm K106 phenolic cavity board and PIR cavity boards including Kingspan Thermawall TW50 and Recticel Eurowall, plus cavity closers and accessories to finish the job.

Why buy your cavity insulation from Insulation UK?

• Great prices - some of the best prices online on cavity insulation boards, batts and slabs, with no minimum order value.
• All the leading brands - a full range of full fill and partial fill cavity insulation from the UK's most trusted manufacturers.
• Fast UK-wide delivery - reliable delivery across the UK, with free shipping on orders over £500 (ex VAT).
• Expert help when you need it - over 50 years' experience in the insulation industry. If you're not sure which product or thickness you need, or you're trying to hit a specific U-value, get in touch and we'll point you in the right direction.