How to correctly install full fill cavity wall insulation

Cavity wall insulation contributes to the thermal performance of a building by limiting heat transfer through the external walls. When correctly specified and installed, cavity wall insulation can also support compliance with Approved Document L (Conservation of Fuel and Power), which emphasises the importance of a building envelope’s thermal efficiency.

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Published : 05 November 2025

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Installing full-fill cavity wall insulation using Celotex Thermaclass Cavity Wall 21 is one approach to improving the thermal efficiency of cavity walls. Compared to other insulation methods, full-fill insulation aims to create a more continuous thermal barrier. This helps to limit thermal bridging and thermal bypass.

This guide will outline the considerations for full-fill cavity wall insulation, explaining what it is, demonstrating how it contributes to a building's thermal efficiency and describing correct installation practices that can help achieve consistent performance.

What is full fill cavity wall insulation?

Cavity walls consist of two masonry leaves separated by a cavity. Full fill cavity wall insulation is a method of filling the cavity with insulation. This contributes to the thermal performance of the wall by limiting heat transfer between the internal and external environments.

Depending on the product used, full fill systems may either completely occupy the cavity or incorporate a narrow residual cavity as part of their design. For example, Celotex Thermaclass Cavity Wall 21 is designed to include a residual cavity of up to 10mm if required.

Mike Vaczi, technical director at Celotex, explains: “The residual cavity helps support ease of installation and accommodate mortar squeeze. It can also be installed as a true full fill solution, depending on project requirements.”

Although there may be a residual cavity, these systems are still classified as full fill because the insulation forms a continuous layer across the cavity without leaving significant air gaps.

What’s the difference between full fill insulation and partial fill insulation?

Full-fill cavity wall insulation is designed to completely fill the cavity. In some systems, such as Celotex Thermaclass Cavity Wall 21, to leave a very narrow residual gap (10mm). The full fill method creates a continuous layer of insulation that helps to reduce heat transfer through the wall and minimise thermal bridging.

Partial-fill insulation, in contrast, occupies only part of the cavity, leaving a wider residual gap (typically 50mm) adjacent to the outer leaf. This gap can provide protection against moisture ingress, but it can also reduce thermal performance compared to a full-fill system.

What type of insulation is suitable for full-fill applications?

Various types of insulation materials can be used for full fill cavity walls, including rigid insulation boards, mineral wool batts and loose-fill materials. The choice of full fill insulation material should be guided by the desired thermal performance and compliance with building regulations.

Celotex Thermaclass Cavity Wall 21 is a polyisocyanurate (PIR) board designed specifically for full fill cavity wall applications. With a thermal conductivity of 0.021 W/m·K and a low-emissivity foil facing, it can enable thinner cavities to achieve target U-values. It also features a tongue-and-groove profile that reduces the risk of gaps between boards, helping to limit thermal bridging and providing protection from wind-driven rain.

How should full fill cavity wall insulation be installed?

The installation of full fill cavity wall insulation must be carried out in accordance with the manufacturer’s recommendations, the relevant BBA certificate, Building Regulations and British Standards.

Mike emphasises the importance of a high-quality installation: “Full-fill insulation systems must be installed in line with the manufacturer’s instructions and relevant standards to achieve the intended performance. When Celotex Thermaclass Cavity Wall 21 is correctly specified and installed, it can contribute to meeting the thermal performance requirements set out in Part L of the Building Regulations.”

Below is a simplified guide to installing full fill insulation. Comprehensive installation instructions for Celotex Thermaclass Cavity Wall 21, including advanced junction detailing, can be found in the product specification guide.

Construct the inner leaf	Build the first section of the internal wall leaf according to the architectural plans.
Install the wall ties	Ensure these are installed to the correct depth and with the right spacing according to the manufacturer’s instructions.
Prepare and orientate the boards	Starting at a corner is recommended, as it makes it easier to create a blocking pattern. Celotex Thermaclass Cavity Wall 21 includes tongue-and-groove joints to simplify installation. When installed correctly, the tongue and groove profile removes the requirement to tape the joints between the boards to prevent water ingress. The tongue should be at the top and either the left or right; be sure to maintain consistency throughout the build. If the board has a foil facing or other type of facing, this should be facing outward. Once the location of the wall ties on the board has been determined, use a self-retracting knife or specialist insulation saw to make a small cut (no more than 15mm deep) in the tongue profile. This should be tilted downward from the inner to the outer leaf.
Install the first row	Position the insulation onto the wall tiles, flush against the internal leaf, ensuring the residual cavity (0–10mm) is maintained. Retaining clips can be used to secure the boards within the residual cavity. Boards should not touch the ground. If ground contact is unavoidable, the edges of the insulation should be sealed. Once the first board is installed, connect the next one using the tongue-and-groove joints. Repeat until the first row is complete.
Install subsequent rows	For Celotex Thermaclass Cavity Wall 21, a staggered pattern of 600mm is recommended between rows. This avoids continuous vertical joints. For stability, to ensure accurate placement and minimise gaps, at least four courses of blocks should be laid on the preceding skin before the insulation is installed.
Avoid damaging insulation boards	Use a cavity board to prevent mortar from dripping on the exposed edges of the insulation. Excess mortar should be removed from the cavity face. • Avoid excess mortar on the leading leaf. Avoid piercing or otherwise damaging the insulation boards. If insulation boards are damaged, they should be replaced.
Continue to the top of the wall	Repeat this process to the top row. The top edge of the insulation should be protected with a cavity tray.
Ensure careful detailing	Detailing at corners, junctions and openings is essential for preventing thermal bridges, thermal bypass and moisture ingress. See the manufacturer’s instructions for guidance about these crucial areas.

When preparing, cutting and installing thermal insulation, eye protection and a suitable dust mask should be worn. Cutting should be carried out in well-ventilated areas and dust should be collected for safe disposal.

The importance of correct full fill insulation specification and installation

Correct specification and installation are critical for achieving the intended performance of full-fill cavity wall insulation. Inadequate installation can lead to air gaps, thermal bridging, thermal bypass, condensation or dampness, all of which may reduce both thermal efficiency and the durability of the wall construction.

When specifying and installing Celotex Thermaclass Cavity Wall 21, it is essential to follow all applicable national Building Regulations and standards. Particular attention should be given to fire performance and height restrictions. It must not be used in external walls of buildings over 11 metres in height.

Celotex provides a range of technical resources to support the correct specification and installation of full fill cavity wall insulation. These include:

Product datasheets and BBA certificates
Installation guides and junction detailing advice
U-value calculation tools

Celotex technical support is also available to provide further guidance on specification and installation requirements.

Categories: Installation , Technical

Author : David Milner Technical Team Leader

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