Selecting the correct cavity closer depends on more than the width of the cavity.
The right product should be selected based on the cavity width, wall construction, insulation, thermal performance, window or door position, required fire performance and installation detail.
A cavity closer should work as part of the complete wall and opening design. Choosing a product simply because it fits the cavity does not necessarily mean that it provides the required thermal, moisture or fire performance.
This guide explains the main factors to consider when selecting a cavity closer for a window or door opening.
There are six key considerations when choosing a cavity closer:
The correct specification will depend on the complete construction detail.
The first step is to establish the cavity width of the wall.
Cavity closers are manufactured for specific cavity-width ranges, so the product must be suitable for the actual construction.
However, cavity width should not be the only selection criterion.
A product that physically fits the cavity may not necessarily provide the required thermal or fire performance.
When checking cavity width, consider the complete wall build-up, including:
Always check the manufacturer’s stated cavity-width range and installation requirements.
Thermal performance is one of the most important factors when selecting a cavity closer.
A window or door opening interrupts the normal continuity of the wall insulation. If the reveal is poorly insulated, it can create a thermal bridge, allowing heat to pass through the junction more easily than through the surrounding wall.
A suitable insulated cavity closer helps maintain insulation around the opening and reduce heat transfer through the reveal.
The 2026 edition of Approved Document L specifically addresses the use of fully insulated and continuous cavity closers at cavity apertures. It also states that insulation should be installed without air gaps and tight to the structure and cavity closers.
When comparing cavity closers, therefore, look beyond whether the product simply contains insulation. Consider how the product performs as part of the complete wall and window or door junction.
Thermal bridging can increase heat transfer through localised areas of the building envelope.
At a poorly insulated window or door reveal, this can result in:
The cavity closer should therefore form part of a continuous insulation strategy around the opening.
The cavity closer needs to work with the insulation used within the wall.
Different wall constructions may use different types and thicknesses of cavity insulation, and the relationship between the insulation and cavity closer is important.
Ideally, the cavity closer should allow the insulation to remain continuous around the opening without unnecessary gaps or compression that could compromise the intended performance.
Approved Document L’s 2026 guidance emphasises continuity of insulation and the installation of cavity closers tight to the insulation and cavity apertures.
Before specifying a cavity closer, check:
The position of the window or door within the wall can affect the cavity closer specification.
For example, a window may be positioned closer to the centre of the wall, closer to the external face or closer to the internal face, depending on the design.
The cavity closer needs to accommodate the intended position of the frame while maintaining the required relationship between:
The window or door detail should therefore be considered at the same time as the cavity closer.
A cavity closer should not be selected independently of the frame and wall junction.
This is an important distinction.
Where fire performance is required, the specified product must have appropriate tested performance for the intended application.
Current Approved Document B states that cavity barriers should be provided at the edges of cavities, including around openings such as windows and doors, subject to the applicable provisions. It also specifies performance requirements for cavity barriers.
This means you should not assume that an insulated cavity closer automatically provides the required fire performance.
When fire performance is required, check:
An insulated cavity closer is primarily intended to close and insulate the cavity around an opening.
A fire-rated cavity closer or cavity barrier has additional tested fire performance and may be suitable where the design requires a cavity barrier.
Some systems are designed to provide both thermal and fire performance.
However, these functions should always be confirmed from the manufacturer’s technical documentation.
Even a correctly selected cavity closer can fail to perform as intended if it is incorrectly installed.
The installation should follow the manufacturer’s instructions and the project’s wall, window and door details.
Pay particular attention to:
A cavity closer is part of a junction detail, so its performance depends on how the complete junction is constructed.
The answer depends on the construction.
An insulated cavity closer should be selected to suit the cavity width and wall insulation, while providing the required thermal and moisture performance.
A cavity closer or cavity barrier with appropriate tested fire performance should be specified.
Do not substitute a standard insulated cavity closer for a fire-rated system without confirming that the product is suitable for the required application.
Look particularly closely at:
The product should be considered as part of the complete thermal-bridge detail.
Cavity size is a useful starting point, but it is not enough on its own.
A simple specification process is:
Establish the actual cavity width and wall construction.
Determine the type and thickness of cavity insulation.
Check where the frame sits relative to the insulation and wall faces.
Check the required thermal performance of the wall and opening junction.
Determine whether a cavity barrier or fire-rated cavity closer is required.
Confirm that the selected product is suitable for the complete construction and intended application.
Install the product according to the manufacturer’s instructions and project-specific construction details.
Some cavity closer systems are designed to accommodate a range of cavity widths, while others are manufactured for specific dimensions.
Do not assume that a product can be adapted to a different cavity width simply because it physically fits.
Always check the manufacturer’s approved cavity-width range and installation requirements.
Some cavity closer systems can be cut or adapted during installation, but this depends on the specific product.
Where cutting or modification is permitted, it should be carried out according to the manufacturer’s instructions.
This is particularly important for products that provide fire performance. Cutting, modifying or installing a fire-rated product outside its tested or assessed configuration could affect its performance.
It can.
The cavity closer forms part of the construction around a window or door opening and can contribute to meeting requirements relating to thermal performance, moisture protection and, where applicable, fire safety.
For buildings in England, relevant guidance includes:
The current 2026 Approved Document L provides guidance for the energy efficiency requirements of Part L, while current Approved Document B provides fire-safety guidance including cavity barriers.
The applicable edition of the regulations and guidance depends on the building work and when the relevant requirements apply.
A cavity closer that fits the cavity may not provide the required thermal or fire performance.
The cavity closer should work with the wall insulation and maintain continuity around the opening.
They are not. Fire performance needs to be specifically established.
The frame position can affect the relationship between the closer, insulation and wall construction.
The closer is part of the complete wall/window or door junction.
Any modification should be permitted by the manufacturer’s technical documentation and remain within the product’s tested or assessed application.
Before specifying a cavity closer, confirm:
Cavity width is an important starting point, but it should not be the only consideration. The correct cavity closer also needs to be suitable for the wall insulation, thermal requirements, window or door position, fire requirements and installation detail.
Start by identifying the cavity width and wall construction. Then check the insulation, window or door position, thermal requirements and whether fire performance is required. Finally, select a product whose technical documentation confirms that it is suitable for the complete construction.
Cavity width determines which products may be physically suitable, but it does not determine the complete specification. Thermal, moisture, fire and installation requirements also need to be considered.
An insulated cavity closer helps maintain insulation around the opening and reduce thermal bridging. The 2026 Approved Document L specifically addresses fully insulated and continuous cavity closers at cavity apertures.
You may need a fire-rated cavity closer or cavity barrier where the applicable fire-safety design requires a cavity barrier around an opening. The requirement depends on the building and construction. The product’s fire performance should be verified from appropriate test or assessment documentation.
No. A cavity closer primarily closes the cavity and can provide thermal and moisture functions. A cavity barrier is intended to restrict fire and smoke spread through a concealed cavity. Some products can perform both functions, but this must be confirmed from the manufacturer’s documentation.
Potentially, but this depends on the product and construction. The selected cavity closer must be suitable for the opening, wall construction, frame position and any thermal or fire requirements.
Yes. An appropriately designed and installed insulated cavity closer can help reduce heat transfer through the window or door reveal by maintaining insulation around the opening.
Provide the cavity width, wall construction, insulation type and thickness, window or door type and position, and any thermal or fire-performance requirements. The more complete the construction information, the easier it is to identify a suitable product.
The correct cavity closer should be selected as part of the complete wall and opening detail.
The key questions are:
Getting these details right helps ensure that the cavity closer contributes to the intended thermal, moisture and fire performance of the building.
For project-specific applications, always check the current Building Regulations guidance, project specifications and manufacturer’s technical documentation before selecting and installing a cavity closer.
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