How to close a cavity around window and door openings in a cavity wall

Thermabate on site

When building a property to achieve high standards of thermal performance it is important that all elements are insulated correctly. One of the areas that is often not addressed properly when insulating a building is the openings around the doors and windows in cavity walls.


A local drop in the surface temperature around openings, or ‘Thermal Bridging’, can give rise to unsightly mould growth and damage to reveal finishes

If the cavity walls are not closed appropriately at openings for doors and windows it can cause issues with condensation and mould growth. Closing the cavities properly ensures that there is continuous insulation around the openings which reduces the heat loss. Linear thermal bridging is the term used to describe this heat loss (psi-value) at junctions between elements and around openings in elements.

Correctly closed cavities can also prevent fire from spreading within the cavity and an appropriate cavity closer can also form an integral damp proof course.

A simple and efficient method of closing cavities around openings in cavity walls and stopping thermal bridging from occurring is by the use of cavity closers. These can be used when building a wall or when replacing doors and windows.

Cavity closers can be cut to fit the size of the opening or preformed and fitted into the opening. They close the cavity, minimising thermal bridging and so reducing heat loss as well as acting as a damp proof course. The use of cavity closers can also help prevent fire from spreading into the cavity by acting as a barrier. If a fire gets into a cavity it could spread throughout a property, proper use of cavity closers can help prevent this.


Kingspan Kooltherm Cavity Closer

Fitting techniques often depend on the type of cavity closer used and if it is a refurbishment or new build. When replacing windows it is an ideal time to check that there are appropriate cavity closers around the opening and fitting them if needed. This can be an effective way of increasing the thermal efficiency of a building as part of a refit.

Kingspan Insulation manufactures two brands of cavity closers, both of which are available in sections, to be cut to the appropriate lengths for the window, or as a unit pre-formed to specified opening sizes. They can also be factory formed to a specific radius for curved or bullseye windows. CAD details are available for both of the cavity closers that Kingspan manufacture which can be found on our website.

These can be used to close cavities of up 100mm wide. These are normally built into the opening as it is created. It is fixed into position by fixing ties and mortar ‘keys’ into the grooves at each end of the box section of the cavity closer, ensuring that it is tight in the cavity and closing it completely. At the head of the opening an appropriate lintel and damp proof course are usually incorporated and if the lintel is insulated then a cavity closer is not required. These cavities closers, when subjected to the British Standard fire test, have passed a 30 minutes UKAS accredited fire resistance test.(currently up to cavities of 100mm).

Kooltherm Cavity Closer
These come in widths of up to 150mm and can be joined together to close cavities of up to 300mm.They can be fitted while the opening is being constructed but normally get fitted after the opening (of an appropriate size) has been constructed. The Kooltherm cavity closer would be fixed through the flange, into the inner masonry leaf. There should then be no gaps between the cavity closer and either wall leaf to ensure that there is no potential path for fire to spread. These cavity closers, when subjected to the British Standard fire test, have passed a one hour UKAS accredited fire resistance test, for thickness up to 150mm and a 30 minute test for two sections joined together for thicknesses up to 300mm. Again at the head of the opening an appropriate lintel and damp proof course should be incorporated which, if insulated, means no cavity closer is required.

By using cavity closers it ensures that the overall thermal performance of the building is not compromised by inadequate construction detailing around openings. For more information about cavity closers and the best ways in which to fit them please download a copy of our structural quick guide which has more information.

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Kingspan Insulation is a market leading manufacturer of optimum, premium and high performance rigid insulation products and insulated systems for building fabric and building services applications.

0 comments on “How to close a cavity around window and door openings in a cavity wall
    • Hi Nancy,
      Yes, cavities can be closed around openings in existing buildings. You just have to cut away and clear any masonry from the openings. See page 7 on the Kingspan Kooltherm Cavity Closer brochure.


    • Hi Ian,

      Thanks for your question. The answer is that it depends on a number of issues including does the wall have a cavity; what is the construction; is there insulation already in the cavity; are you insulating the wall in any other way? We do have a cavity closer (Kingspan Kooltherm Cavity Closer) which is suitable for closing cavities in an existing wall, but you would need to remove the windows to install the closer (this would be true for any retro-fitted cavity closer).
      The best bet would be to talk through your construction with one of our technical advisors and they’ll run through the options for your particular build-up. They can be reached on 01544 387 382.

    • Hi Brian,

      Without knowing the exact details of your project and build-up, we would recommend that you talk to Building Control about this. It is worth noting that a cavity closer also performs the function of acting as a DPC and reduces the heat loss at the junction between the window/door and wall. This can help avoid mould forming.

  1. Is the cavity closer hydrophobic as well as thermally and fire rated and is that the reason that it can replace the vertical dpc

  2. I’m taking a small window out and bricking the opening up , do I need to open the cavity up again to ensure a free air flow or can I just go ahead and brick it inside and out?

  3. What is the linear thermal transmittance value (W/m.k) for the sill and reveal junctions of a window if you use Kingspan cavity wall closures, in relation to SAP 2012 Table K1 – ref E3 + E4?

    • Hi Adrian

      Take a look at the Kingspan Kooltherm K106 (90mm in 100mm Cavity) Standard Details. This literature includes Kingspan Kooltherm Cavity Closer alongside the lower-lamdba Kingspan Kooltherm K106 Cavity Board, so might be of some help. We don’t have any details for the cavity closer alone, as the whole build-up needs to be taken into account to produce the psi value.

      If you need further guidance, I suggest contacting our technical team on 01544 387 382 for more advice.

    • In order to demonstrate the effectiveness of Kingspan Kooltherm Cavity Closer, we commissioned BRE Scotland to carry out a thermal modelling study of jamb details for differing cavity wall constructions. Overall there is a significant performance improvement over the Accredited Details, with calculated Psi values ranging between 0.027W/mK and 0.007W/mK depending on the frame size and overlap with the cavity closer.

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