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What is a Cavity Closer and Why Do We Need Them?

A cavity closer is a small, but essential part of the overall energy-saving strategy of any new or refurbishment project. Cavity closers will reduce opportunities for cold bridging where heat may escape around a window or door frame and control the passage of moisture.

Fitting cavity closers to your door and window openings will help to ensure compliance with both Part C and Part L of the UK Building Regulations that deal with water ingress and energy management.

Here’s our guide to cavity closers, and why they are important to your project.

What is a Cavity Closer?

A cavity closer is basically an insulated core, usually a polystyrene foam board, within a plastic casing. It can be cut to fit any size cavity and used alongside a thermally broken, or insulated lintel, and also where individual masonry leaf lintels are used.

Some cavity closers, such as the Cavalok Multi-width and the YBS closer, come ready-grooved so they can be cut on site to suit the width of the cavity.

Why Do We Need Cavity Closers?

Quite apart from Building regulation compliance, there are several very good reasons for not skipping fitting cavity closers.

cavity closer helps moisture control

Moisture Control

In a maritime climate zone such as the British Isles, there is an ongoing issue of moisture control. The main role of an insulated cavity closer is to prevent water ingress.

The area around a window is vulnerable to water, both from rain acting directly upon the frame and also to any water that has managed to make its way inside the cavity.

Cavity trays and weep vents should take care of a lot of the issues within a cavity, but a properly fitted window cavity closer will be able to keep most of the wet off your window and door reveals.

cavity closer helps thermal efficency

Thermal Efficiency

Condensation is another perennial problem around windows and doors. Insulated cavity closers can prevent this from happening by helping to maintain a stable temperature across the cavity.

Continuity of insulation is key to preventing heat loss. When a standard-size window or door frame will only cover about 30mm of any given cavity, a closer makes up the difference.

In some build projects, there might be a foam board insulation specified or mineral full-fill cavity insulation. Neither of these insulators will do the same job as a proprietary cavity closer.

fire protection assistance from cavity closers

Fire Event Protection

Part B of the UK Building Regulations is concerned with the protection of building occupants in a fire event. In this case, the unseen spread of fire and smoke within concealed spaces such as a wall cavity can be delayed or prevented with the use of cavity closers around openings.

For specific protection against the spread of fire, a mineral wool full-fill cavity closer will likely be specified as part of the overall protection strategy. Fit these at party walls and where a wall meets the roof structure.

A standard plastic cavity closer will not be fire-rated.

Sustainable Benefits

Cavity closers, such as the Cavalok product are often made from what is called post-consumer uPVC, which means that it is made from 100% recycled plastic.

The insulation within the casing is 98% air and just 2% plastic, expanded without the use of solvents, CFC gases, or other ozone-depleting chemicals, which makes it zero-rated for global warming potential.

How Do You Fit a Cavity Closer?

There are two ways to fit a cavity closer on a new-build project, either as a former or fitted after the walls have gone up.

Make sure to cut the closer to the correct width. Many closer systems are pre-scored on the polystyrene insulation side to help ensure a nice straight cut is achieved.

Cavity Closer Former

  • Cut the cill to the structural opening size, then cut the two jambs to length plus 50mm.
  • Pin the jambs to the cill and cut and pin a batten the same length as the cill to use as a spacer for the jambs.
  • Set the former plumb and level and proceed to build it into the masonry leaves.

Use holding lugs or brackets to secure the jambs into the mortar bed on the inner leaf as you go.

Cavity Closer Retro-Fit

  • Clean out the cavity to the depth of the cavity closer and remove any loose mortar.
  • Cut the jambs to length plus 50mm.
  • Cut out the flange at the bottom of the jamb and slide the closer into place.
  • Cut and fit the cill as tight as possible.
  • Nail the flange part of the cavity closer to the inner leaf with galvanised clout nails.

Use this method when fitting replacement windows and doors to upgrade the water and thermal efficient performance of the building.

Best Practise

The cavity closer must fit snugly within the cavity to ensure maximum benefit, so seal any joints with a good quality silicone sealant.

To minimise waste on site it is fine to butt two shorter lengths of a cavity closer together but try to marry up two machine-cut ends for a snug fit. It’s recommended that you use aluminium tape over any butt joint to seal it from moisture.

YBS Cavity Closer Installation Guide

Download the Cavity Closer Installation Guide here.


Next Steps

Cavity closers are inexpensive elements and are great for protecting the cavities in an opening before the windows or doors arrive on site.

Aside from anything else, it makes the openings clean, tidy, and ready to be fitted out.

More Information on Cavity Closers, including Installation and Specification Downloads, can be found in the Insulation Section on our website. Please do not hesitate to contact our Technical Team if you have further questions.


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