Cavity Closers: What They Are, Why You Need Them, and How Not to Get Them Wrong

A bricklayer turns up to fit your windows three months after the build started, takes one look at the openings, and tells you the cavity closers are missing. The walls are up, the lintels are in, the brickwork is pointed. The only way to fit a proper closer now is to take the windows back out, pull the closers in, and refit. That is a five-figure mistake on a build where the closers themselves cost a few hundred pounds. Cavity closers are a first-fix item. Get them in as the wall goes up or pay tradespeople twice to fix it later.

What it is and what it's for

A cavity closer is a prefabricated insulated frame fitted into the cavity at every window and door opening in a cavity masonry wall. It replaces the plain mortar-and-blockwork "return" that used to close cavities at openings in older buildings. The product looks like a long uPVC rail with two flanges and an insulating core in between. The flanges sit on the inner faces of the outer and inner leaf, and the core fills the cavity itself.

It does three jobs at once. First, it stops the cavity becoming a cold bridge at the reveal (the strip of wall around the opening), where, without insulation in the cavity, heat would leak straight through and condensation would form on the inside of the plaster. Second, it acts as a vertical damp-proof course at the jambs (the vertical sides of the opening), stopping rainwater that has crossed the outer leaf from tracking sideways into the inner leaf. Third, it slows fire from spreading through the cavity at the opening. Fire-rated variants are tested to provide 30 or 60 minutes of certified fire resistance.

In England and Wales the relevant Building Regulations are Approved Document L (thermal: the cavity closer must give a minimum thermal resistance of 0.45 m²K/W at the reveal), Approved Document B (fire: barriers required at specific cavity locations), and Approved Document C (moisture: windows must sit back at least 30mm from the inside face of the outer leaf, with a continuous DPC at the reveal). Scotland's Technical Handbooks and Northern Ireland's Building Regulations impose equivalent rules.

That number is the minimum Thermal Resistance Path that an insulated reveal detail has to deliver under Accredited Construction Details. It is not a marketing figure. Reveals are responsible for up to 30% of total heat loss in a poorly detailed building, and 0.45 m²K/W is roughly the threshold below which condensation and mould start appearing on plasterboard around windows in a typical UK climate. Every BBA-certified proprietary insulated closer on the market meets it. Plain uPVC ducting, expanding foam, and improvised solutions usually do not.

NHBC Standards Chapter 6.1 (the warranty body's technical requirements for masonry walls) goes further. It requires "a proprietary cavity closer assessed under Technical Requirement R3" at every opening where the cavity is not closed by returned blockwork. The closer must run in one continuous piece up each jamb, with the rigid cavity insulation butted tightly against it. NHBC inspectors check this. Building control inspectors check it under Part L's photographic evidence requirement, which since 2021 has obliged builders to supply images of construction details to verify thermal continuity.

Types, sizes, and specifications

Cavity closers come in two families. Bar-length closers are the standard product: 2.4m or 2.5m straight lengths fitted in the cavity as the masonry goes up. Frame formers are a different, less common system where the closer attaches to the window or door frame and is then built into the wall around it. For almost every domestic extension, bar-length closers are what you want. Frame formers are mostly used on commercial fenestration packages.

Within bar-length closers, the variables are cavity width, insulation core, fire rating, and whether the closer is single-flange (sits flush on one face) or double-flange (with a flange on both inner and outer leaves). Double-flange is the standard for new build. Single-flange variants exist for retrofit or check-reveal details.

UK cavities run in standard widths. 50mm cavities are typical of older properties and simple extensions. 75mm is common in mid-2000s construction. 100mm is the current default for new build extensions, which gives enough room for either 75mm partial-fill rigid insulation with a 25mm clear cavity, or 100mm full-fill mineral wool. 125mm and 150mm cavities are used in modern energy-efficient builds where the U-value target demands more insulation. A multi-width closer with pre-cut grooves can be snapped down to suit 50mm, 75mm, or 100mm. Useful when the cavity width varies along the wall (it shouldn't, but old extensions sometimes have surprises).

Closers are made in either 2.4m lengths (most brands) or 2.5m (Manthorpe). A standard window opening of 1200mm height needs one length per jamb plus a head section, with around 75mm of run-on at the base of each jamb so the bottom edge sits below the sill in the cavity. Two 2.4m lengths cover most domestic windows.

The thermal conductivity figures in the table look minor (0.031 vs 0.039 W/mK) but they translate into different reveal psi values, which feed into the SAP calculation for the whole house. A higher-performance closer with PUR core can drop the reveal psi value by enough to reduce the calculated heat loss noticeably. For Future Homes Standard and Passivhaus-adjacent builds, the difference matters. For a standard rear extension to current Part L minimum, any BBA-certified insulated closer will pass.

Brands and what to specify

The cavity closer market in the UK shifted significantly in mid-2025. The most commonly specified premium product was Kingspan Thermabate, a uPVC profile with a rigid PUR foam core. Architects and merchants specified it by name for years. Kingspan discontinued Thermabate on 13 June 2025. Many online guides and older specification documents still recommend it. Some retailers are still selling old stock. Do not specify Thermabate for new projects. Kingspan now points customers to Kooltherm K107 or to a request-for-quotation route.

Current options in mainstream UK supply:

• Timloc Thermo-loc Platinum+: uPVC extrusion with Platinum EPS insulation. Sizes 50–150mm, 2.4m lengths. BBA certified. Available with FR30 and FR60 fire-rated variants. Currently the closest direct replacement for Thermabate's specification slot.
• Manthorpe G240 / G247: Long-established uPVC + EPS closer. G240 is the standard fixed-width range (50mm, 100mm, 125mm, 150mm). G247 is the multi-width version (50–100mm with pre-cut grooves). Sold widely through builders' merchants in 2.5m lengths.
• Cavalok (Eurocell): 100% post-consumer recycled PVC with polystyrene insulation. BBA certified, NHBC approved. Sizes 50–300mm, the widest range on the market. The BigBlok variant is for cavities of 125mm and above. FlameBlok is the 60-minute fire-rated variant.
• ARC Building Solutions Contract-Closer: Heavy-duty double-flange closer, popular with national housebuilders. Sizes 100mm, 125mm, 150mm. ARC also makes the Rockfibre (mineral wool core) and Insulated DPC roll for retrofit and party-wall situations.
• YBS EasiClose / Soprema Easi-Close: Value-end of the market, stocked at Screwfix and Toolstation. Standard insulated, BBA certified. Fine for general use; no fire rating.

For a typical rear extension in standard low-rise housing, Cavalok, Manthorpe G240, or YBS EasiClose at the standard insulated specification is sufficient at the window and door reveals. Where the extension joins a separating wall (a party wall to a neighbouring property, or the wall between an attached garage and the house), use a fire-rated variant such as Timloc FR30, Cavalok FlameBlok, or ARC Rockfibre.

How to work with it

Cavity closers are light. A 2.4m length weighs around 1kg. A bundle of ten is easily man-handled by one person. They cut with a fine-tooth handsaw or a sharp Stanley knife: the uPVC slices cleanly, and the EPS core compresses and tears under the blade. There is no dust to worry about with EPS-cored closers. Mineral wool fire-rated variants do create wool fibres when cut, so wear gloves and a P3 dust mask for those.

Storage on site is straightforward. Keep them flat on the ground or stacked, off the ground on a pallet, under a tarp. They do not deteriorate from a few weeks outdoors but the EPS core can yellow slightly in direct UV, which is cosmetic only. Keep them away from solvents and strong adhesives. uPVC reacts with some chemicals, and you do not want a softened flange when it comes to fixing.

The installation sequence is the part most often messed up.

1. The bricklayer measures the cavity width before ordering. A 100mm closer in a 75mm cavity won't fit; a 75mm closer in a 100mm cavity leaves a 25mm gap that defeats the point.
2. Each jamb closer is cut to length: opening height plus 75mm of run-on at the base. The bottom 75mm has the flanges trimmed back so only the cavity-width foam core remains. That section drops into the cavity below the eventual sill, anchoring the closer and providing continuity of insulation under the sill.
3. The closer is positioned in the opening as the brickwork rises around it. Some products include cavity closer clips that hold the closer to one leaf as the bricklayer builds up. Others are tied in with the next mortar course.
4. Flanges must be mechanically fixed. Masonry screws (such as Spax masonry screws or Rawlplug Concrete Screws) into plugged holes every 225mm up the jamb. Masonry nails alone are not enough; they pull out of soft mortar over a winter cycle. Closers held by friction or mortar grip alone shift when the window goes in, leaving gaps.
5. As cavity insulation goes in, it must butt tightly against the closer with no gap. Cut the insulation to fit, don't compress the closer to fit the insulation.
6. The window or door is fitted with a minimum 30mm overlap of the inside face of the frame past the inside face of the outer leaf. This is the Part C setback requirement. The closer foam sits between the back of the frame and the inner leaf, completing the thermal seal.
7. External and internal gaps around the frame are filled with low-expansion frame foam and finished with a quality frame sealant. Do not use standard expanding foam; it pushes the closer out of position as it cures.

Where the closer goes, and where a lintel replaces it

A typical window opening needs three closer runs: the two jambs and one across the head (the top of the opening). The head closer is often eliminated because the lintel that spans the opening already incorporates the cavity-closer function.

Catnic and IG Lintels make one-piece galvanised steel lintels that wrap the cavity, with an integral insulation strip across the cavity gap. The Catnic Thermally Broken Lintel range publishes psi values of 0.05 W/mK or lower at the head. If the structural engineer has specified a one-piece thermally-broken cavity lintel, you do not need a separate head closer. The lintel does the job.

If the lintel is a two-part system (separate inner and outer lintels with no integral insulation), or a concrete or steel beam without a closer, the head needs its own cavity closer cut to the opening width and fitted into the cavity below the lintel before the next masonry course goes on. This is the easiest position to forget. If you only see closers at the jambs and not at the head, the head is a cold bridge and a fire path.

The sill detail varies. Many builders run a continuous DPC across the sill on top of the inner leaf and trust the closer's run-on flanges to bridge the cavity below. Some specifications add a separate sill closer. Either way, the cavity should not be open at the sill. It needs either a closer, a DPC tray, or returned blockwork closing it off.

How much do you need

For a typical 30m² rear extension with three windows and one or two doors, total cavity closer requirement is around 50–60 linear metres.

A worked example for a single window opening 1200mm wide x 1200mm tall, with a thermally-broken lintel at the head:

• Two jambs at 1200mm + 75mm run-on each = 2 × 1.275m = 2.55m
• Head closer = not required (lintel does the job)
• Sill closer = optional depending on detail; if used, opening width = 1.2m
• Total per window = 2.55m to 3.75m depending on sill detail
• Buy three 2.4m lengths to allow for cuts and waste

For a French door at 1.8m wide x 2.1m tall:

• Two jambs at 2.1m + 75mm = 2 × 2.175m = 4.35m
• Head closer if lintel is not thermally broken = 1.8m
• Total per door = 4.35m to 6.15m
• Buy three 2.4m lengths

Add 10% wastage for cuts, mistakes, and the inevitable damaged length the merchant slipped into the bundle. Order in even pack quantities. Most products ship in boxes of 6, 8, or 10 lengths, and ordering single lengths is more expensive per metre than buying a pack.

Cost and where to buy

That is the inc-VAT range for standard insulated closers (50–100mm cavity) at Toolstation, Screwfix, Insulation Superstore, and most builders' merchants in 2026. Pack pricing brings the per-metre rate to the lower end of the range; single lengths sit at the upper end. Wickes and DIY chains stock cavity closers but typically at higher per-metre rates than dedicated insulation suppliers.

Wider cavities cost more:

Fire-rated variants are roughly double the price of standard:

For a typical 30m² rear extension with mostly standard closers at reveals and a few metres of fire-rated at any party-wall or garage junction, total cost is:

A few hundred pounds. Compared to the cost of taking out windows to fit closers retrospectively (window removal, refitting, brickwork repair, sealant), getting the right product on site at first-fix is one of the most cost-effective decisions on the whole build:

For ordering: Toolstation and Screwfix are reliable for small quantities and same-day collection. Insulation Superstore is the strongest online specialist for full pack quantities and the wider Cavalok and Timloc ranges. Travis Perkins, Jewson, and independent builders' merchants stock Manthorpe and Cavalok by default and will price-match if you ask. Wade Building Supplies regularly discounts Cavalok multi-width closers below the four-pounds-per-metre mark.

Alternatives and edge cases

In Northern Ireland and parts of Scotland, an established alternative to a proprietary cavity closer at the jamb is returned blockwork. The inner leaf is returned around the opening to meet the outer leaf, with a 50mm minimum strip of PIR insulation behind it and a continuous DPC at the reveal. This works and complies with regulations where building control accepts the detail. It is less common in England, where proprietary closers dominate. If your bricklayer is from an NI tradition and proposes returned blockwork, it is not wrong, but check the inspector is comfortable with the detail before the brickwork goes up.

For Passivhaus and high-performance builds, custom OSB and PIR window boxes outperform standard uPVC closers on air-tightness and reveal psi value. The window frame fixes into a timber and rigid-insulation surround that's foamed, taped, and sealed to a level that no off-the-shelf uPVC product can match. This is specialist detailing and not relevant to standard extension work (the proprietary closer hits Part L easily) but it's the route to take if your target is sub-15 kWh/m²/yr or air-tightness below 1.0 ach@50Pa.

For sliding patio door thresholds where a standard cavity closer cannot fit, Compacfoam structural insulation blocks are the specialist solution. Most builders cut PIR board to size at this location instead. Either approach works as long as the cavity is fully closed and the DPC is continuous.

Where you'll need this

• Walls and blockwork: fitted at every window and door opening as the brickwork rises, before lintels or windows go in
• Steels and lintels: coordinate with one-piece thermally-broken lintels, which replace the head closer
• Windows and doors: the closer must already be in place at first-fix; window setback (30mm minimum) overlaps the closer flange
• Building control inspection (structure): Part L photographic evidence requirement now expects images of cavity closers in place

Cavity closers appear at every opening on every cavity-wall extension, loft conversion with new dormers, garage conversion that introduces a new window, or any extension or renovation project that creates an opening in a cavity masonry wall. The product is invisible after second fix, and the consequences of getting it wrong are invisible until the first cold winter.

Common mistakes

Forgetting the head. Builders sometimes fit closers at both jambs and skip the head entirely, assuming the lintel handles it. If the lintel is not a one-piece thermally-broken design, the cavity at the head is wide open and the strip of plaster above the window will go cold and damp within one heating season.

Closer not mechanically fixed. Held in only by friction, mortar grip, or a few nails into mortar joints. The window fitter knocks the closer out of alignment when fitting, the cavity insulation pushes it sideways, and gaps appear around the frame. Always specify masonry screws into plugged holes through the flanges.

Insulation not abutting tightly. A 5mm gap between the cavity insulation and the closer is enough to make the reveal a cold bridge. Cut the insulation to fit, do not compress the closer. NHBC inspectors check this with a torch.

Standard insulated closer at a party wall. Rigid PIR or EPS cannot run continuously through a separating wall. It would breach the fire compartmentation. The party wall junction needs a mineral wool or fire-rated closer that breaks the rigid insulation at the wall line. Standard insulated closers at this junction fail Part B and any NHBC inspection.

Specifying Thermabate. Architects' boilerplate specifications and older online guides still call out Kingspan Thermabate. The product was discontinued in June 2025. Substitute Timloc Thermo-loc Platinum+ or Cavalok BigBlok at the specification stage rather than buying remaining stock from a clearance retailer. Once the stock runs out, replacement parts and matching profiles will be unobtainable.

Trusting that BCO will catch it. Building control inspection of cavity closers is inconsistent in practice. Some BCOs check, some don't. Part L 2021 photographic evidence has tightened this, but the legal responsibility for compliance sits with the building owner, that is, you. If your builder skips the closers and the BCO doesn't notice, the wall is still non-compliant and the problem is yours to fix when the property is sold or surveyed. Insist on closers at first fix. Photograph each opening before it is plastered. Keep the photos with the build records.