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A Simple Airtightness Win at Window Reveals: Before/After With VaraSeal SL 200

On a recent site visit we completed an air permeability test, during the test we identified significant air leakage around the french and patio doors. We invited the site manager to so us around some less complete plots still at a pre-plasterboard stage. The window and door cavity closer being used differed from the type we had previously seen on thier developments: the inner plastic “cap” that normally closes the reveal was missing. This new cavity closer detail leaves a clear path for air to move from behind the plasterboard straight into the cavity at the jambs and head.


Close-up of window jamb with exposed cavity and metal strap, no airtight tape applied.
Open reveal at window jamb—no inner closure; clear air path from service void into cavity.

Why does that matter? Two reasons:


  • Air leakage & Part L risk. Uncontrolled air paths at the window perimeter make it much harder to hit your airtightness target at final test.

  • Moisture risk. Warm internal air leaking into a cold cavity can raise interstitial condensation risk at sensitive junctions.


The Recommended Detail


We suggest sealing the window perimeter with VaraSeal 200 SL (200 mm) run continuously at both jambs and head, and used at the sill where practicable.


The photo below shows a small sample of the VARA Seal installed as a demonstration to the site team.

Demonstration of wide airtight tape spanning from uPVC frame over cavity sock onto blockwork.
Sample strip of VaraSeal 200 SL bridging from frame to blockwork to create an airtight reveal.

How to apply (quick sequence):


  1. Prep the reveal: dust off blockwork; trim excess foam; surfaces should be dry and reasonably clean.

  2. Start on the frame. Use the tape’s split-release liner (10 mm relief) to bond the first edge to the window frame.

  3. Bridge the gap. Remove the remaining liner and carry the tape over the cavity/sock and onto the blockwork.

  4. Consolidate. Press firmly with a roller; pre-crease corners and overlap pieces by ~50 mm at changes of direction.

  5. Make it continuous. Run the tape up both jambs and across the head; treat the sill where access allows.

  6. Coordinate with other layers. Ensure compatibility with DPCs/weep holes and any internal VCL/plasterboard returns.


This approach provides a tapable, durable, and continuous air barrier at the window perimeter—even when the cavity closer doesn’t encapsulate the reveal.


Why VARA Seal SL 200?


  • Bridges irregularities between frame, cavity sock, and blockwork.

  • Split-liner makes it easy to set alignment on the frame before spanning the reveal.

  • Wind-tight and air-tight to reduce both infiltration and exfiltration at the junction.

  • Speed of install, a single person can standardise this as a repeatable step before plasterboard.

  • The SL (split liner) range is available in 80m, 150mm and 200mm widths.


Ready to seal those reveals properly?


Check out the VARA Seal SL 200 product page and/or order your VaraSeal 200 SL now using the below. With upto 10% off manufacturers RRP!!!



If you have any air leakage questions or queries, give us a call on 01489 565920 and one of our experts will be happy to chat through what solutions we can offer. Alternatively you can arrange one of our experts to visit your project and offer onsite tailored air leakage advice and solutions, call us now to book your visit.

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