Video Transcript

What This Detail Is – And What You’ll Learn Today

Today’s junction is an external masonry cavity wall window head, using a steel lintel with built-in EPS insulation, a cavity tray and weep holes for moisture control, and airtightness measures on the internal reveal. By the end of this session, you’ll understand:

• How weep holes and cavity trays work together to manage moisture above openings.
• Why the minimum frame overlap and set back matters for thermal performance and weather protection.
• How the airtightness tape connects the window to the internal wall build-up.
• The difference between frame overlap and frame set back
• How insulation and structural elements link for thermal continuity.

This detail and the associated student pack can be downloaded for free – stick around to find out more about the download. 

Glossary of Terms

• Cavity tray — A damp-proof barrier installed above openings. It collects any moisture entering the cavity and drains it through weep holes to the outside.
• Weep hole — A small opening in the outer leaf, typically at 450 mm centres, allowing water and moisture from the cavity tray to escape.
• Lintel — A horizontal structural member above the window opening. In this case, a steel lintel with EPS insulation to reduce thermal bridging.
• Frame overlap and set back — The frame overlap is the distance the window or door frame overlaps the insulation layer within the cavity. The frame set back is the required distance the window needs to be set back from the outer face of the external leaf depending on the exposure zone. Severe exposure zones require a check rebate. 
• Airtightness tape — A flexible adhesive tape used to seal the junction between window frame and surrounding structure, preventing air leakage.
• Insulation to reveal — Insulation placed within the window reveal to maintain thermal continuity and improve thermal performance around the frame.
• Parge coat — A thin coat of mortar applied to the inner face of blockwork to improve airtightness.

Walk Through the Detail

Starting from the top, the steel lintel spans the window opening and carries the outer leaf of brickwork and the inner leaf of blockwork above. The lintel includes EPS insulation bonded to its underside, reducing heat loss across the steel.

Above the lintel sits the cavity tray, a continuous DPC that slopes gently to the outer leaf. Any moisture that gets behind the brickwork is caught by this tray and directed outwards through the weep holes. The weep holes are small openings in the mortar joint — they don’t drain the window frame itself, but the cavity tray above it. They should be spaced no more than 450 mm apart and kept clear during construction.

Below the lintel, the window frame overlaps the cavity insulation by at least 30 mm and is set back from the external brickwork face. The overlap ensures continuity in the insulation layer and the set back ensures that the joint is protected from wind-driven rain.

On the inside face, airtightness tape seals the junction between the window frame and the plastered reveal. The tape sticks to a clean, dry surface — usually onto the parge coat or blockwork. It’s then covered by insulated plasterboard, completing the airtight and thermal line around the opening.

The full-fill cavity insulation continues up past the lintel, ensuring thermal continuity around the head. Internally, a 6 mm parge coat and 52.5 mm insulated plasterboard finish the wall.

Student Questions

1. How do weep holes work? Weep holes let water escape from the cavity tray above the lintel. Any moisture entering the cavity drains down onto the tray, then out through these small openings. They prevent water from building up and leaking internally.
2. What does “minimum frame overlap 30 mm” mean? This ensures the window frame covers at least 30 mm of the cavity, meaning the window will be overlapping with the insulation making to ensure a continuous thermal line. The window is also set back which provides protection against rain and improves the visual depth of the frame within the wall.
3. Does the airtightness tape stick straight onto the brick and blockwork? Yes — it should adhere to a prepared, surface such as the blockwork, parge coat or a primed frame edge. The goal is to form a continuous airtight seal between the frame and the wall construction. Different taping systems are available for different applications.

Key Points for This Detail

• Cavity trays and weep holes must be included above all window and door openings in cavity walls.
• Weep holes at 450 mm centres maximum, kept clear during construction.
• Frame overlap of very minimum 30 mm to 50mm for adequate thermal performance 
• Frame set back by 38mm from the external face. (If in severe exposure zones a check rebate would be needed). .
• Airtightness tape applied to clean, dry surfaces and compatible with adjoining materials.
• Insulation continuity around reveals reduces cold bridging and condensation risk.
• Lintel specification (gauge, bearing, insulation) should follow the structural engineer’s requirements.
• Cavity closure/fire stop materials to comply with Approved Document B.

As always Project specifics matter. Substrate, moisture, exposure, finishes, acoustic and fire requirements, warranties/insurer requirements and manufacturer guidance can all shift the final arrangement.  There is always more than one way to design a detail.