Video Transcript

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

This is one of those details that looks simple… until you try to understand how the moisture layer, airtightness layer, insulation and fixings all overlap.
Most problems at window cills come from not knowing which layer takes priority — or where one stops and the next starts.

In this episode, we’ll pull the whole junction apart so you can finally see how all the layers connect.

This is a window cill detail for a masonry cavity wall using a solid timber window frame.

By the end of this session, you’ll understand:

• How the DPC at the cill protects against moisture
• Where the sealant should be positioned
• What the insulated cavity closer does and how it compares to a cavity barrier
• How the airtightness layer connects at the base of the window
• How all these elements combine for a dry, durable and efficient junction

Glossary of Terms

Let’s start off by introducing you to some of the common elements we will come across when looking at a window sill detail. 

• DPC (Damp Proof Course) —  A waterproof barrier laid beneath the window cill and lapped up the sides to stop moisture tracking into the wall or frame.
• Flashing — A weatherproof covering (often metal) that directs water away from the window junction.
• Sealant — Flexible silicone or polymer mastic used to seal small movement joints between the frame and surrounding materials, preventing water and air leakage.
• Airtightness tape — Adhesive tape used to create a continuous airtight connection between the window frame and wall structure.
• Cavity closer — A rigid or semi-rigid insulated component that closes the open end of a cavity around windows and doors. It reduces heat loss and provides fire resistance (30 minutes in this example).
• Galvanised window strap — Metal strap fixing the window securely to the surrounding structure, often with expanding foam or mineral wool to fill gaps.

Walk Through the Detail

This is a busy part of the wall where the window, structure, waterproofing and airtightness all meet. Getting this junction right is essential to prevent water ingress, reduce thermal bridging and maintain airtightness. Let’s go through the components and clear up how it all works.

Starting from the outside:
The window cill sits proud of the brickwork below, with a drip groove on its underside to throw rainwater clear of the wall face. Directly beneath, the DPC runs under the cill and laps up the sides of the window frame. This is critical — it stops any water that makes its way through the cill from entering the wall.

At the junction between the frame and wall, where the packers are, a flexible sealant is applied externally. It seals the packing and visible gap between the cill and frame. The DPC and sealant perform complementary roles: one is the hidden waterproof layer, the other is the external weather seal.

The window is positioned central to the cavity to maintain the thermal line.

Inside the cavity, the insulated cavity closer runs along the jamb and around the window, in other words it sits both horizontally below the window, and vertically on either side of the window. It closes the cavity edge, providing thermal continuity and (in this example) 30-minute fire resistance. The cavity closer isn’t exactly the same as a cavity barrier, though both contribute to fire safety — the closer is a finished edge component, while a cavity barrier is purely for fire separation.

On the inside face, airtightness tape seals between the window frame and internal reveal. The blockwork leaf has a parge coat for airtightness, and the wall is finished with insulated plasterboard for thermal performance.

The galvanised window straps secure the frame to the structural blockwork, while expanding foam or mineral wool ensures the gap around the frame is fully filled without creating air paths and reducing thermal bridging.

Student Questions

1. Having a hard time picturing how all of this comes together.
   Think of the cill detail as a system with layers that each do a job: the flashing sheds rain, the sealant stops surface water, the DPC blocks any deeper moisture, and the cavity closer seals the open edge of the wall. Together, they form a continuous moisture and airtight envelope around the window base.
2. What is this (galvanised strap and foam)?
   The galvanised strap fixes the window securely to the structure, and the expanding foam or mineral wool around it fills gaps to improve thermal and acoustic performance.
3. Does the sealant go over the DPC or next to it?
   The DPC runs below and behind the visible joint, while the sealant finishes the external face, bridging the small gap between the frame and cill for weather protection.
4. Is the insulated cavity closer similar to the cavity barrier from earlier?
   They’re related but not the same. The cavity closer seals the end of the cavity at an opening and provides thermal and fire resistance. A cavity barrier stops fire and smoke spreading through longer cavity runs — it doesn’t form a finished edge.

Key Points for This Detail

• DPC must be continuous below the cill and lapped up the sides to prevent moisture ingress.
• Sealant goes over the DPC sealing it in place, providing a flexible surface joint.
• Flashing or cill drips direct water away from the wall face.
• Airtightness tape connects internal airtight layer to window frame.
• Insulated cavity closer provides thermal and fire performance at the reveal.
• All fixings should be stainless or galvanised to resist corrosion.
• Check compatibility between sealant, frame material, and DPC type.

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.