TB/08 – Window Cill Detail

This window cill detail illustrates a typical junction within an insulated masonry cavity wall.

The drawing is intended for architectural designers, building professionals, contractors, developers and self-builders preparing coordinated Building Regulations information or a project specification.

For the adjoining side junction, see the related reveal drawing. Guidance on moisture resistance in walls is available in Approved Document C.

The detail shows:

• External brickwork and Celcon Solar aircrete block inner leaf
• Partial-fill cavity wall insulation
• 50 mm clear cavity
• Proprietary fully insulated cavity closer beneath the window
• Continuity between the cill closer, jamb closers and wall insulation
• Flexible DPC beneath the window cill
• DPC turned up at the rear and ends to form a cavity tray
• Minimum window-frame overlap
• Projecting precast concrete, stone or proprietary cill
• Outward fall and underside throating groove
• Internal airtight and external weather-resistant seals
• SAP junction reference E3
• Calculated psi value of 0.042 W/m·K

The accompanying specification notes cover cavity-closer installation, insulation continuity, DPC positioning, stop ends, cill projection, water discharge, frame fixing and sealing around the completed window.

What This Window Cill Detail Includes

• One downloadable PDF construction drawing
• A3 landscape sheet
• Drawn at 1:10 scale when printed at 100% on A3 paper
• Drawing reference: TB/08
• Detail title: Window Cill Detail

Important Information

This is a standard construction detail intended to provide general design and specification guidance. It must be reviewed and adapted to suit the selected window system, wall construction, cavity width, insulation, cill material and exposure conditions.

The window, cavity closer, DPC and cill must be installed in accordance with the relevant manufacturers’ instructions. Cill dimensions, projection, end details, fixings and weather sealing must be coordinated with the window and external wall specification.

The stated psi value applies only where the illustrated geometry, dimensions, materials and insulation arrangement are followed. It should be checked for use within the project’s SAP or thermal-bridging assessment.

The drawing does not include a project-specific window specification, structural calculations, SAP assessment or Building Regulations approval. Purchasing or using the detail does not guarantee approval by Building Control.

The PDF is supplied for use by the purchaser on their own projects. It may not be resold, redistributed, shared as a standalone product or reproduced for commercial resale without written permission.