TB/09 – Window Jamb Detail

This window jamb 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 upper junction, see the matching lintel drawing. Guidance on insulation continuity and thermal bridging is available in Approved Document L.

The detail shows:

• External brickwork and Celcon Solar aircrete block inner leaf
• Partial-fill cavity-wall insulation
• 50 mm clear residual cavity
• Proprietary fully insulated PVC-U cavity closer
• Continuity between the wall insulation and cavity closer
• Position of the window frame within the insulated reveal
• Minimum 30 mm frame overlap
• Insulation of residual gaps around the window frame
• Internal airtight and external weather-resistant perimeter seals
• DPC continuity between the head, jambs and cill
• SAP junction reference E4
• Calculated psi value of 0.045 W/m·K

The accompanying specification notes cover cavity-closer selection, thermal continuity, DPC returns, frame positioning, mechanical fixing, perimeter insulation, airtightness and weather sealing.

What This Window Jamb 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/09
• Detail title: Window Jamb Detail

Important Information

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

The window frame must be mechanically fixed to the surrounding masonry and should not be supported solely by the cavity closer unless the closer is specifically designed and approved for structural fixing.

The stated psi value applies only where the illustrated geometry, 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 or reproduced for commercial resale without written permission.