Details

The window reveal is a drawing of its own

14 September 2026

Most leaks, cold spots and cracked plaster start at the joint between frame and wall. It deserves a 1:5 detail.

General arrangement drawings show a window as a rectangle in a wall. On site, the rectangle turns into a frame, a gap, a membrane, insulation, plaster, a sill and a drip - and each layer is installed by someone different.

Decide where the frame sits

The frame should sit in the plane of the insulation, so the insulation line runs through it without a break. A frame set at the outer face of a wall insulated on the inside, or at the inner face of an externally insulated wall, leaves a cold bridge all the way round.

What the detail must show

1. Frame position and fixings
2. Air seal on the inside and a vapour-open, rain-tight seal on the outside
3. Insulation overlapping the frame by 30-40 mm
4. External sill with a fall and a drip at least 30 mm clear of the wall face
5. End caps on the sill so water cannot run behind the render
6. Internal reveal finish and how it meets the frame

Drawn at 1:5, these layers fit on one sheet and answer most of the questions the builder would otherwise solve on the day, usually with sealant.