Detail Post – Window Details

This timber cladding details post includes excerpts and details from our book Understanding Architectural Details – Residential.

The book is now in its fourth edition, and was updated to reflect the changes to the Building Regulations in June 2022. All of the construction details in the book comply with the latest u-value requirements.

 

Introduction to Window Details

 

Approved Document Part A states that the number, size and position of openings should not impair the stability of a wall, it provides detailed requirements limiting the size of openings and recesses. However, other regulations set out minimum dimensions of openings to ensure there is enough ventilation and daylight into a space. Approved Document Part F, for example, sets out minimum areas required for purge ventilation. Planning permission may also require adherence to the Building Research Establishment (BRE) Daylight and Sunlight. This guidance sets out the daylight and sunlight requirements for specific rooms.

When designing openings, it is good practice to use cavity closers or DPCs backed with insulation to avoid cold bridging at the reveal.

Within a typical masonry cavity construction, it is recommended to install window sets within the zone of the insulation layer to prevent cold bridging and reduce thermal conductivity from the window and frame. The window nevertheless needs to be installed back to the structure. This is usually done through galvanised mild steel installation straps fixed back to the blockwork. These are fixed at regular intervals to secure the window to the structure whilst reducing thermal bridging. The gaps between the window and frame can then be filled with PU expanding foam or mineral wool insulation to further reduce thermal bridging. All junctions can then be closed with silicone sealant or compressible seals.

 

Some projects may require a specific external or internal reveal depth and therefore the window frame can be fixed directly through the frame onto the structural inner or outer load bearing masonry. However, these details are at a higher risk of cold bridging and should use suitable external or internal wall insulation to minimise possible areas of condensation.

At the head of an opening the brickwork or blockwork requires support by a lintel. Steel section lintels are often used, they are a galvanised strip that also act as a damp proof tray, or can be dressed with damp proofing material. The lintel is filled with EPS insulation to reduce thermal bridging. However, given the steel lintel spans between the inner and outer leaf of the cavity wall, there is still heat flow through the material. A more effective lintel arrangement that can further limit cold bridging is two independent concrete lintels. However, this will create a visible concrete lintel externally and is usually only used in conjunction with a rendered facade.

 

At the base of a window, a cill is required to help evacuate water hitting the window. The function of the cill is to protect the wall below a window, so they are shaped to slope down and project beyond the external face of the wall in order to allow water to run off. It is good practice for the cill to project at least 45mm beyond the face of the wall below and have a drip on the underside of the projection. Cills are usually made from reconstituted concrete or preformed painted aluminium sections.

Cavity Trays

Some lintels have an integral cavity trays while others need a separate cavity tray which is usually formed using DPC materials on site. The cavity tray sits directly above the lintel, or sometimes a couple of courses above it. Any moisture trapped inside the cavity will be caught by the cavity tray and directed to the weep holes and out to the external face of the wall.

Window Details Examples

The following images give examples of window details in various construction types, from masonry to timber frame.

Masonry cavity wall window head detail

Masonry cavity wall window head detail

Masonry cavity wall window head detail

Masonry cavity wall window head detail

Masonry window sill detail

Masonry window sill detail

Masonry window sill detail

masonry cavity wall window jamb detail

Masonry cavity wall window jamb detail

masonry cavity wall window jamb detail

Masonry cavity wall window jamb detail

Timber frame window head detail

Timber frame window head detail

Timber frame window head detail

Timber frame window head detail

Timber frame wall window sill detail

Timber frame wall window sill detail

Timber frame wall window sill detail

Timber frame wall window sill detail

 

Timber frame window jamb detail

Timber frame window jamb detail

Timber frame window jamb detail

Timber frame window jamb detail

Understanding Architectural Details Residential is packed full of plenty of window details. In addition, you can download all of the CAD dwg files and all of the SketchUp files of the details featured in the book! What you waiting for? Get your copy now!

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1 Comment

  1. On the masonry window sill detail, what is the panel (timber board?) beneath the rigid insulation/ above the blockwork and cavity closer for?

    Reply

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