Technical Study: Window Details

Windows are usually the most striking element on a building and can be completely transformed depending on how they are detailed and installed. However, with the increased consciousness on thermal insulation, the detailing of windows and their installation has become progressively more complicated. Below we will explore a typical masonry cavity wall, the different elements required to install a modern window and how these all come together on site.
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01 Hidden Gutters on Chimney House
How a window is designed and detailed has a huge impact on the overall project…

Key Elements Within A Window Installation

02 Parts of a window

Structure – The building’s structure may help in choosing a window style and material type. Based on these decisions, a different installation method may be required. For ease, we will be looking at a modern mixed aluminium and timber window installation within a masonry cavity wall. However, a timber or PVC window could also use this similar installation method.

 

Straps – Metal straps are used in modern window installation to create a thermal break between the window frame and structure. The gaps between the structure and window can then be filled with insulation to further reduce the cold bridge between the two elements.

External Sill – At the base of a window, a sill is required to help evacuate water hitting the window. It is good practice for the sill to project at least 45mm beyond the face of the wall below. This combines with a good drip on the underside to prevent water from dripping onto the wall. Sills are usually made from reconstituted concrete or preformed painted aluminium sections.

 

Head – 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. Lintels filled with EPS insulation or thermally broken lintels further reduce thermal bridging. Concrete lintels can also be used for both the inner leaf and outer leaf of masonry. Although these will be visible externally if render is not used.

 

Cavity closers – When designing openings, it is good practice to use cavity closers or DPCs backed with insulation toreduce cold bridging at the reveal. This is also a requirement in cavity wall constructions to prevent the spread of fire although it may not be a requirement if the cavity is fully filled with a Class A1 insulation as this will do the same job as a cavity closer. 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.

Window Construction Sequence

Below is a series of images illustrating key moments when installing a window and how different elements come together.  For ease, we have used a standard masonry cavity wall.

03 Stages of window detailing
Stage 1:

Structure – The location and size of the structural zone is key in designing windows. The thickness and type of structure should work with the opening size required and construction type. Some structures may bridge into the façade such as the thermally broken steel lintel shown.

 

Stage 2:

Insulation and external face – Once a structure has been chosen, this is insulated and faced with a rain resistant material. Extra insulation can be added internally to help reduce cold bridging and increase thermal performance.

 

Stage 3:

Choosing placing of window for desired appearance, solar gain, solar shade and thermal bridging – Depending on the desired position of the window within the wall build-up, different straps or thermal breaks may be required. Further information on the benefits and disadvantages of each placing can be found in the following section.

 

Stage 4:

Install aluminium straps and packers, these create a thermal break between the windows and the other materials – Aluminium straps are a great way to further reduce thermal bridging whilst fixing the windows securely to the structure.

 

Stage 5:

Window and cill installation – The window is then installed along with the aluminium external cill. If choosing a pre-cast concrete cill, this may need to be installed prior to the window fitting and will require a DPC due to possible water soaking into the concrete.

 

Stage 6:

Fill gaps with insulation – All gaps are then filled with either PU expanding foam or foam/mineral wool insulation.

 

Stage 7:

Air tightness tapes – Airtightness tape can now be installed to ensure a tight seal between the structure and window, reducing the internal air leakage.

 

Stage 8:

Internal finishes and seals – Internal finishes such as plasterboard and cills can now be installed. The window is then sealed with silicone internally and externally to the finishes.

 

Window-Sequence
Window-Sequence-internal

Window positioning within a wall

Image above: House Pepingen by Lens°ass Architect positions windows flush with the external brickwork to frame the external landscape.

Some projects may require a specific external or internal reveal depth depending on the internal use or external shading required. Below is a list of typical window positioning’s and a short description of their advantages and disadvantages.

Centred

Centred window detail
Centred window jamb detail

This is the most common modern positioning of a window as it usually reduces possible thermal bridging. It also creates a reveal both internal and externally for an internal sill and some shading externally.

Deep External Reveal / Full Brick Reveal

Deep reveal window detail
Deep reveal window detail
A deep external reveal is great for sun facing windows as it helps to reduce solar gain. This positioning also helps to make the window opening stand out as there will be stronger shadows on the façade. This detail requires the window to be installed directly on the structure which can increase cold bridging, however this can be reduced by using structural thermal break insulation between the structure and window frame. If using a deep reveal, some designers choose to return the brick so that the reveal exposes a whole brick. A full brick reveal can only be achieved when the cavity is wider than the brick return.

Flush with External Face

Flush window detail
Flush window detail

On facades of a building where solar gain is not an issue, such as the north face, a distinctive window position is to install it flush with the external brick face. This creates a very large internal sill, possibly for a reading or desk area. Installing a window with aluminium straps allows for the window to project off the structure and align flush with the external brick.

 

Image above: Buchholzer Grün Housing by Busch & Takasaki Architekten has a full brick reveal, given the window a deep reveal.

Building Regulations Requirements for Windows

 

There are a range of building regulations to refer to when designing windows. Below is a summary of some of the more import regulations to be familiar with when designing windows. 

Structure: Building Regulations 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. 

Fire Safety: Building Regulations Part B – This regulation describes how windows should be designed if used for fire escape purposes and as an Automatic Opening Vent (AOV) to help smoke escape the building to allow escape. 

Ventilation: Building Regulations Part F –  Part F explains how to provide adequate means of ventilation. In terms of windows, this may include trickle vents within the window for background ventilation and how large the window should be to provide enough purge ventilation. 

Protection From Falling – Building Regulations Part K – This regulation along with BS EN 12600 and BS 6206: 1981 advises on the critical areas of glazing which when broken could cause someone to fall. It also advises on the glass specification and guarding. 

Access To Buildings – Building Regulations Part M – Part M explains where to place window handles so they are accessible to all users. 

Overheating – Building Regulations Part O – This regulation requires the limit of unwanted solar gains in summer. This can be done by designing smaller windows on the southern façade or designing southern facing windows with external solar shading. 

Security – Building Regulations Part Q – Part Q requires reasonable provision to resist unauthorised access to a building. Windows can be an area of a facades, especially on the ground floor which is easily accessible. Window sets and glazing should therefore be designed to comply with regulation and British Standard PAS 24:2012.
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 and may require windows to be made larger to meet the regulations.

Author

Written by Aida Rodriguez-Vega, architect and researcher. At the Detail Library, Aida keeps busy by carrying out technical research and drawing new details for the ever-growing library.

For More Inspiration…

For More Inspiration…

Resources

If you want more inspiration on windows such as different, types, shapes, sizes, window sills and surrounds, how to position them in the façade and window frame systems, be sure to follow Detail Library on Pinterest where we have a huge collection of window ideas and precedents.      

Our Details

For more information on how to detail windows check out the fully resolved details via the link below.      

Some of our most recent examples include:

DL149 – Window Section Detail with Centred Frame, masonry wall

DL153 – Window Flush with External Wall Section Detail

DL158 – Window Plan Detail with Full Brick Reveal Wide Cavity

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