Technical Study: Window Details
Key Elements Within A Window Installation
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.
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.
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 positioning within a wall
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
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
Flush with External Face
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, BA(Hons) MArch DipArch ARB, 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…
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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Other Resources
There are plenty of further resources to help when designing windows.
- https://www.gov.uk/government/collections/approved-documents
- https://www.designingbuildings.co.uk/wiki/Window
- https://architizer.com/blog/product-guides/product-guide/eantka-windows/
- https://www.firstinarchitecture.co.uk/detail-post-window-details/
- https://detail-library.co.uk/what-is-a-window-schedule/
- https://www.trada.co.uk/start-here/windows-and-doors/
- https://www.insulationsuperstore.co.uk/help-and-advice/product-guides/insulation-projects/what-are-cavity-closers-and-how-do-you-install-them/
You can also contact manufacturers directly for further information. Below is a list of suggested suppliers:
Image Credits
Image 1
Image URL: https://divisare-res.cloudinary.com/images/c_limit,f_auto,h_2000,q_auto,w_3000/v1568033431/chxepf0hs4mptavuf2pu/adn-architectures-severin-malaud-single-family-house.jpg
Architect credit: ADN Architectures
Photographer credit: Séverin Malaud
Image 2
Source URL: https://www.archdaily.com/872525/house-pepingen-lens-degrees-ass
Architect credit: Lens°ass
Photographer credit: Bieke Claessens
Image 3
Source URL: https://www.archdaily.com/961134/buchholzer-grun-housing-busch-and-takasaki-architekten
Architect credit: Busch & Takasaki Architekten
Photographer credit: Caspar Sessler
