What to Consider When Specifying an Oriel Window

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Being used since the Middle Ages in Europe and the Middle East, Oriel windows continue to offer both aesthetic and practical benefits. The contemporary oriel window comes in various shapes, styles, and designs since the development of structural glazing. Consequently, with advancements in technology and miscellany in oriel windows and modern glazing, specifying an oriel window is all the more challenging for architects.

What are Oriel windows?

An oriel window is a widely used window that protrudes from the facade and overhangs from the face of the building but does not reach the ground floor, unlike bay windows. These are often used to create a bright and paradisal corner in the house, to read and repose.

The window is a structural glass assembly of frameless, high-specification glass elements secured with structural-grade silicone. To make things easier, architects can consider certain things when specifying an oriel window for the next project.

What to consider when designing and planning for an oriel window?

As per the specialists, there are three major steps to specifying an oriel window: designing the cantilevered box frame for the window seat, making a choice on glazing specifications, and finally installing and finishing the oriel window.

For a smooth design process, clear communication between the architect, glazing specialist and the engineer is required. The following things will define the design style and glazing specifications for an oriel window:

Building Type – New versus Old

Specifying an oriel window for new construction is not complicated, as opposed to an old building. Old buildings tend to have certain peculiarities that are unconventional and may require bespoke design solutions and specifications.

Aesthetics

Based on the appearance expectation of the client, architects can choose from a variety of glass specifications and finishing styles, well within the engineering limits, considering the structural nature of the oriel windows.

Opening Elements

To incorporate an openable element for either ventilation or another reason, a framed window will need to be installed within the structural glass design. To make this possible, steel supports will be required. However, this often compromises the aesthetic value of the oriel window.

Glazing specifications for Oriel windows

Glazing specification is a very significant part of specifying an oriel window. Safety, thermal performance, and aesthetic value are the three main factors when determining a glass specification.

Safety

Internally laminated glass should ideally be used for base and roof elements. Upon any impact with the glazed roof or base, in an unforeseen accident, the laminated glass sheet stays intact even after breaking into pieces. Therefore laminated glazing is essential to ensure the safety of inhabitants.

Thermal Performance and Insulation

Glass specification varies when considering the thermal performance, and is based on the location of the building and its orientation. For example: A South or west-facing window in London could take advantage of a solar coating to prevent overheating of the internal spaces. On the other hand, a window on the North facade does not need solar coating and may benefit from the sunlight and keep the interiors warm in winter. Choosing double glazing is a common practice in specifying an oriel window. The standard Ug value of double-glazed units is 1.0 W/m2K. Depending on the thermal insulation demands of the project, triple-glazing or single-glazing may also be used.

Aesthetic Value

Considering the load-bearing nature of oriel windows, the glass panels are usually thicker, which makes them suitable for seating. As a result, the thick glazing has a green tint and low visibility. Using low-iron glass can reduce this effect and increase transparency for a clear viewing experience.

Size of Structural Glazing for Oriel Windows

Structural glass assemblies have no upper or lower limits in size. However, in the UK, the largest piece of glass is six meters on the longer side and three meters on the shorter. Procuring glass larger than this size will result in additional costs of importing from overseas. Any piece larger than 2.4m in both dimensions or longer than 4m in either dimension, comes under ‘Oversized Glazing’. These sizes available in the UK, but yet difficult to source and transport, also add to the total cost, sometimes increasing it by 30%.

Installation and Finishing of Oriel Windows

Some of the other major concerns are the height of the window, depth and structure of the base, weight of the roof element, and wind load. Generally, the depth of the protrusion is no less than 1/10th of the window height. The vertical glazing should incorporate extra interlayers for strengthening, to achieve a frameless look. 

The base structure is designed by the builder or contractor, in either steel or concrete. This base structure requires appropriate finishing catering to the inhabitant’s desires and to achieve a cohesive design. Common examples are back-painted glass, or metal pressing, covering the base structure.