In last week's Explainer, we looked at spandrel panels, so this week it makes sense to look more at the building cladding systems where they are employed in.

Curtain Wall

In the context of architecture and design, the term curtain wall refers to a non-structural exterior cladding. In the early days of high-rise construction, the structural load of a building was supported by thick load-bearing walls made of masonry. This would change in the late 19th and early 20th century, as timber-frame and load-bearing masonry construction was supplanted by more efficient media like structural steel and concrete.

Curtain wall system with windows and spandrel panels being installed on a Toronto construction project, image by Jack Landau

No longer bound by the constraints of load bearing exterior walls, architects were able to apply lighter materials such as glass as an exterior finish, allowing for greater design flexibility as well as penetration of natural light. Typically found in office and hotel projects, curtain walls are affixed over the concrete slabs of a building, visually hanging off the skeleton while physically affixed to it with metal plates. In the image below, a section of curtain wall is hoisted into place at The One in Downtown Toronto, with two crew members waiting to receive it and then affix it to the metal plate clips that can be seen embedded into the concrete slab edge.

A curtian wall section hoisted into place at The One, image by UrbanToronto Forum contributor jer1961

While the earliest implementations of non-structural curtain walls date back to the mid 19th century, like 16 Cook Street in Liverpool, England, the technology continued to evolve and was most notably embraced by architects in Germany's interwar period. Examples of interwar period German curtain wall systems include the Kant-Garage in Berlin and the Bauhaus Dessau.

Kant-Garage, Berlin, image via Wikimedia Commons

In the decades since, curtain walls have evolved further, growing from steel mullion and plate glass assemblies into modern systems that utilize materials such as extruded aluminum for mullions that require very little upkeep. In modern day Toronto, curtain wall, which is more expensive than window wall, is typically used in commercial applications like office towers. With higher insulation value than window wall, larger expanses of open window can be used and still keep a building warm in the winter or cool in the summer.

Window Wall

Window walls, instead of being clipped onto and hanging off the edges of floor slabs, are installed between the slabs above and below, essentially sitting on the slab, with caulking used to seal the gap between them.

Window wall in a Toronto condominium, image by Marcus Mitanis

Since window wall glazing is not as physically strong as curtain wall glazing, thicker mullions are typically installed for structural support, standing proud of the glass, with maintenance on them required more often. Buildings that implement a window wall system tend to present a rougher exterior texture, as opposed to the generally smooth surfaces associated with curtain walls. 

Virtually all buildings employ a window wall system in this shot, image by Marcus Mitanis

Mullions (and transoms)

The aluminum elements that separate windows, spandrel panels and other elements are known as mullions. They can be used decoratively, but their primary purpose is to provide structural support. In the past, they were mostly used in buildings like churches to support brick or stone archways above the windows. Properly, mullions are vertical while their horizontal counterparts are known as transoms. Together they act as support for a building's glazing. They are also installed to accommodate window openings, usually for condominium buildings. Modern mullions are most often comprised of aluminum, but can be steel, wood, or fibreglass, whereas stone was the primary material pre-20th century. 

Aluminum mullions used to separate windows in a Toronto building, image by Marcus Mitanis

Have any other construction and development related terms that you would like to see featured on Explainer? Share your thoughts and questions in the comments section below!

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From 2015 to 2017, UrbanToronto and its sister publication, SkyriseCities, ran an occasional series of articles under the heading Explainer. Each one took a concept from Urban Planning, Architecture, Construction, or other topics that often wind up in our publications, and presented an in depth look at it. It's time to revisit (and update where necessary) those articles for readers who are unfamiliar with them. While you may already know what some of these terms mean, others may be new to you. We are publishing or updating and republishing Explainer on a weekly basis. This article is an update and combination of three that first appeared in 2015 and 2016.

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Do you have other planning terms that you would like to see featured on Explainer? Share your comments and questions in the comments section below!

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