The mullion-transom construction is a facade system made of load-bearing profiles. Large-area openings but also entire facade and roof surfaces can be produced by it. In addition, filigree glass facades with narrow elevation widths and all-glass corners can be realized.
The mullion-transom facade is one of the curtain walls – it carries no static loads except its own weight. The construction of the mullion-transom facade is based on the connection of storey-high vertical mullions and horizontal transoms. The floor-to-ceiling vertical mullions are attached to the floor slab by means of connecting shoes and take over the load transfer. The horizontal ledgers are connected to the mullions by bolting, plugging or welding. This construction forms the functional layer. By using different materials such as steel, wood or aluminum and variable profile dimensions, the mullion-transom design can be adapted to almost any installation situation.
The infill elements, also called infill, are attached to the mullions and transoms with pressure bars. They merely bear their own weight and form the weather protection layer. Various infill elements can be selected for the mullion-transom façade – transparent, translucent or opaque glazing, fixed or openable panel elements, window and door openings, and infill elements with special functions such as solar energy harvesting. The infill panels not only close the load-bearing framework, but also provide ventilation and light. Functionally and also structurally, the infill elements are clearly separated from the falsework and do not take on any load-bearing task.
The mullion-transom construction consists of at least two sealing levels. The room-side seals of the filling elements form the primary sealing level and ensure that condensation that forms on the inside is drained away. Elastic seals are installed between the supporting framework and the filling elements – these prevent the water from entering the rebate spaces of the profiles. The rebate spaces are additionally provided with thermal insulation. By means of mechanically fixed and glued foils or frames, the connections to the carcass are made waterproof, airtight and thermally insulated all around.
The mullion-transom façade is either delivered to the construction site pre-assembled or only assembled on site at the construction site. The components include the mullions and transoms, which form the supporting framework, and the filling elements, but also drives and sensors, as well as sunshade structures. There are several ways to integrate a solar shading system into the mullion-transom façade: integrated into the glass, on the inside, or on the outside.
For tall buildings, it is advisable to dispense with external solar shading due to wind loads, and to use internal or glass-integrated solar shading.
Advantages
Due to the slim profiles and thus view widths architects like to resort to the mullion-transom façade. The minimum profile width of mullions today is 35 mm. Thus, filigree glass facades can be realized. Since not only the mullions and transoms made of different materials are possible, but also the infill panels, a large number of combinations are available and a great freedom of design is given. In addition, highly thermally insulated systems can be realized – the heat transfer coefficient of the frame (Uf value) can be 0.50 W/m²K. Furthermore, the mullion-transom construction is suitable not only for facades but also for skylights.
Disadvantages
The appearance of the otherwise filigree post-and-beam construction is impaired by any openings – because each opening requires an additional frame.
Since the mullions are load-bearing elements, but should also have a filigree view, the mullions must necessarily go down. This poses a problem when a shallow facade depth is desired. Due to the high proportion of glazing and the possibility to realize glass surfaces over several floors, it is more difficult to clean it with a conventional perforated facade. In addition, the mullion-transom façade costs about 30% more than a conventional perforated facade.
Element facade
Unlike the mullion-transom façade, the element façade consists of completely prefabricated façade elements that are merely brought into position and fixed on site. The degree of prefabrication aims to reduce the costly assembly and set-up times on site and make them more calculable. It is a significant advantage that manufacturing is moved to an early stage and assembly can take place regardless of the weather.
Like the mullion-transom façade, the element façade is designed to be storey-high and consists of a load-bearing frame in which glazing and panels are inserted.
Double facade
The double facade consists of two facade levels. The outer level (secondary facade) has the function of absorbing environmental impacts that occur. The inner level (primary facade) forms the room closure and also has the function of thermal insulation. An intermediate space is created between the outer and inner levels, which can vary in size. This intermediate space serves as a buffer zone, as solar radiation heats the air there. Double facades are particularly useful for buildings exposed to wind and noise. Post-and-beam facades can also be designed as double facades.
Pre-hung ventilated facade
In the pre-ventilated rear façade (VHF), the façade cladding, also called outer skin or weather protection, is not applied directly to the masonry but mounted on a substructure. An air layer is created between the insulated masonry and the facade cladding. This air layer ensures constant rear ventilation of the outer skin and thus permanent removal of existing moisture, as well as a barrier that prevents capillary transport of moisture into the masonry.
Structural glazing facade
The structural glazing façade is a glass façade in which the glass elements are held in the supporting framework solely by adhesives. Any wind loads that occur are absorbed by the special bonds. There is no need for pressure bars in the structural glazing façade. It can also be implemented as a mullion-transom façade.
Mullion and transom facade is mainly installed in:
But of course there is also the possibility of installing the mullion and transom facade on other buildings, such as single-family homes, which have rooms over several stories.
Simplify the planning of a mullion-transom façade with the free BIM plugin. The plugin for Archicad and Revit allows you to transfer the listed products directly into your planning environment. Take advantage of selecting the matching mullion-transom facades from different manufacturers.
Optics and design
The mullion-transom design can be used to create vertical facades, sloped facades and polygon facades, as well as panoramic facades with all-glass corners. As the most common designs for profiles, you can find the wood, steel, or cover shell optics.
Construction and arrangement of mullions and transoms
The typical profile face widths are between 35 and 60 mm.
There are several options when arranging mullions and transoms – the transoms can be in the same plane as the mullions, resulting in a flush appearance. The other option is for the transoms to be set forward or back from the mullions, creating a louvered façade.
Materials
The supporting framework of mullions and transoms and the pressure bars are made of wood, steel or aluminum. A combination of materials is also possible. Different materials can be selected as infill panels – from glass to sandwich elements to steel and aluminum. Glass facades can be realized with profile construction glass, cast glass or glass blocks – all types of glass are available as single, double or triple glazing.
Requirements
Special requirements for statics as well as fire, sound and heat protection are taken into account by different manufacturers. For example, there are solutions for increased static loads, suitable systems for passive houses or solutions with integrated photovoltaic system.
Manufacturers
Among the well-known and proven manufacturers of mullion-transom facades in Germany are Schüco, Raico, Hueck and Wicona.
You can find these and other manufacturers directly on the Plan.One comparison platform. Within the product category Facade elements you will find mullion-transom facades under the filter Facade type. For this purpose, discover different manufacturers and their products. Additionally, choose between materials, such as alumnium, aluminuium wood and steel.
Relevant information on the subject of mullion-transom facades can also be found on YouTube. For example, the manufacturer Schüco presents a detailed recording on the installation of an aluminum mullion-transom facade. But also the manufacturer Batimet convinces with an informative video on the wood-aluminum mullion-transom façade.
Are you an architect or building designer? Then find out how the Plan.One BIM plugin makes your work easier and how planning a mullion-transom façade can be easily implemented in your planning environment.
Janina Pörtner
Author and architect with a focus on architectural communication and mediation
