Large Window Wall Basics

Mulling

In the trade, the practice of making “big window walls” is known as mulling, and the structural members used to join them together are called mullions. Mulling windows means joining two or more framed window units together directly and installing them as a single unit in a large window opening. This greatly improves the potential and practicality for achieving a higher glass-to-wall ratio, compared to installing a series of individually framed windows or a single, very big window. But creating a huge rough opening for a wall of windows is an engineering and design challenge. After all, you’re replacing solid wall framing members with an assembly of glass and wood frames.

Challenges

Many of the challenges surrounding mulled windows are addressed in the factory. There, the individual units are fastened together securely into finished assemblies for more modest big windows, or into two or more subassemblies that are designed to be joined together in the field for larger installations. The maximum size of a subassembly is determined in part by shipping restrictions that make it difficult to transport a unit larger than 8-feet-by-8-feet. Building codes also are critical in the big window wall world, as the beefy headers needed to distribute the load over an enhanced window opening places hard limits on how big your window wall can be. In recent years the challenge of mulling windows, even in residences, has become even more complicated by the fact that window assemblies are growing larger and larger. This makes it more difficult to address key engineering questions, such as: How does the assembly handle wind loads and air and water resistance? How does it perform in a coastal environment? Or does the window wall meet American Architectural Manufacturers Association (AAMA) standards?

Suitable Window Types

Just about any style of window (or windows) can be mulled together and installed. The most commonly employed type is the casement window. Often, casements are combined with other types. One very popular configuration is to mull a crank-out casement window on each flank of a picture window. Double-hung windows also are suitable for mulling and, like casement windows, can be installed in rows or stacked, or combined with other window types. Awning windows, which are hinged at the top and crank open, are successful mulling choices, especially for installation in a transom application above another window type. Another popular window type for mulled installations is the half-round, which is normally mulled above another window of the same width. Trapezoid windows are used occasionally, but less frequently, in mulled assemblies, as are gliders.

Design

Designing a big window wall involves technical engineering as well as a sense of aesthetics and proportion. Each mulled assembly is unique in some regard, and every installation site includes its own set of challenges. The individual window units you select matter visually, but they also affect the window wall’s rated ability to withstand stresses. It is strongly recommended that a professional designer or building engineer with fenestration expertise be employed to help, since it can get complicated, and the stakes of a failure are high.