How To Detail Window Mullions and Transoms Part 1

Continuing on from our last episode about window detailing, today we are going to look specifically at mullion and transom placement. Mullions are the vertical divisions, while transoms are the horizontal divisions in the panes of glass and are used to reduce the overall size of the sealed glass units as well as to integrate openers into a window system.

Window mullion / transom placement has to work from both the interior and the exterior. From the inside it is critical to place the mullion / transom in a position that does not obstruct your view to the exterior when either standing or sitting. It is often most desirable to have the panes of glass be as large and as unobstructed as possible, so the mullions or transoms in the glass should be kept to a minimum. It is also very important to have any horizontal transom line follow a consistent datum line throughout the room or visual field, particularly as the windows turn corners. It is also important to note that operable windows have an additional thickness of framing around them, called the sash, and therefore opening windows work best if they are placed at the top, bottom or sides of any large window composition.

Today’s Slides:

  • Louis Pereira

    Great topic for us design geeks!  I employ very similar methods that you’ve discussed recently for placing mullions, transoms, establishing window/door heights etc.
    One topic I would appreciate to get your thoughts on are ‘corner’ window conditions.  Not necessarily the ‘glass-to-glass’ corners – due to colder climates it’s not recommended – but rather two windows separated only by a support column and how you typically finish the inside and outside corners.