Hollow metal windows, or borrowed lites, are available in many sizes and configurations. The most common hollow metal window frames are four-sided frames and have one or more sections of glass installed. When multiple sections of glass are specified they are separated by “mullions” when built.

Mullions and Frames

Hollow metal windows can be fire rated. The fire rating of the window unit often depends more on the glass to be used than the actual hollow metal frame material. This means that the fire rating limit will depend mostly on the approval achieved by the type of glass used. Many larger windows with fire ratings may require more mullions than non-rated or lower-rated units.


A mullion is essentially a closed section of hollow metal that divides up two pieces of glass.

A mullion is usually built with the same jamb depth dimension as the outside sections that anchor to the wall. The mullion profile also matches the profile of the outer sections as well. However, custom options are available and can be custom fabricated. One typical scenario where mullion profiles may be different than the outer jambs is when blinds are installed in the window and the blinds need to run the full height of the window frame.

Fire Rated Window Frame

Window frames can have multiple mullions that run vertical and horizontally within the frame. But caution is advised: just because something can be drawn does not mean it can be fabricated.

Talk with a sales consultantto get a free quote on your project today!

(images used from Steelcraft Technical Data Manual)