A mulled window unit is a window unit containing two or more single windows joined together. Mulling creates a larger window that often shares the head and sill to give it the look of a single unit. Multiple window sizes and shapes can be combined to create a unique look.
Mulled windows are reinforced with a piece known as mulling to provide the extra strength needed to stand up against wind conditions as a single unit. The material used to reinforce mulled windows varies according to the reinforcement needs, but aluminum is most commonly used.
There are two basic methods of mulling windows. The first uses a T-styled construction shape that contains a continuous head and window sill. The second construction method uses an H-shaped design, which lacks the shared head and sill. The T-styled design is more sound structurally and better prevents water and wind infiltration.
The mulling process can be done at the factory or in the field, with an experienced contractor combining the separate windows together. Factory mulling is generally considered to be the superior method, as the factory uses experienced assemblers, whereas field contractors may be inexperienced at performing the job. This inexperience can lead to badly placed reinforcement strips that can lead to wind or water infiltration through the joined areas. Badly set windows may also shake in place when doors are opened and closed.