Sliding storm door or window assembly

In a window assembly including a four-sided frame having first and second adjacent lower channel members, a first window having a sash located within said first channel member, and a second window having a sash slidably located within said second channel member, each first and second sash enclosing a glazing panel, the improvement wherein each first and second sash is formed of a lightweight material and includes interconnected horizontal and vertical sash members, at least one of said vertical sash members including a channel part, and a rigid insert fitted within said C-shaped channel part substantially filling the space provided thereby, said insert constituting means for stiffening and strengthening each sash.