It's Called a Window Sash (But What Does It Do?)

Window Sash
Window Sash. Getty / Dorling Kindersley

A window sash is the unified framed part of the window which holds the sheets of glass in place, including the glass and relevant components such as grillework (mullions).  

5 Components Make One Sash

The saying "It's the sum of its parts" truly applies to window sashes.  Several pieces comprise a sash.  If the pieces were disassembled, no sash would exist.  A sash for a modern window might include as many components as these:

  1. Glass:  Two sheets of glass for a double-paned window, one sheet for single panes.
  2. Frame:  Frame is a convenient term for the wood, fiberglass, vinyl, or metal parts that hold the glass together.  It is actually composed of rails (horizontal pieces) and stiles (vertical pieces). 
  3. Grille:  Within that frame may be smaller sections of window that go under different terms but collectively called grillework.  Older, single-pane windows may have true mullions, in which panes are separated by wood.  Newer double-pane windows may have a "mullion look," called GBG or grille-between-glass, in which false mullions are placed between the sheets of glass and serve no structural purpose.  GBGs provide aesthetic appeal and allow for easier cleaning of the glass.
  4. Gaskets:  Glass must be sealed into the framework.  Gaskets made of rubber or TPVs (thermoplastic vulcanizates) serve this purpose.
  5. Gas:  Usually argon but sometimes krypton, this colorless, odorless gas is injected between the sheets of gas for its insulating properties.  Single-pane windows have no gas.

    Double-Hung Windows

    Window sashes are typically found in the double-hung window style, in which one sash is positioned above a lower sash. The lower window sash has the capacity to slide up and down until it is nearly parallel with the upper sash.

    It is not uncommon in older windows for the upper sash to be fixed in place (while the lower sash remains operable).  Newer double-hung windows, though, tend to have upper sashes that move up and down.  One benefit of this is that it allows for the window to be open, without the hazard of people--namely small children--falling out of the window.  An open upper sash is too high for children to reach.

    Casements

    While mostly associated with double-hung windows, sashes can be found on any type of window which moves (such as the casement window).  

    Sash Fogging and Leakage

    Double-glazed IGU (insulated glass unit) windows come with one problem:  fogging.  Fogging is the result of improperly installed gaskets or gaskets that have deteriorated over the years.  Impaired seals allow gas to escape, greatly impacting the window's insulating properties.  As condensation builds, light and views are obscured.  

    Window defogging companies can fix this problem by drilling 2mm holes in the glass, injecting cleaning fluids, letting them dry, and them plugging up the holes.

    Replacement Kits

    If the window sash breaks, it is possible to purchase a sash replacement kit. This avoids the cost and mess of total window replacement. Also, because it is designed for homeowners to be able to use, it helps avoid labor costs associated with window or sash replacement.