What Is a Window Sash?

Window Sash

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A window sash is truly the sum of its parts. If all of the pieces of a window sash were disassembled, it would probably be unrecognizable. Older windows can be disassembled, but newer windows increasingly come with parts that are factory-built and sealed. PVC window frames are welded together. Grilles are sealed behind glass. And the separate sheets of glass are no longer separate; rather, they are built as one unit, called an insulated glass unit (IGU).

What Is a Window Sash?

A window sash is the part of the window that you move up and down when you want to open and close the window.


Two sheets of glass comprise a double-paned window. Sometimes this is called glazing, as in double-glazed window or triple glazed window. Single-paned windows are less common than in years past, with double-paned windows being more the norm.

Insulated Glass Unit (IGU)

The insulated glass unit is the heart of the modern window. IGU is a window industry term that refers to a double- or triple-paned collection of glass, gas, seals, and gaskets. The window frame and peripheral items are not included with an IGU.

Window Frame

Window frame is a convenient term for the wood, fiberglass, vinyl, or metal parts that hold the glass together. In older windows, the window frame is actually composed of rails (horizontal pieces) and stiles (vertical pieces). 

Window Grille

Within the window 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. The industry term for this is 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.


Glass must be sealed into the framework. Gaskets made of rubber or TPVs (thermoplastic vulcanizates) serve this purpose.


Argon and krypton are colorless, odorless gasses that are injected between the sheets of gas for its insulating properties. Single-pane windows have no gas. Seals allow for equalization of gas and outside pressure. IGUs come with the gas already inside.


Because of the difference in pressure between the IGU and the exterior, seals are built into the IGU to equalize pressure.

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. In this case, it may be referred to as a single-hung window. Newer double-hung windows, though, tend to have upper sashes that move up and down. However, modern single-hung windows are readily available.

One benefit of this action 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 most children to reach.

Fun Fact

Sliding sash windows became prevalent in England after the Great Fire of London, in 1666, when new building regulations stipulated that timber window frames had to be recessed behind stone or brick.

Casement Window Sashes

While mostly associated with double-hung windows, window sashes can be found on any type of window which moves, such as the casement window. Care must be taken with casement windows when they are open. A high wind can move casement window sashes and break them off.

Window Fogging and Leakage

Double-glazed IGUs often 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 between the glass panes, 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 then plugging up the holes. Window defogging is not a common practice, since the procedure often does not last for long.

Window Sash 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 entire window or individual sash replacement.