How to Understand Windows & Window Terminology – Courtesy of Joyce I Thrush & Son® I Your Hometown Contractor

Window terminology can be confusing when you’ve decided to replace some old drafty windows and are beginning to do your comparison shopping.  You are quickly discovering that a replacement window has many parts, more than just glass and a frame, and the window terminology can get very confusing.  In this article, we will go over a few of the more common window industry terms.  

Window Parts

  • Sash:  The portion of a window that includes the glass and the framing sections directly attached to the glass, not to be confused with the complete frame into which the sash sections are fitted. 
  • Sill: The lowest horizontal member in a door, window, or sash frame. 
  • Frame: The fixed frame of a window which holds the sash or casement as well as hardware. 
  • Glazing: The glass or plastic panes in a window, door, or skylight. 
  • Header: The upper horizontal member of a window frame. Also called head. 
  • Bottom rail: The bottom horizontal member of a window sash. 
  • Lift: Handle for raising the lower sash in a double-hung window. Also called sash lift. 
  • Light: A window; a pane of glass within a window. Double-hung windows are designated by the number of lights in upper and lower sash, as in six-over-six. Also spelled informally lite.
  • Muntin: A secondary framing member (horizontal, vertical, or diagonal) to hold the window panes in the sash. This term is often confused with mullion. 
  • Mullion: A major structural vertical or horizontal member between window units or sliding glass doors. 

Energy Efficient Window Terminology

  • Argon: An inert, nontoxic gas used in insulating glass units to reduce heat transfer. 
  • Gas fill: A gas other than air, usually argon or krypton, placed between window or skylight glazing panes to reduce the U-factor by suppressing conduction and convection. 
  • R-value: A measure of the resistance of a glazing material or fenestration assembly to heat flow. It is the inverse of the U-factor (R = 1/U) and is expressed in units of hr-sq ft-°F/Btu. A high-R-value window has a greater resistance to heat flow and a higher insulating value than one with a low R-value. 
  • U-factor (U-value): A measure of the rate of non-solar heat loss or gain through a material or assembly. It is expressed in units of Btu/hr-sq ft-°F. The U-factor may be expressed for the glass alone or the entire window, which includes the effect of the frame and the spacer materials. The lower the U-factor, the greater a window’s resistance to heat flow and the better its insulating value. 

Educating yourself in window terminology and learning about all of your options is a great way to ensure that you are getting the best window for your budget and your home. To learn more window terminology, please visit our glossary at

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s