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Windows 101

With so many choices in materials, styles, operation, function and energy efficiency, window shopping has become increasingly more difficult. Even if you go by the numbers, those can get confusing too. This quick replacement window tutorial will help make sense out of the sea of window parts, window technical specifications and window ratings.

Anatomy of a Window

´╗┐There are over 100 components in every window, but here are the top 10 most common window parts referenced when window shopping:

Head: the horizontal frame forming the top of the window.

2Upper Sash: the assembly of stiles and rails made into a frame to hold the glass. (The upper sash will slide up and down in a double-hung window, but will not move in a single-hung window.)

3Jamb: the main vertical frame forming the sides of the window.

4Brick Mould (exterior): Outside casing around window to cover jambs and through which nails are driven to install the window.

Casing (interior): Inside casing is a flat, decorative moulding that covers the inside edge of the jambs and the rough openings between the window unit and the wall.

5Muntin Bar (Grid or Grille) - any small bar that divides the window glass (panes). Muntins can either be true divided light or simulated divided light.

True divided light: A term that refers to windows in which multiple individual panes of glass or lights are assembled in the sash using muntins.

Simulated divided light: A method of constructing windows in which muntins are affixed to the inside and outside of a panel of insulating glass to simulate the look of true divided light.

6Rail - the horizontal part of the sash

7Lower Sash - the assembly of stiles and rails made into a frame to hold the glass.

8Stile - the vertical part of the sash

9Glazing - The glass panes or lights in the sash of a window. Also the act of installing lights of glass in a window sash.

10Sill - the main horizontal frame forming the bottom of the window (on the outside of the window).


Window Math | Window Test Factors

The National Fenestration Rating Council (NFRC) is the organization that tests and rates all windows in the US. The NFRC energy performance label can help you determine how well a product will perform the functions of helping to cool your home in the summer, warm your building in the winter, keep out wind, and resist condensation. By using the information contained on the label, consumers can reliably compare one product with another, and make informed decisions about the windows they buy.

NFRC logoNFRC performance labels list the manufacturer, describe the product, provide a source for additional information, and include ratings for one or more energy performance characteristics.


U-factor measures how well a product prevents heat from escaping. The rate of heat loss is indicated in terms of the U-factor (U-value) of a window assembly. U-Factor ratings generally fall between 0.20 and 1.20. The lower the U-value, the greater a window's resistance to heat flow and the better its insulating value.

Solar Heat Gain Coefficient

Solar Heat Gain Coefficient (SHGC) measures how well a product blocks heat caused by sunlight. The SHGC is the fraction of incident solar radiation admitted through a window (both directly transmitted and absorbed) and subsequently released inward. SHGC is expressed as a number between 0 and 1. The lower a window's solar heat gain coefficient, the less solar heat it transmits in the house.

Visible Transmittance

Visible Transmittance (VT) measures how much light comes through a product. The visible transmittance is an optical property that indicates the amount of visible light transmitted. VT is expressed as a number between 0 and 1. The higher the VT, the more light is transmitted.

Air Leakage*

Air Leakage (AL) is indicated by an air leakage rating expressed as the equivalent cubic feet of air passing through a square foot of window area (cfm/sq ft). Heat loss and gain occur by infiltration through cracks in the window assembly. The lower the AL, the less air will pass through cracks in the window assembly.

Condensation Resistance*

Condensation Resistance (CR) measures the ability of a product to resist the formation of condensation on the interior surface of that product. The higher the CR rating, the better that product is at resisting condensation formation. While this rating cannot predict condensation, it can provide a credible method of comparing the potential of various products for condensation formation. CR is expressed as a number between 0 and 100.

* This rating is optional and manufacturers can choose not to include it.


Energy Star Efficient Windows Chart


Other Window Criterion


R-Factor is a measure of the resistnace of the insulated glass unit to heat flow. A high R-factor window has a greater resistance to heat flow and thus, a higher insulating value than a window with a low R-Factor.

Ultra Violet Light

The invisible rays of the light spectrum at its short-wavelength violet end. U.V. Ras are found in everyday sunlight and can cause fading and damage to paint, carpet, furniture and fabrics.

Visible Light

This is the fraction of the visible specturm of light weighted by the sinsitivity of your eyes that is transmitted through the window. Simply put, it represents the amount of daylight that the window lets in.

Low-E Stacked

Low-E Stacked Glass keeps you warmer in the winter and cooler in the summer. The overall U-Factor (insulating value) and R-Factor (resistance to heat and cold transfer) of windows with Low-E stacked glass is considerably better than stand clear insulating glass and also better than standard Low-E products.

Argon Gas Filled

Argon is a colorless oderless gas that is 6 times heavier than air. Argon insulates better than just air and also reduces the amount of convection that goes on between the panes of glass.