Window & Glass Glossary

A measure of the rate of heat transfer through a window. Lower values indicate better insulation.

U-factor measures how well a window prevents heat from escaping. Values typically range from 0.15 to 1.20, with lower numbers indicating better insulating performance. For Idaho's cold climate, look for U-factor of 0.30 or lower.

Source: NFRC (National Fenestration Rating Council)

Solar Heat Gain Coefficient - measures how much solar radiation passes through a window.

SHGC ranges from 0 to 1, with lower values meaning less solar heat enters. For Idaho's heating-dominant climate, a moderate SHGC (0.30-0.40) allows beneficial winter solar gain while limiting summer heat.

Source: NFRC (National Fenestration Rating Council)

The amount of visible light that passes through a window, expressed as a number between 0 and 1.

Higher VT values mean more natural daylight enters your home. Most residential windows have VT between 0.30 and 0.70. Higher VT reduces the need for artificial lighting during the day.

Source: NFRC (National Fenestration Rating Council)

A rating from 1 to 100 indicating how well a window resists condensation forming on the interior surface.

Higher CR ratings mean better resistance to condensation. Windows with CR of 50 or higher perform well in most conditions. This is especially important in Idaho where temperature differences between indoors and outdoors can be significant.

Source: NFRC (National Fenestration Rating Council)

A measure of thermal resistance - the reciprocal of U-factor (R = 1/U).

While U-factor is the standard for windows, R-value is commonly used for insulation. A window with U-factor 0.25 has an R-value of 4. Higher R-values mean better insulation.

Source: U.S. Department of Energy

The rate of air infiltration around a window, expressed in cubic feet per minute per square foot.

Lower AL values mean less air leakage. ENERGY STAR requires AL of 0.30 or less. Air leakage affects both energy efficiency and comfort. Quality installation is as important as the window's AL rating.

Source: NFRC (National Fenestration Rating Council)

National Fenestration Rating Council - the organization that sets standards for window, door, and skylight energy performance ratings.

NFRC provides independent, third-party certification of window energy performance. Their label appears on all certified windows and includes U-factor, SHGC, VT, and other ratings. ENERGY STAR uses NFRC ratings to determine qualification.

Source: NFRC (National Fenestration Rating Council)

A U.S. EPA program that certifies products meeting strict energy efficiency guidelines.

ENERGY STAR certified windows meet climate-specific performance requirements. For Idaho (Northern zone), windows must have U-factor ≤0.27 and any SHGC. ENERGY STAR windows may qualify for federal tax credits.

Source: U.S. Environmental Protection Agency

Low-emissivity glass with a microscopically thin coating that reflects heat while allowing light to pass through.

Low-E coatings can be applied to different surfaces of the glass to optimize for heating or cooling climates. For Idaho, soft-coat Low-E on surface 3 (interior side of outer pane) provides excellent winter performance.

Source: U.S. Department of Energy

A window unit consisting of two or more panes of glass separated by a spacer and sealed together.

IGUs create an insulating air space between glass panes. The space is often filled with argon or krypton gas for improved insulation. Double-pane IGUs are standard; triple-pane provides better performance in extreme climates.

Source: Glass Association of North America

An inert gas used between panes in insulated glass units to improve thermal performance.

Argon is denser than air and reduces heat transfer by conduction and convection. It's colorless, odorless, and non-toxic. Argon fill typically improves U-factor by 10-15% compared to air-filled units.

Source: U.S. Department of Energy

Safety glass that has been heat-treated to be 4-5 times stronger than regular glass and breaks into small, less dangerous pieces.

Tempered glass is required by building codes in hazardous locations: doors, sidelights, windows near floors, bathrooms, and stairways. It cannot be cut after tempering.

Source: CPSC (Consumer Product Safety Commission)

Safety glass made of two or more glass layers bonded with a plastic interlayer that holds fragments together if broken.

Laminated glass stays intact when broken, providing security and protection. It's commonly used for shower enclosures, skylights, and impact-resistant windows. Also provides UV protection and sound reduction.

Source: CPSC (Consumer Product Safety Commission)

A window that meets building code requirements for emergency exit, typically required in bedrooms.

Per IRC building codes, egress windows must have: minimum 5.7 sq ft opening, minimum 24" height, minimum 20" width, sill no more than 44" from floor. Idaho follows IRC with local amendments.

Source: International Code Council (IRC)

A spacer bar between glass panes made of materials less conductive than aluminum to reduce heat transfer at the edge.

Warm edge spacers reduce condensation and improve overall window energy performance. They're made from foam, silicone, or composite materials. Look for "warm edge" technology in premium window units.

Source: Window & Door Manufacturers Association

The frame that holds the glass in a window and may be fixed or operable.

In double-hung windows, both upper and lower sashes move. In single-hung, only the bottom sash moves. Casement windows have a single sash that swings open. Sash quality affects air infiltration and longevity.

Source: Window & Door Manufacturers Association

Window replacement that removes the entire existing window including the frame down to the rough opening.

Full-frame replacement is necessary when the existing frame is damaged or you want to change window size. It costs more but allows for insulation of the rough opening and fixing any hidden issues.

Source: FGIA Installation Standards

A replacement window that fits inside the existing frame, leaving the original frame in place.

Insert windows are faster and less expensive to install than full-frame replacements. They're ideal when the existing frame is in good condition. However, the new window opening will be slightly smaller.

Source: FGIA Installation Standards

The design and placement of windows and other openings in a building.

Fenestration includes windows, doors, skylights, and curtain walls. Proper fenestration design balances natural light, views, ventilation, and energy efficiency. The term comes from Latin "fenestra" meaning window.

Source: FGIA (Fenestration and Glazing Industry Alliance)

The glass or plastic panes in a window, or the act of installing them.

Single glazing uses one pane, double glazing uses two, and triple glazing uses three. Multiple glazing layers with gas fills and Low-E coatings dramatically improve energy efficiency.

Source: Glass Association of North America