Key takeaways
- SHGC and U-factor are the two numbers that matter most for comfort. Lower is better for both.
- Retrofit keeps the old frame and the stucco; new construction replaces everything.
- Tempered and laminated glass are both safety glass, but they work differently.
Glass and energy
- Low-E glass
- Glass with microscopically thin metal coatings that reflect heat while letting visible light through. More coating layers usually means more heat control. See best windows for hot summers.
- SHGC (Solar Heat Gain Coefficient)
- How much of the sun’s heat passes through a window, on a scale from 0 to 1. Lower numbers block more heat, which matters most on west and south walls.
- U-factor
- How easily heat flows through the whole window. Lower is better. California’s Title 24 sets maximum values for replacement windows.
- VT (Visible Transmittance)
- How much daylight a window lets in, from 0 to 1. Higher numbers mean brighter rooms.
- NFRC label
- The temporary sticker on a new window showing its tested U-factor, SHGC and VT, from the National Fenestration Rating Council. Inspectors often check it.
- ENERGY STAR
- A federal program that certifies windows meeting efficiency criteria for each climate region.
- Dual-pane (double-pane)
- Two layers of glass with a sealed space between them that insulates far better than a single pane.
- Triple-pane
- Three layers of glass and two sealed spaces, for more insulation and noise reduction. Heavier and more costly than dual-pane.
- Argon and krypton gas
- Inert gases sealed between panes that insulate better than air.
- Warm-edge spacer
- The strip that separates the panes at the edge, made from materials that conduct less heat than old aluminum spacers, reducing condensation at the edges.
- Seal failure
- When the seal around an insulated glass unit breaks, letting moisture in. It shows as fog between the panes. See signs you need new windows.
Window parts
- Frame
- The fixed outer part of the window that attaches to the wall.
- Sash
- The part that holds the glass and, on operating windows, moves.
- Sill
- The bottom of the frame, sloped to shed water.
- Jamb
- The sides of the frame.
- Head
- The top of the frame.
- Meeting rail
- Where the two sashes of a hung or sliding window meet, usually holding the lock.
- Grilles, grids or muntins
- Bars that divide the glass into smaller panes, either between the glass, on the surface, or as simulated divided lites.
- Simulated divided lites (SDL)
- Grilles bonded to the inside and outside of the glass, often with a spacer between the panes, so they look like true separate panes.
- Mullion
- A vertical or horizontal bar joining two windows into one unit.
- Balance
- The spring or mechanism that holds a hung window’s sash in place when opened.
- Weatherstripping
- Flexible seals that close the gaps between sash and frame.
Installation
- Retrofit (insert) installation
- Fitting a new window inside the existing frame without disturbing the exterior wall. See retrofit vs new construction.
- New construction (full-frame) installation
- Removing the entire old window down to the studs, then repairing, flashing and installing a new window.
- Nailing fin
- A flange around a new construction window that is nailed to the wall framing and covered by flashing and siding or stucco.
- Flashing
- Waterproof material that directs water away from the opening and out of the wall.
- Rough opening
- The framed hole in the wall that the window fits into.
- Title 24
- California’s building energy code, which replacement windows must meet. See window permits in Contra Costa.
Styles
- Casement
- A window hinged on the side that cranks open. See casement windows.
- Awning
- A window hinged at the top that opens outward from the bottom. See awning windows.
- Double-hung
- Both sashes slide up and down. See double-hung windows.
- Single-hung
- Only the bottom sash moves. See single-hung windows.
- Slider
- Sashes glide sideways on a track. See sliding windows.
- Picture window
- A large fixed window that doesn’t open. See picture windows.
- Bay and bow
- Windows that project out from the wall. See bay and bow windows.
Safety and codes
- Tempered glass
- Glass heat-treated to be much stronger and to break into small, blunt pieces. Required near doors, in some wet areas and often in fire hazard zones.
- Laminated glass
- Two layers of glass bonded with a clear interlayer that holds together when broken. Adds security and reduces noise.
- Fire Hazard Severity Zone
- Areas mapped by CAL FIRE by wildfire risk. High and very high zones can require specific glass on windows.
- Egress window
- A bedroom window large enough, and low enough, to climb out of in an emergency, as required by code.
- STC and OITC
- Sound ratings. Higher numbers mean the window blocks more noise. OITC focuses on lower-pitched traffic noise.
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