No window is truly soundproof, but the right glass and a tight installation can cut road and aircraft noise substantially. What works is laminated glass, two panes of different thickness, a wider gap between panes, and a window that seals tightly when closed. Compare windows by their STC rating, and for low, rumbling noise like freeways and jets, by their OITC rating. Extra panes and gas fills help far less than most people expect.
STC and OITC: the two ratings to ask for
- STC (Sound Transmission Class) is a single number for how well a material blocks sound, measured across 125 to 4,000 Hz. Higher is better. It was developed for interior walls and handles voices and mid-range sound well.
- OITC (Outdoor-Indoor Transmission Class) covers 80 to 4,000 Hz, a lower starting point that captures the rumble of traffic, trucks and aircraft. Higher is better. For freeway and flight-path noise, OITC is the more useful number.
The two are not interchangeable, and a window's OITC is usually lower than its STC. Ratings come from lab testing under ASTM E90, with ASTM E413 used to calculate STC and ASTM E1332 for OITC. Glass-only ratings are higher than whole-window ratings, because the frame, seals and installation all let some sound through. Ask for the rating of the complete window, not just the glass.
How glass build-up changes sound ratings
Glass maker Cardinal publishes estimated acoustic ratings for insulating glass units. A selection from its tables shows what actually moves the numbers. These are glass-only estimates; a finished window will rate lower.
| Glass build-up (outer / gap / inner) | STC | OITC |
|---|---|---|
| Double-pane: 3 mm / 13 mm / 3 mm | 31 | 26 |
| Double-pane: 3 mm / 19.5 mm / 3 mm (wider gap) | 33 | 28 |
| Double-pane: 6 mm / 13 mm / 6 mm (thicker glass) | 35 | 30 |
| Double-pane: 6 mm / 13 mm / 7 mm laminated | 40 | 33 |
| Double-pane: 7 mm laminated / 13 mm / 7 mm laminated | 42 | 35 |
| Triple-pane: 3 mm / 6.5 mm / 3 mm / 6.5 mm / 3 mm | 31 | 25 |
Two things stand out. The triple-pane unit rates the same STC as an ordinary double-pane unit, and lower on OITC, because its gaps are narrow. And laminated glass beats every other change. Cardinal also notes that argon gas has no measurable effect on acoustic performance.
What makes a window quieter
Laminated glass
Laminated glass is two layers of glass bonded to a plastic interlayer, usually PVB. The interlayer damps vibration, so the glass doesn't ring along with the noise outside. Usually only one pane of a double-pane unit needs to be laminated. Laminated glass also holds together when broken, which is why it is sold as security glass too. Anlin, for example, states that its First Guard laminated glass provides greater sound reduction along with forced-entry resistance. Our Anlin Coronado guide lists the glass options available in that line. A sound rating applies only to the glass build-up that was tested, so make sure your quote specifies the same one.
Panes of different thickness
Every pane of glass has frequencies where it vibrates easily and lets sound through. When both panes are the same thickness, those weak spots line up. Using different thicknesses spreads them apart. Canada's National Research Council, in its digest on sound through windows, says a thickness ratio of about 2 between the layers is most effective. Anlin's Sound Package uses this principle; Anlin states it reduces outside noise by up to 50% compared to typical dual-pane windows.
A wider airspace
The same NRC digest says that for double glazing, each doubling of the airspace raises STC by about 3. That is why a secondary window or an interior storm panel set several inches inside the main window can be so effective: it creates a very wide gap.
Air sealing and the operating style
Sound travels through any air gap. The NRC digest notes that a window with good weatherstripping typically rates 3 to 5 STC points lower than an equivalent sealed window, and that cracks can drastically reduce noise reduction. In practice:
- Fixed picture windows seal best.
- Casement and awning windows close against compression seals and generally seal tighter than sliders and hung windows.
- The gap between the window and the wall has to be sealed fully. A noise-rated window installed with gaps around the frame will underperform.
- Lightweight frames transmit more sound. The NRC digest advises avoiding them where noise reduction matters.
Bay Area noise: freeways, rail and flight paths
Homes along the major freeway corridors (I-880, US-101, I-280, I-80, I-580), near Caltrain and BART lines, and under approach paths to SFO, Oakland and San Jose airports deal with steady low-frequency noise. That is where OITC matters most, and where laminated glass and mismatched panes earn their cost.
SFO has run a noise insulation program for eligible homes near the airport, covering work such as windows and doors. If you live in San Mateo County close to SFO, check with the airport whether your address qualifies before you pay for upgrades yourself.
Many older Bay Area homes still have single-pane aluminum sliders, which leak air at the tracks and block very little sound. Replacing them with any modern double-pane window helps. Upgrading the glass on the street-facing side helps more. For more on glazing types in general, see our guide to single, double and triple-pane windows.
What windows can't fix
Once the windows are upgraded, noise finds the next weakest path. Common ones:
- Exterior doors, especially hollow-core doors and doors with worn weatherstripping.
- Through-wall vents, old wall-mounted AC units, mail slots and pet doors.
- Thin walls. Stucco and fiber cement walls are heavier than vinyl-sided walls, and insulation in wall cavities helps. If you are re-siding, filling empty cavities while the walls are open is worth discussing.
- Open windows. Sound-rated windows only work closed, so think about ventilation and cooling for rooms that face the noise.
How to choose sound-reducing windows
- Find the noise source and side. Most homes only need upgraded glass on one or two elevations.
- Ask for STC and OITC of the complete window in the configuration you are buying, not glass-only numbers.
- Prioritize laminated glass and dissimilar pane thickness over a third pane.
- Prefer fixed, casement or awning operation on the noisiest walls where the room layout allows.
- Insist on a fully sealed installation. Our retrofit vs new construction guide explains how the install method affects the perimeter seal.
Our window replacement team can quote noise-rated glass for just the rooms that need it. For apartment and HOA buildings near freeways, see our multifamily windows and siding page.
Frequently asked questions
What STC rating should a soundproof window have?
There is no official cutoff, so compare products against each other. Ask for the STC and OITC of the complete window. For traffic and aircraft noise, the OITC number is the better guide, and laminated glass with panes of different thickness usually rates highest.
Are triple-pane windows better for noise than double-pane?
Not automatically. Cardinal's acoustic tables rate a typical triple-pane unit with narrow gaps the same STC as an ordinary double-pane unit. A double-pane unit with one laminated pane rates considerably higher.
Does argon gas reduce noise?
No. Argon improves insulation, but Cardinal states that argon filling has no measurable effect on acoustic properties.
Can I open soundproof windows?
Yes, sound-reducing windows come in operable styles, but they only reduce noise when closed and latched. Fixed windows seal best, followed by casements and awnings.
Will new windows block all outside noise?
No window removes all noise. Good windows can make a large difference, but doors, vents and walls also carry sound, and low-frequency rumble from trucks and jets is the hardest to block.
Sources
The information provided here is intended for general reference and should not be considered professional advice. Before starting any project, consult a licensed contractor. Product features, specifications, and warranties may have been updated since this was published. Any mention of brands or products reflects personal opinion and does not constitute an endorsement or warranty.
