Sound: STC, OITC and the glass that earns them.

On a busy street the window is the thinnest part of the facade. Most of the noise that reaches a bedroom comes through it or around it. In multifamily buildings, student housing and hotels, noise is one of the most common complaints, and it is hard to fix after the building is occupied. This guide explains the two ratings used to describe a window's sound performance, what in the glass earns them, and where to write the requirement so it survives to site.

Transmission loss.

Sound level is measured in decibels, dB. A window's job is to reduce the level between outside and inside. Take a commuter train that registers 105 dB to someone standing on the platform. Inside a nearby building it registers 80 dB. The 25 dB difference is the transmission loss of that facade.

Two things make window acoustics harder than that example suggests.

• The decibel scale is logarithmic. A drop of 10 dB is heard as roughly half as loud. A few points of rating make a real difference in the room.
• Transmission loss changes with frequency. Glass stops high-pitched sound far more easily than low-pitched sound. A window can do well against voices and poorly against a bus engine.

So a window's performance is a curve across many frequencies. STC and OITC are two ways of reducing that curve to one number.

STC and OITC.

	STC	OITC
Full name	Sound Transmission Class	Outdoor-Indoor Transmission Class
Frequency range	125 to 4,000 Hz	80 to 4,000 Hz
Weighted toward	Speech and mid frequencies	Low frequencies: traffic, trains, aircraft, construction
Written for	Interior walls, floors, ceilings and doors	Exterior walls, windows, doors and facades

Both numbers come from the same laboratory measurement. The window is built into an opening between two rooms, sound is generated on one side, and the loss is measured band by band on the other. The test method is ASTM E90. The measured curve is then fitted to a reference curve to give STC, under ASTM E413, or weighted against a typical outdoor noise spectrum to give OITC, under ASTM E1332.

STC is the older and better-known rating, and most product literature leads with it. OITC was developed for the noise that facades actually face. Its range starts lower, at 80 Hz, and it gives more weight to low frequencies. Those are the sounds that travel furthest and are hardest to block.

For the same window, OITC is almost always the lower of the two numbers. A rating of STC 47 and OITC 40 describes one window, not two.

For street noise, specify OITC. STC tells you how the window handles voices.

Which one to specify.

If the noise is traffic, rail, aircraft or construction, specify OITC and list STC beside it. If the concern is mostly mid- and high-frequency sound, such as a schoolyard or a busy pedestrian street, STC is a fair guide. For a mid-rise building on an urban street, low-frequency noise is nearly always the problem. The acoustic consultant's report will usually give the required facade performance as an OITC value.

What in the glass earns the rating.

A window's sound rating comes mostly from the glass. Four things do most of the work.

Mass.

Thicker glass is heavier, and heavier glass is harder for sound to move. Going from the thin lites of standard glass to thicker ones is the first step up.

Unequal thicknesses.

Every lite of glass has a frequency at which it vibrates freely and lets sound through. Two lites of the same thickness share that weak point. Two different thicknesses cover for each other. This is why acoustic insulated double-pane glass pairs a thinner lite with a thicker one, instead of two lites of the same thickness.

The cavity.

A wider space between the lites improves low-frequency performance. Gas fill makes no measurable difference to the sound rating. Mass and lamination do the work.

Lamination.

Laminated glass is two or more lites bonded to a plastic interlayer, usually polyvinyl butyral, PVB. The interlayer is soft. It damps the vibration of the glass, where a monolithic lite of the same thickness would ring. Lamination is the biggest single step available, and it is the only one that also brings other benefits. Laminated glass holds together when it breaks. It resists forced entry. It also blocks nearly all ultraviolet light, which protects finishes and furnishings.

On a glass quote, a laminated lite is written as a short code for its layers and its interlayer. The code matters less than the test: ask for the sound report that names the glass.

Two panes or three.

Triple glazing is chosen for energy, not for sound. A standard triple with three thin, equal lites is not an acoustic glass. A triple becomes quiet when its outer and inner lites are laminated and unequal. The best sound and energy numbers then arrive in the same unit.

The ladder, on one window.

These are tested and certified values for one window type, the Vi6 casement. Each row changes the glass and nothing else.

Step	Glass	STC	OITC	U-factor
Standard	Insulated double-pane	33	27	0.27
Quieter	Insulated double-pane, asymmetric	39	34	0.27
Quiet	Laminated acoustic double-pane	46	38	0.27
Quietest	Laminated acoustic triple-pane	47	40	0.24

Read down the OITC column: 27, 34, 38, 40. The first step, to thicker and unequal lites, is worth 7 points. Laminating both lites and widening the cavity adds 4 more. The laminated triple adds the last 2 and lowers the U-factor as well. The U-factors shown are with an aluminum spacer. The warm-edge version of this laminated triple is certified at U 0.18 and carries the same STC 47 and OITC 40.

Thirteen points of OITC, from 27 to 40, is a large step. Every 10 dB is heard as roughly half as loud.

The frame and the seals.

Glass is most of the story, not all of it. Sound behaves like air. It gets through any gap air gets through.

• Seals. An operable window depends on continuous gaskets and on hardware that pulls the sash tight against them at several points. One gap at a corner undoes expensive glass.
• Corners and joints. Welded corners leave no path at the miter.
• Frame material. The frame is part of what was tested, so a whole-window rating already counts it. A glass-only number does not.
• Product type. The rating belongs to the type that was tested. On the Vi6 series the acoustic triple gives STC 47 / OITC 40 in the casement, STC 46 / OITC 40 in the fixed window and STC 44 / OITC 39 in the in-swing terrace door.

Ask whether a rating is for the glass alone or for the whole window. A glass-only number says nothing about the frame and the seals. Ask for the laboratory report number.

The opening around the window.

A tested window installed in a leaky opening will not perform as tested. The perimeter joint needs the same care as the window.

• Fill the joint between frame and wall completely. Voids behind the trim are sound paths.
• Seal both the inside and the outside of the joint with a flexible acoustic sealant.
• Watch for flanking paths: through-wall air-conditioning sleeves, trickle vents, and unsealed gaps at slab edges can each leak more sound than the window.

Where to write it down.

Acoustics goes wrong when it is left until complaints arrive. The steps below follow the normal design phases.

1. Design development. Bring in the acoustic consultant early. A noise survey gives the required OITC for each facade. The street side and the courtyard side rarely need the same glass, and pricing them separately saves money.
2. Construction documents. State the required OITC and STC, the test standard, and that the rating must be for the whole window of the type specified. Put the glass makeup on the window schedule by elevation.
3. Construction administration. Inspect the perimeter seal before the interior trim covers it. Where the specification calls for it, the consultant can measure the finished rooms.

Building types.

• Multifamily. The building code deals with sound between dwelling units. Facade noise is usually left to the project's own requirements or to a local environmental review. In New York City that review is CEQR: where a site carries a noise (E) designation, the window and wall together must give a set attenuation, usually 28 to 35 dBA, worked out from their OITC ratings, with windows closed and another means of ventilation.
• Student housing and schools. ANSI/ASA S12.60 sets acoustic criteria for classrooms, including limits on background noise from outside. Programs such as CHPS and LEED for Schools reference acoustic performance as well.
• Hotels. Guest rooms are bedrooms. Treat street-facing rooms the way you would treat street-facing apartments, and check the operator's own standard.

Three common mistakes.

1. Specifying STC for traffic noise. A high STC can hide weak low-frequency performance. Ask for OITC.
2. Assuming a triple is the quiet option. Without lamination and unequal lites it may be no better than a good double.
3. One glass for the whole building. The noise is not the same on every side. Matching the glass to the elevation costs less than upgrading everything.

Where Vistaza fits.

Vistaza's High Performance uPVC windows are sound-tested by Intertek to ASTM E90 as whole windows, and each rating is tied to a report number and a glass makeup. The highest is STC 47 / OITC 40 on the Vi6 casement with the laminated acoustic triple, report J4759. The warm-edge version of that glass is NFRC-certified at U 0.18, so one unit answers both the energy and the noise requirement. Other types carry their own tested values, such as STC 39 / OITC 33 on the Vi5 casement and STC 37 / OITC 32 on the sliding door, each with asymmetric double-pane glass, one lite thicker than the other. The sound page shows the full ladder.