STC Ratings for Windows

Have you ever bought windows and wondered what the STC Rating for Windows means? Windows allow us to see out and allow light in while providing protection from the elements and security from intruders. Unfortunately, they also permit noise to sneak in and out.

Window STC Ratings identify how well the glass and window assembly work to block sound transmission into or out of buildings. The Sound Transmission Class reveals how well the window performs at preventing 16 select frequencies from moving through. The greater the rating, the better the assembly prevents transmission loss (TL).

This article will identify how well different glass and window assemblies stop noise leakage, and also what to look for when selecting soundproofing windows.

STC Ratings for Windows

Understanding STC Ratings

The Sound Transmission Class (STC) rates how well a material or a barrier like a window, door, or wall will block the movement of sound. It measures the transmission loss (TL) of 16 different sound frequencies between 125 and 4,000 Hz.

The results are graphed and compared against a reference graph. The higher the STC, the better the window or barrier prevents the movement of sound.

STC window rating depends upon several factors – glass thickness, laminated glass, the airspace between panes, and how and what the frame is constructed of. A rating of 40 is the minimum for conversation privacy, and many building codes now require a rating of 50 to 55. A window with a rating of 40 should decrease the sound transmission by 40dB depending on the frequency being evaluated.

Sound travels in waves and will find the path of least resistance if it can. The higher the STC value, the better it is at preventing soundwaves from moving in or out of space.

Windows, along with walls and doors are the common culprits in the soundproofing saga. The type of glass and frame are the key factors in improving the transmission loss of windows.

Sound Absorption vs. Transmission

Soundwaves travel through the air and encounter objects or buildings. Some of the sounds will transmit through, some will be absorbed, and some may be reflected in a different direction. The challenge with windows is to decrease the sound transmission through the panes and the frames.

Thick panes of glass, or monolithic, will block more sound than thin panes. A 1/8” has an STC of 21 to 28, 1/4” between 30 and 31, and 1/2″ up to 37. However, laminating two thin panes with a layer of polyvinyl butyral (PVB) or ethylene vinyl acetate (EVA) in between will do better – STC 35 to 40.

Known as laminated glass the PVB or EVA layer absorbs some of the sound waves the glass doesn’t reflect, decreasing the sound transmission. The laminated glass is considered one pane, even though two pieces make it up.

Two or three panes of glass or glazing can increase the window’s ability to absorb sound if there is at least a 1/2-inch airspace between the standard glass panes – STC 26 to 32. The airspace absorbs more sound waves and further decreases the sound transmission. Laminated panes with the 1/2-inch or more airspace will prevent even more sound movement – 35 to 41.

Triple glazing may not be as good at blocking noise as double glazing if the airspace between the panes is less than 1/2-inch. The narrower space will cause more reverberation and boost them into and through the other panes. Triple glazed windows need to be thicker than double pane to allow for proper spacing, which may not fit aesthetically in the window frame.

Considerations for STC Ratings for Windows

The glass and glazing techniques are two of the factors that affect how well a window prevents sound transmission. The airspace between the panes, plus the frame material and construction are the other factors to consider.

Different thicknesses also block different sound frequencies, and combinations of materials and thicknesses work together to stop a broader spectrum of noise transmission. All components of a window’s construction should work to improve the STC.

Glass

Single sheets of glass, or monolithic glass, range from 1/8” to 1/2″ thick glass and have STC values from 21 to 37. Laminated glass, also known as impact glass, is composed of two layers of glass sandwiching a polyvinyl butyral (PVB) or ethylene vinyl acetate (EVA) sheet.

The thickness of the glass sheets may be the same, or dissimilar; which means the two laminated glass layers are different thicknesses. The width of the glass sheets influences soundproofing effectiveness.

Glazing

Glazing refers to panes of glass in a frame. Older windows and storm windows are often single glazed – one layer of glass. Double or dual glazing is two layers of glass, and triple is three layers.

The layers of glass may be the same thickness, or dissimilar. They may also be two panes of laminated glass which is often referred to as soundproof windows and have an STC between 45 and 50, or higher.

Air-Space between the Panes of Glass
The air-space between panes or glazing is an essential factor in absorbing sound, provided it is 1/2″ or greater. If the gap between the panes is less, it will act as a springboard for the sound waves to and through the next layer. The wider the air-space, the more sound it will absorb.

Frame Material

The frame material plays a vital role in how well a window blocks sound transmission.

Wood is made up of cells that have airspaces which dampen sound and absorbs sound vibrations, making wood better than metal at blocking sound. The main drawback for wood frames is maintenance.

Metal is a solid material without airspaces to absorb sound and vibrations, so it doesn’t stop as much noise as wood frames. Metal frames are stronger than wood or vinyl and have more mass than vinyl and some wooden frames. Most industrial and commercial buildings use metal (aluminum) frames for ease of maintenance.

Vinyl frames are lightweight but don’t have the mass or airspaces to absorb sound or vibration. They absorb less noise than metal frames unless significantly thicker than a metal frame with a similar STC. Vinyl, however, requires less maintenance than wood frames.

Frame Construction 

Windows are either fixed or openable, and fixed windows are commonly 3 to 5 STC points higher since they are made not to open. The tighter the glazing fits into the frame the more air it will block, and thus the more noise. Glass needs to be sealed into the frame, and moveable sashes must be sealed and properly fitted to compress tightly to prevent noise movement.

The construction of the frame is crucial since it must seal with the glass to stop the noise. Any gaps or cracks will allow sound waves to wiggle through. Hardware for windows that open must be able to pull or push sashes tight so they compress against weather-stripping seals to prevent gaps.

 

STC Ratings for Windows

The STC rating of windows depends on the thickness of the glass, use of PVB or EVA, the number of panes, the frame material, and weather-stripping seals. Having your windows tested for sound transmission may cost a pretty penny, but it could save you a bundle if you don’t have to replace your windows. If you do replace your windows, make sure you get something at least 6 STC points better.

Single-Paned Window

Single pane windows have been used for centuries to keep the elements and nature out while letting light in and providing vistas of the landscape from within buildings. Whether pour or rolled to a consistent thickness, single pane glass also kept out the noise.

Today single glazing is still used, although it is commonly used as storm windows to protect other glass. A by-product of the storm window is improved sound control. The thickness of the pane affects how well it reduces noise transmission.

Single pane 1/8” has an STC between 21 and 28, and is commonly 27.

Single pane 1/4″ has a sound transmission class between 30 and 32, and averages 31.

Single pane 3/8” averages 31, but could have an STC of 33 or 35.

Single pane 1/2″ has a rating of 35 to 37, and is usually given as 36.

Double-Pane Windows

Double pane glazing usually refers to two single panes with sealed airspace in between. The noise blocking efficiency depends upon the thickness of glass and the width of the air gap.

The thicker the glass and wider gap translate into better sound control. 1/8” glass with a 1/4” airspace and another 1/8” sheet of glass has an STC of 28, and normal voice levels can still be heard and distinguished. The same glass thickness with a 1/2” airspace will improve the rating to 32, which will mute a quiet conversation but allows loud voices through.

Using two thicker 1/4″ plates and the 1/2″ airspace between increases the value to 35, and prevents loud talking from being understood. If the same 1/4″ plate is used with a 1″ airspace the STC to climbs to 37.

Loud speech is still audible, but not understandable. The examples demonstrate the importance of the air gap and glass thickness for reducing sound transmission.

Triple-Pane Glass

Triple pane windows may sound good. Unfortunately, they often don’t meet the sound improvement expectations. Manufacturers cut corners and fit the three panes in the same frame space as two panes, which decreases the air space between panes and amplifies noise penetration. Depending on the thickness of the glazing and the air gap between panes, the STC values can range from 26 to 41.

Better results can be achieved using triple glazing if the air gaps are 1/2″ or wider. Using panes of different thicknesses as well will improve the STC further, and also enhance its effectiveness with lower frequencies.

If paying for triple glazed windows, make sure the air space is at least 1/2″. You should be able to tell as the windows will be thicker.

Triple Pane Insulated

Insulated triple pane windows frequently have an STC rate of 39. They consist of three panes of glass with gas-filled airspaces to provide a thermal barrier.

The layers of glass are often different thicknesses (1/8” to 3/8”) which improves the sound blocking ability provided the gap is 1/2″. If the panes are exchanged with laminated glass, the STC rate will increase too.

Typically, insulated triple pane windows have three plies of 1/8” glass with 1/2″ sealed air spaces filled with insulating glass. The glazing is also low-E coated to decrease thermal influences further. The sealed glass unit is 1-3/8” thick, which is almost double the standard double glazed window unit.

Dissimilar panes

Dissimilar panes mean the window has two or more sheets of glass, and each is a different thickness. One pane is designed to block lower frequency sound, and the next layer stops higher frequencies. The end result is better control of transmission levels through the window.

Most dissimilar glazing results in an STC of 32 to 38, which is similar to a double-glazed window. Laminated glass with dissimilar panes will improve the rating even more and can reach 51 or higher on the STC scale.

Laminated panes

Laminated glass has a layer of PVB or EVA sandwiched between two layers of glass. Also known as impact glass since the vinyl material will hold the glass together even if it is cracked. Laminated glass initially was used as safety glass or for areas prone to natural disasters. The bonus is the improved soundproofing laminated glass offers.

Single glazed laminated windows may have an overall thickness from 1/4″ up to 3/4″ depending on the thickness of the glass and vinyl plies. Two sheets of 1/8” glass with a 3/100” PVB sheet in between makes a 1/4″ pane with an STC of 35, 3 points better than 1/4″ plate glass. Double those dimensions and the value improves to 39, which is also higher than a 1/2″ plate of monolithic glass.

Acoustical Windows

Acoustic glass is insulated laminated panes and can be used to configure double, triple, or dissimilar windows. They offer an STC between 39 and 51, or higher, and will mute even loud voices so words aren’t distinguishable.

Different glass thicknesses deflect different frequency ranges making dissimilar laminated glass the ideal acoustic window. Additionally, the thickness of the vinyl also affects how well the window absorbs sound. Laminated glass is the best for decreasing sound transfer, it is also more expensive.

Some acoustic windows are sealed units and others aren’t. Most windows with overall glazing thickness less than 1-1/4” are usually sealed and can even be insulated. An STC of 39 can be achieved with 1/4″ laminated combined with a 3/16″ plate separated by 1/2″ of air.

The overall thickness is 15/16”. Increase the airspace to an inch, and the rate moves to 42. Increasing the air gap further to 4-inches and the rate jumps to 49, but the window unit is 4-3/4 thick. The acoustic ability of the window can be improved even more (above 50) by using laminated glass for both the inside and outside panes.

Best Windows for Sound Reduction

Single pane windows can have an STC rate of 21 to 37 depending on the thickness of the glazing. Double pane typically has a rate between 26 and 32, but it can be improved to 37 by using thicker glass and a wider gap between plates. The type and thickness of the glass combined with the airspace between determine how well a window will stop sound transmission.

The best sound reducing windows have dissimilar laminated plates with a wider air gap between the glazing. Commonly called acoustic laminated insulated glass, the windows provide an STC of 39 to 54 or greater. The cost, however, may be prohibitive.

How to Increase Window STC Rating

There are different ways to improve the STC rating of windows. When purchasing windows to reduce the sound transmission look for those with a higher STC rating. Laminated plies with wider air spaces between the panes tend to perform better. Thicker glass adds more mass which absorbs more sound, and the PVB or EVA helps block frequencies too.

Wooden frames have cell voids that absorb sound, metal frames have more mass to absorb noise, and vinyl frames are too lightweight to absorb much sound. Selecting the frame that aesthetically blends and improves sound control is a good plan. Ensuring sashes seal tight and weather-stripping prevents air movement will also help control sound transmission.

Remove the window trim to make sure there are no cracks or crevasses for noise to penetrate around the frame will have positive results. If there are, fill them with sound absorbing material.

Additionally, make sure partition walls are insulated to absorb sound. A window with an STC of 50 won’t make much difference if the wall is only rated at 30.

Another way is to add a storm panel that fits tightly into the window frame and is well sealed to prevent noise penetration. Building a window plug to temporarily or permanently block the window and decrease sound movement is another possibility. For other suggestions, check out my post about how to soundproof a window.

Conclusion

The STC Rating for Windows identifies how different glass types and thicknesses, plus the airspace between glazing in a window, and the frame material can decrease sound transmission. The best windows to block sound are dissimilar laminated panes with a wide air gap between the glass plates.

They are also known as acoustic windows. I hope you found this article informative and useful. If you know someone who might be interested in the topic, pass it on. Your comments and suggestions are appreciated.

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