Window Rating: Sound Transmission Class (STC) Explained & Glass Comparisons

Everything Makes, Absorbs or Blocks Sound

Every animal, object, and thing does one of those three things. Animals and humans all make sounds. Snow & furniture in a house absorbs sounds (think about how much a house echo's when it is completely empty). Retaining walls and house walls block sounds, and windows also block sounds... but how effective are they? 

Windows Do Many Things

Windows are a crucial element to a homes overall appearance and performance. They are a major design component that ideally matches the style of the house. Then they also serve an important role in regards to energy efficiency... they block out solar heat (or let it in depending on if you have a passive house), they reflect the heat back into your house during winter months, the block out harmful UV rays and of course they let you look out into the beautiful world. If you would like to learn more about windows NFRC ratings and their purpose, see my post about windows NFRC ratings. Another key element is a windows sound rating, known as its sound transmission class (STC) rating. 

Understanding STC

STC is a rating system used in a buildings wall assembly to calculate the sound isolation The higher the STC rating, the better sound isolation the wall assembly is to achieve. STC ratings are commonly used in interior partitions, ceilings/floors, commercial buildings, movie theaters, recording studios, and of course what we are discussing today, WINDOWS. In residential buildings, a STC rating of 38-42 is considered good soundproofing. A STC rating of 50-65 is considered a commercial/professional grade soundproofing. A typical setting that would have this would be a  recording studio or a movie theater. 

Window STC Range

A windows STC rating is going to have a much lower range than a framed wall assembly and infinitely lower than a recording studio, nonetheless there are ways to improve a windows sound rating. The primary way to do so is by changing the insulated glass unit (IGU) glass lite make-up. For instance, a standard double pane IGU is made with two panes that are both 1/8" thick and with an airspace & spacer bar. To improve the STC rating, you can order the window with the IGU to have "offset" glass thicknesses. So instead of a typical 1/8" over 1/8" glass make-up, you can order the interior pane 3/16" or 1/4" thick. That additional glass thickness helps reduce the sound waves from transferring through the window/glass. 

An old single pane window will have a range between 15-21 whereas a standard double pane glass unit (1/8" over 1/8") will have a range of between 26-30 respectively. If you increase that inside pane to 3/16" or 1/4" thick it increases the STC rating by one or two points. Contrary to popular belief, triple pane does not produce a higher STC rating than the 1/8" over 3/16" offset dual glazed IGU. If you want to obtain the best STC rating for a standard window, making the inner pane laminated glass (like your car windshield) produces the best STC rating you can get with a standard window and dual glazed IGU. By having the laminated glass as the interior pane, it typically provides an STC rating of around 35. 

IGU with Laminated Glass and Regular 1/8" Pane.

Specialized Sound Proofing Windows

This is the last category of windows specifically designed to block out sound. These window manufactures are few and far between as it is a very specialized engineered system. Milgard Windows is one of those companies who manufactures one. They have coined it their "Quiet Line Series."

Here is what Milgard says about this line:

"Every Quiet Line Series window is double glazed with a built-in sound panel, essentially two windows combined into one. But rather than gluing together two separate components, we carefully design each quality window through single-extrusion construction for superior sound control and better weather protection."

These Quiet Line windows provide the ultimate solution for homes located near train tracks, busy intersections, or airports. The STC ratings for these windows start at 41 and can go up to 48 on picture windows with the correct offset glass thicknesses. That is incredible performance compared to a typical double pane window with offset glass. You can view Milgard's data on all of their STC ratings for the Quiet Line HERE.

The basic construction behind these "soundproof" windows is having two different sets of sash's built into one main perimeter window frame. If you want the ultimate sound control, go with a window like the Quiet Line Series. Just keep in mind that the overall frame with of these windows is pretty thick so if you have an older 2x4 construction house it may not work for you. Ideally, you need a 2x6 constructed wall (or more) for it to fit into the wall properly and to allow for interior trim work.

Conclusion/Pricing

STC is not a well known term and any spec house is not the least bit concerned with this. However, if you are building a custom home or in need of replacement windows, consider looking into the price for offset glass to improve the sound rating. The cost to make a window simply 1/8" thick over 3/16" thick is only about $50-$100 more per window... of course this depends on size of window and window style, etc. If you want the best sound rating within a double pane window (regular frame - not sound proof window), then you are looking for 1/8" over 7/32" laminated glass. The cost to do so varies greatly depending on size of window and window manufacture... however, expect to spend an additional $200-$500 PER window for that glass combination.