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Soundproof Window Basics
Soundproof windows work to a large extent by creating space between the panes of glass of a window - typically 2" to 3" in distance from one another. This additional airspace kills much of the noise that would pass through a normal window. Soundproof replacement windows will also typically use thicker glass panes, often either a .50" or .75" thick (although the effect of thicker glass is often overhyped by soundproofing companies). Finally, high end seals and glazing are used to insure the window is "working" as optimally as possible.
Airspace Between The Glass
The amount of air space between the two panes of glass is the single most important factor in how effectively sound is deadened between the time it hits the outisde pane to when it enters the buidling. The ideal spacing between the panes is 3", although anything over 2" should have a big effect on how effectively the exterior noise is blocked from entering a room.
Laminated glass can have a nice effect on reducing noise - laminated glass options use offset thicknesses for the two glass panes. So you might have a 0.25" and a 0.375" pane thicknesses. Laminated glass also makes the window much more shatter resistance (good for storms and security) and blocks nearly all UV rays from entering the home (low-e glass blocks roughly 85%). Laminated glass will increase the price by about $125 per window (average price on a standard size unit).
Window Seals & Glazing
Seals and glazing have a significant effect on the STC rating. Poorly made windows often cut corners on glazing or sealing and this can lower the STC rating.
Window glass thickness has some effect on how well noise is dampened. One of the important points here is that you want the two glass panes to have different thicknesses, which dampens sound. Glass with a thickness of 0.50" or 0.75" will dampen noise more effectively than glass with a 0.25" thickness. Some manufacturers tout their thicker (and more expensive) glass as the main way that sound is dampened. This is not correct, it is the airspace between the panes that has the greatest effect.
Hollow sashes will do a worse job of reducing sound than sashes than are solid wood or have been filled with a low expanding foam. The more chambers in a vinyl, aluminum or fiberglass frame will also have some effect on reducing sound. Certain window manufacturers use multi-chambered extrusions for their standard frame, while others only use the multichambers on their higher end models. Always inquire as to whether this features is available on the model you are considering.
Sound transmission class is a measure of how well a window kills sound. While normal windows have an STC window rating of 30 to 40, soundproof windows typically have an STC rating of 45 to 54.
Many homeowners want fixed windows if they are concerned with noise, but this isn't always the case. For movable windows, we suggest casements as they will typically have better compression seals when the window is closed. They are therefore preferable to double hungs and sliders.