#1 The windows must be airtight. A lot of sound can follow just a little air.
#2 Glass density [thickness] must vary from pane to pane and the thicker the better. For example, insulated, triple-pane glass, with each pane having the same strength and glass thickness equals a Sound Transmission Class (STC) of 29. A window with Insulated glass having one double-strength pane and one 3/16" pane yields an STC of 31. Even further, a window with a single pane of 1/4" laminated glass equals an STC 32.
#3 Distance between the panes is also very important. The greater the distance, the better! In the two scenarios found in #2 that involve multiple panes of glass, the insulated glass units only have about 3/4" of space between the panes, which is fairly typical. That is why a secondary [storm] window is the cost effective way to better sound blocking. For example, an airtight high performance storm window mounted on the exterior or interior normally will have around 2 1/4" of space between it's glass and the primary window glass. A Mon-Ray high performance storm with 3/16" or 1/4" laminated glass over a single pane window with single or double strength glass will have a combined STC of 40 to 43. Off the charts!
Further Educational Information for You!
The higher the STC rating is in number, the better. Consider this: a typical 2" x 4" studded wall that includes fiberglass insulation and drywall, sheathing and is installed on a non-masonry exterior wall that consist of wood, or standard siding (vinyl or aluminum) will have an STC rating of approximately 40. Walls that include a masonry exterior will typically be in the neighbordhood of 50 on the STC scale.
The "holy grail" of space between window panes is 5". However, this is terribly difficult to achieve in most situations. A 1/2"-thick laminated glass secondary window with 5" of space on most primary windows will have a combined STC rating of 50+.
Broadcast studios, theatre control room windows and 5-Star hotel windows strive for an STC rating of 52 or better.
Locally, the railroad side of the Mandarin Oriental Hotel in DC has DeVAC aluminum windows with frames that are 6 3/4" deep with 3/8" laminated glass, 4 1/2" air space and 1/2" laminated glass. And yes...that was a Window Man project.
WARNING: Lab testing of windows is conducted at normal room temperature and in a controlled environment, typically around 70 degrees. This is not typical weather. This is not a typical winter or summer in the NOVA and D.C. region! 90-degree summer days and 10-degree winter nights are not that uncommon, and that is when you'll discover the Vinyl Window Mistake you've made...when it's too late. Vinyl windows contract significantly in cold weather which can reduce their ability to block air/noise. Vinyl windows also struggle against harsh summer sun and heat causing that window to "jamb" and become difficult to operate.
Sound Abatement
If noise is a problem in your home, there are several different glass options that can help reduce noise transmittance.
