Thermal insulation and acoustic soundproofing are closely related, but they are not the same. Standard R-15 fiberglass batts provide excellent thermal resistance but do very little to stop the low-frequency rumble of diesel trucks or airplane engines. To stop sound, you need *mass*.
Fiberglass vs Cellulose Mass
Sound travels via vibration through the air. Lightweight fiberglass is mostly air, so sound waves pass through it relatively easily. Blown-in Cellulose, conversely, is manufactured from pulverized, fire-retardant recycled newspaper. It is three times denser than fiberglass by weight.
The "Dense-Pack" Retrofit
If your house is already built, contractors can employ a technique called "Dense-Packing." We drill 2-inch holes into the exterior wall cavities from the outside and forcefully inject cellulose at high pressure until the cavity is completely solid. The sheer density of the packed paper acts as a massive acoustic dampener, frequently dropping exterior noise transmission by 40% to 50%.
Sound travels best where air travels. If you can hear highway noise clearly, you have air leaks. Caulking windows and spray-foaming top plates in the attic not only stops drafts but acts as a primary acoustic seal against high-frequency street noise.