Mineral wool or dense packed cellulose
In most areas we install Rockwool EVO (formaldehyde free) mineral wool sound insulation batt. This product, along with air sealing utility and framing connections, significantly reduces the transmission of sound between living spaces.
Dense pack cellulose is also a great sound attenuation material though due to cost we prefer to use this in scenarios where there is a significant need for quiet. This approach is also useful in interior framed walls where there are many utilities sharing the same space making the installation of a batt product less than ideal.
In 2023 we plan to transition to locally produced wood fiber batts manufactured by TimberHP