The most obvious con of injection foam is the cost. It’s much more expensive to install than fiberglass or spray foam, but it may be your only option (unless you want to remove entire walls around the home). Another con is that it involves drilling into existing material, which has to be repaired after the foam is injected. Some injection foams are made from products that aren’t environmentally friendly, and improper installation can lead to trapped moisture, which can lead to expensive mold remediation down the road. 

You do not always need to remove old insulation before adding new. If the existing insulation is in good condition—not wet, physically damaged, moldy, or infested with pests—you can add new insulation directly on top of it for a compounding effect. You can also mix different types, such as adding blown-in insulation around existing fiberglass batts.

However, removal is recommended if the old insulation is compromised by water damage or pests. You should also remove it if you plan to install a different type of insulation, like switching from roll or batt insulation to spray foam. If the existing insulation is very old or damaged, you will likely get more benefit from removing it and starting fresh.

It's essential to use insulation materials specifically designed for air ducts. Common types of duct insulation materials include fiberglass, foam board, and reflective foil. These insulation materials have the right properties to withstand the temperature and airflow conditions within the ductwork. Using the appropriate insulation not only ensures efficient temperature control but also prevents potential safety hazards or the release of harmful particles into the air.

Mineral wool has an excellent life expectancy of up to 50 years, provided it's kept dry and free of pests. Compared to fiberglass, with a life span of 20 to 30 years, mineral wool stacks up well for long-term cost savings. Sheep wool maintains a similar life span to fiberglass, while estimates for hemp wool insulation life expectancies are all over the board, ranging anywhere from 20 to 100 years.