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Fiberglass is the best insulation for HVAC ducting because it's inexpensive, widely available, versatile, and long-lasting. Fiberglass also offers acoustic insulation in addition to thermal insulation, which can contribute to a more peaceful home environment. You can use other materials like mineral wool, but fiberglass is the industry standard.

The life span of insulation is between 20 and 100 years. At that time, existing insulation can fall victim to gravity, resulting in compression, which reduces its insulating protection or R-value. If you can't simply add more insulation over the existing material, removing the old to increase the effectiveness of the new can reduce your energy bills and make the project worth the time and money investment.

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.

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.