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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.

Several types of insulation are effective for crawl spaces, with spray foam and rigid foam boards being two of the best options. Closed-cell spray foam insulation is a strong choice because it effectively repels moisture, resists biological growth, seals the air, and provides excellent thermal protection, especially in hard-to-reach areas. Rigid foam insulation boards are another excellent material as they insulate well, resist water damage, and will not settle over time like fiberglass or cellulose.

Installing sheets of plywood over attic insulation is a popular choice, especially in storage spaces. That said, insulation works by trapping hot air—so it needs extra space to expand and be effective, or you might reduce the energy efficiency of your home. 

It’s important that you never compress the insulation by installing flooring directly on top of it. In many houses, you’ll need to install a subfloor before putting plywood over insulation in your attic.