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The lifespan of crawl space insulation depends on how well you maintain your insulation and the type of insulation. Fiberglass will need replacing every 15 to 20 years on average if you don't have a vapor barrier in place. If you do have vapor barriers in place, then your insulation could last much longer. Spray foam insulation can last anywhere from 80 to 100 years. 

Plaster itself has a higher R-value than drywall. However, there are other considerations surrounding plaster and insulation. As a general rule, it's easier to obtain consistency in insulating value and quality when working with drywall rather than plaster. Plaster walls often don't contain any insulation behind the wall covering by design, and careful evaluation is necessary when choosing to add it.

Unless your cellulose insulation is wet, moldy, or showing signs of mildew or pests, there’s no need to remove it before adding new insulation on top of it. Inspect your insulation thoroughly before adding the new insulation, and if you’re unsure how to assess the presence of mold or mildew, talk to a mold remediation specialist near you.

Because of the moisture concern, open-cell foam requires a vapor barrier. The barrier protects against high humidity and moisture exposure in rooms such as basements, bathrooms, or exterior walls. It wouldn't be necessary in dry interior spaces.