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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. 

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.

If you plan on heating your garage, you must install a vapor barrier when drywalling the garage. A vapor barrier protects your garage from frost and mold that can occur through condensation. If your garage is unheated, you don't need to install a vapor barrier. The only exception is if you regularly crank the heat in your car before getting in it. In this case, you may still want to install a vapor barrier, as your car can heat your garage.

The best type of insulation for your home depends on your specific needs, climate, and the area you are insulating. For new construction, structural insulated panels are a highly energy-efficient option that is easy to install during the building process. In very warm climates, reflective insulation can be a cost-effective choice. For cold climates, fiberglass is an excellent option due to its high R-value and resistance to settling over time. Spray foam works well in humid environments because of its moisture-resistant properties. For standard attics, loose-fill, spray foam, or blown-in insulation are all suitable options.