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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.

The most affordable way to insulate a garage involves choosing cost-effective materials like fiberglass, which comes in batts or rolls and is relatively easy to install. You can often install this type of insulation without needing to add drywall, which helps reduce costs. While a DIY approach might seem cheapest, hiring a professional installer can be highly cost-effective. Experts can accurately determine your garage's specific insulation needs and ensure the best results for your investment, often using batt insulation for the bulk of the project.

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.

Each of the three types of wool insulation has particular disadvantages. While their R-value is higher, they all cost more than fiberglass versions. Installing mineral wool and sheep wool requires protective gear. Because of its vulnerability to insects, chemicals are used in its production. Hemp wool is more costly to install than other types because additional tools and labor are involved.