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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 insulative type of insulation for behind drywall is spray foam insulation, but this is the most expensive insulation available. The most common insulation—sometimes considered the best for the value you get for the money—is fiberglass roll or fiberglass batt insulation, which is significantly more affordable but also far less insulative. The best kind of insulation for your house will also vary based on the climate in your area, your budget, and more. A professional insulation company can help you decide which type is best for your home.

Heat lost through uninsulated basement walls can account for up to 30% of the total energy lost through heat transfer to the outdoors. While homes in mild climates won't gain as much benefit from having an insulated basement as those in cold climates, the effects are still noticeable on your energy bill.

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.