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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.

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.

A moisture or vapor barrier helps separate the warm air inside your home from the cold air outside of it. Without it, your walls could collect condensation, mold, or other damage. Check your local building codes to see if including an interior vapor barrier is necessary for your building project or renovation, especially for rooms that are likely to be humid, like bathrooms.

The cost to install a radiant barrier ranges from $700 to $2,800, depending on the size of your roof and the type you choose. Most people pay an average of $1,700 to install a radiant barrier. Foil options, both single- and double-sided, cost between $0.10 and $0.90 per square foot, while spray-on options start at $0.60 and go all the way up to $1 per square foot—which can add up quickly.