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It’s best not to install a radiant barrier directly under insulation because it requires an air gap on at least one side to work properly. Instead, you should install the radiant barrier over your existing insulation to create an air space to reflect the radiant heat.

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.

For standard 2x4 walls, the ideal insulation has an R-value between R-13 and R-15. This level of insulation, typically around 3.5 inches thick, fits well within the wall cavity. Common options include fiberglass batts and spray foam. The best choice for your home can depend on your climate; use R-value maps, like those from Energy Star, to determine the recommended level for your region. For colder climates, you might also consider adding exterior insulation for greater efficiency. Always check with your local building codes to ensure you meet any required standards.

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.