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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.

If you need to insulate behind finished walls, the best option will likely be to have an insulation professional install blown-in insulation. The installation requires small holes cut in your wall, after which the installer can blow loose-fill insulation into the void. After the installation, you’ll have minor drywall patching, which shouldn’t cost more than $100. Compared to ripping down the existing drywall, using a different type of insulation, and replacing and finishing the drywall, this should save you hundreds or even thousands, depending on the size of the room you’re insulating.

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.

If you’re using faced insulation with paper, face the paper toward the inside of the room during the installation process. 

A good way to remember this is that the paper should face the “warm side” as it’s the vapor barrier, meaning it should face toward where the heat will be coming from.