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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.

Professionals can install a radiant barrier in a day or two, but DIY installation will take several days. The time it takes to install a radiant barrier depends largely on the size of the space and the installation method. Larger homes can take a pro two to three days to finish the job. It’s also quicker to lay foil over existing insulation.

According to a report from Energy.gov, radiant barrier installations can save between 5% and 10% on your energy bill each month that your HVAC cooling is in operation. That fact can mean savings of up to more than $325 per year for homes that experience frequent direct sunlight and high summer temperatures. 

The life span of insulation is between 20 and 100 years. At that time, existing insulation can fall victim to gravity, resulting in compression, which reduces its insulating protection or R-value. If you can't simply add more insulation over the existing material, removing the old to increase the effectiveness of the new can reduce your energy bills and make the project worth the time and money investment.