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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.

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. 

While fiberglass is dangerous to work with, it’s a perfectly safe material once it’s installed. If you’re tackling the installation yourself, make sure you take the proper safety precautions and wear the right gear to avoid skin and eye irritation and lung complications. Working with a professional insulation company can mitigate these risks.

The most affordable way to insulate behind unfinished walls is to install fiberglass batts or rolls. These total around $0.75 per linear foot, with the width matching the gaps between standard studs perfectly. However, it’s important to consider the R-value of your insulation and the climate in your area, as fiberglass insulation isn’t the best in terms of preventing heat transfer. Plus, the most affordable option may cost more on electricity bills over time. The most affordable way to insulate behind finished walls is to leave them up and use blown-in insulation to avoid drywall replacement.