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The primary downside of radiant barriers is their limited effectiveness in cold climates. They are designed to reflect radiant heat, making them less beneficial in cooler conditions where heat retention is more important. 

Radiant barriers require proper installation to maximize effectiveness. Incorrect placement can lead to minimal benefits. They also do not provide insulation, meaning they should be used in conjunction with traditional insulation for optimal energy efficiency. 

The lifespan of crawl space insulation depends on how well you maintain your insulation and the type of insulation. Fiberglass will need replacing every 15 to 20 years on average if you don't have a vapor barrier in place. If you do have vapor barriers in place, then your insulation could last much longer. Spray foam insulation can last anywhere from 80 to 100 years. 

In addition to adding insulation to your attic, you may want to add a radiant barrier. A radiant barrier works by using reflective material, such as foil, on the attic floor. As heat penetrates the roof and enters the attic, the radiant barrier reflects the heat upward and back through the roof. If you receive a lot of direct sunlight on the roof, using both a radiant barrier and insulation in your attic can provide significant benefits.

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.