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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.

If you plan on heating your garage, you must install a vapor barrier when drywalling the garage. A vapor barrier protects your garage from frost and mold that can occur through condensation. If your garage is unheated, you don't need to install a vapor barrier. The only exception is if you regularly crank the heat in your car before getting in it. In this case, you may still want to install a vapor barrier, as your car can heat your garage.

Insulating a detached garage won't reduce your home's heating or cooling bills—and insulating an attached garage will only have a small effect on your home's energy efficiency. However, insulating an attached garage can reduce the transfer of heating and cooling to and from the outdoors, which can reduce the amount of energy your home needs to stay comfortable. Also, having an insulated garage is just more pleasant. Balance the cost of insulating your garage with how you'll use the space to determine if the project is worth it.

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.