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Blown-in insulation works, but only for unfinished attics. Also, it isn’t the longest-lasting nor the most effective option. For better options, look into spray foam insulation or natural fiber batts. They typically have a longer lifespan, better heat resistance, and stronger sound-proof abilities. Spray foam insulation costs about $1.25 to $1.50 per square foot, but can go up to more than $4 per square foot. Fiber batt pricing varies depending on the material.

The primary reason that insulation saves you money over time is that it reduces your energy bills by making your home more energy efficient. A crucial part of an insulation system is a properly installed vapor barrier. Local building codes dictate whether your insulation requires a vapor barrier based on the type installed. While some high-quality wool insulation may not technically need a vapor barrier, most municipalities still require one to meet code.

Installing sheets of plywood over attic insulation is a popular choice, especially in storage spaces. That said, insulation works by trapping hot air—so it needs extra space to expand and be effective, or you might reduce the energy efficiency of your home. 

It’s important that you never compress the insulation by installing flooring directly on top of it. In many houses, you’ll need to install a subfloor before putting plywood over insulation in your attic.

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.