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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.

It’s best not to install a radiant barrier directly under insulation because it requires an air gap on at least one side to work properly. Instead, you should install the radiant barrier over your existing insulation to create an air space to reflect the radiant heat.

R-30 insulation is better for higher thermal resistance, but whether it’s "better" depends on the application. R-30 is much thicker and suited for areas like attics or floors in cold climates, where more insulation is required. R-13, on the other hand, is thinner and works well for 2x4 walls or milder climates where less insulation is sufficient. Choosing between R-13 and R-30 depends on the area being insulated and the climate. For example, R-30 provides more insulation but may not fit everywhere in your home.