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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.

If you feel your energy bills are too high or your HVAC system is working harder than necessary to maintain the correct temperature in your home, you may have an under-insulated home. If you’re unsure, consider having your system’s efficiency assessed by a local independent energy auditor.

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.

Adding insulation to ceilings is an excellent way to increase the energy efficiency of your home. If your home's layout is such that you need insulation in the ceilings rather than in the roof, it's likely that at least some insulation already exists in the space. However, each additional inch of insulation you can add boosts the R-value by between 2.2 and 4.2.