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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.

While insulation is required for all exterior walls in a house, it is not typically required for interior walls. Local and state building codes mandate that new construction homes have adequate insulation in exterior walls and ceilings to suit the local climate.

However, there are several benefits to adding insulation to interior walls. It is excellent for sound-dampening between rooms and can help with temperature control by maintaining a regulated temperature in specific rooms and lowering heat transfer between adjacent areas. Insulating interior walls can also contribute to the overall energy efficiency of your home.

A moisture or vapor barrier helps separate the warm air inside your home from the cold air outside of it. Without it, your walls could collect condensation, mold, or other damage. Check your local building codes to see if including an interior vapor barrier is necessary for your building project or renovation, especially for rooms that are likely to be humid, like bathrooms.

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.