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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. 

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.

For standard 2x4 walls, the ideal insulation has an R-value between R-13 and R-15. This level of insulation, typically around 3.5 inches thick, fits well within the wall cavity. Common options include fiberglass batts and spray foam. The best choice for your home can depend on your climate; use R-value maps, like those from Energy Star, to determine the recommended level for your region. For colder climates, you might also consider adding exterior insulation for greater efficiency. Always check with your local building codes to ensure you meet any required standards.