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When it comes to fiberglass insulation versus foam board, there are a few notable differences, both aesthetically and functionally. Fiberglass insulation is soft and flexible, meaning it can be squeezed into tight spaces more easily than foam board. Fiberglass insulation also has a higher R-value, making it a better pick for colder climates. However, foam board insulation can resist moisture and pest damage much better than fiberglass.

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.

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.

Monolithic slabs don’t require insulation, but many homeowners choose to have their contractors install insulation under the slab during construction. This can help improve home efficiency even more and keep indoor temperatures more stable. Slab foundations are more energy-efficient than uninsulated crawl spaces and basements, and insulated slab foundations are more energy-efficient than insulated crawl spaces and basements.