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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. 

Yes, you can install insulation behind drywall yourself, but only if your walls are not finished. If your studs are exposed, installing fiberglass insulation in roll or batt form is very straightforward and just requires a few tools and some safety equipment. If your walls are already finished and you’re looking to add insulation, it’s better to hire a professional, as you’ll likely need blown-in insulation, which can be dangerous to work with and a challenge to get into the proper areas in your wall voids.

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.

Insulating an unheated garage can still be worth it. For instance, insulating an attached garage can help with heat loss from your house in general, and it can also make it easier to store electronics in your garage without worrying about them being exposed to extreme temperatures.

If you use your detached garage as a workspace, insulation can still make a difference. Plus, you can use space heaters and a window unit AC to help condition the space while you work without installing heating.