Yes, crawl space insulation is worth it if installed correctly. Installing insulation in your crawl space will improve the air quality, help stabilize your home's temperature, lower your energy bills, and prevent pests from entering your home. Though you’ll pay more to install insulation initially, the savings you get from installing insulation can help offset those initial costs over time.

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.

The best type of attic insulation depends on where you live, your budget, and what stage of construction the attic is in. For example, if you're building a house, then structural insulated panels are a worthy investment since they're easy to install during construction and they're the most energy-efficient type of insulation.  

Additionally, if you live in a very warm climate, then you can potentially save quite a bit of money by opting for reflective insulation, which is more affordable than comprehensive insulation but doesn't work well for cool climates. Blown-in, loose-fill, and spray foam insulation are all great options for attics in moderate climates.

Ideally, you can add new insulation to your existing insulation. However, some circumstances require you to remove old insulation before adding any new insulation: 

• If the insulation is over 15 years old

• If the insulation is wet or moldy

• If it is vermiculite insulation

• If you see signs of pest infestation

Removing old insulation is best left to a pro who can safely dispose of potential moldy or asbestos-ridden insulation.