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The most affordable way to insulate behind unfinished walls is to install fiberglass batts or rolls. These total around $0.75 per linear foot, with the width matching the gaps between standard studs perfectly. However, it’s important to consider the R-value of your insulation and the climate in your area, as fiberglass insulation isn’t the best in terms of preventing heat transfer. Plus, the most affordable option may cost more on electricity bills over time. The most affordable way to insulate behind finished walls is to leave them up and use blown-in insulation to avoid drywall replacement.

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.

R-30 insulation is better for higher thermal resistance, but whether it’s "better" depends on the application. R-30 is much thicker and suited for areas like attics or floors in cold climates, where more insulation is required. R-13, on the other hand, is thinner and works well for 2x4 walls or milder climates where less insulation is sufficient. Choosing between R-13 and R-30 depends on the area being insulated and the climate. For example, R-30 provides more insulation but may not fit everywhere in your home.

Insulating a detached garage won't reduce your home's heating or cooling bills—and insulating an attached garage will only have a small effect on your home's energy efficiency. However, insulating an attached garage can reduce the transfer of heating and cooling to and from the outdoors, which can reduce the amount of energy your home needs to stay comfortable. Also, having an insulated garage is just more pleasant. Balance the cost of insulating your garage with how you'll use the space to determine if the project is worth it.