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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.

Structural insulated panels and rigid foam boards have the highest R-value of any insulation that contractors commonly use, with R-values reaching up to around 8.0. These types of insulation are best in extremely cold climates, but they’re generally preferred for new construction, as they can be challenging to retrofit. Blown-in and loose fill insulation provide the next best insulative value, provided the installation is done properly. It’s important to choose an insulation type that meets your needs based on climate, as maximizing R-value may not always be financially worthwhile if you live in a moderate climate.

If you plan on heating your garage, you must install a vapor barrier when drywalling the garage. A vapor barrier protects your garage from frost and mold that can occur through condensation. If your garage is unheated, you don't need to install a vapor barrier. The only exception is if you regularly crank the heat in your car before getting in it. In this case, you may still want to install a vapor barrier, as your car can heat your garage.

Air sealing isn't necessary for attic insulation, but it's incredibly helpful since it can help you identify and patch any existing air leaks. Failing to find these leaks before adding insulation can render your insulation less effective, leading to higher energy bills and potentially costly repairs down the line.