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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.

Bubble wrap is better for moving since it can be cut to size, whereas dish foam comes in precut sheets that are harder to manipulate. However, if you’re packing items like dishes, wine glasses, picture frames, or mirrors, dish foam will protect them better. Ensure you measure your box beforehand so you know what size dish foam to purchase. 

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.

The life span of insulation is between 20 and 100 years. At that time, existing insulation can fall victim to gravity, resulting in compression, which reduces its insulating protection or R-value. If you can't simply add more insulation over the existing material, removing the old to increase the effectiveness of the new can reduce your energy bills and make the project worth the time and money investment.