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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.

One of the clearest signs that your insulation isn’t working as well as it should be is a noticeable difference in temperature throughout your home. One room might feel icy while another is hot as Arizona—this indicates that some parts of your home are either retaining or losing heat more than others. Additionally, a sudden increase in your energy bills, evidence of pests or rodents, or evident water damage are other signals that warrant a thorough inspection of your insulation and potential replacement.

2x4 walls can hold R-13 or R-15 batts of insulation, which is usually about 3.5 inches thick. You should also check with your local building codes to see if there’s a required thickness you need to adhere to. 

You can also use R-value maps to determine the best thickness and ratings for where you live. For example, someone in Texas doesn’t need as thick of insulation as someone in Montana. Energy Star has a good one you can reference.

The average U.S. homeowner pays $1,850 to insulate their home. However, the cost of insulation depends on the type, the size of your home, and who does the installation work. 

The cost of foam board insulation depends on whether it's made from expanded polystyrene (EPS) or extruded polystyrene (XPS). EPS has the lower price point, usually costing between $0.25 and $0.50 per board foot, while XPS costs between $0.40 and $0.50 per board foot.