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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.

While there are many different kinds of insulation, spray foam, blown-in, foam board, and batt and roll are the four types you’re most likely to see. That doesn’t mean they’re alike, though. Each one varies drastically in terms of cost, materials used, and how they’re installed—not to mention, how well they limit heat transfer.

Natural fiber batts and spray foam insulation both work great for soundproofing. Newer materials, such as hemp batts, are more expensive but more effective for insulation and sound blocking. They are also more sustainable. There is also a professional system called Blow-In Blanket Insulation (BIBS) that is soundproof and moisture-resistant. However, BIBS must be installed professionally and can be a costly investment.

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.