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Blown-in insulation works, but only for unfinished attics. Also, it isn’t the longest-lasting nor the most effective option. For better options, look into spray foam insulation or natural fiber batts. They typically have a longer lifespan, better heat resistance, and stronger sound-proof abilities. Spray foam insulation costs about $1.25 to $1.50 per square foot, but can go up to more than $4 per square foot. Fiber batt pricing varies depending on the material.

The best type of attic insulation depends on where you live, your budget, and what stage of construction the attic is in. For example, if you're building a house, then structural insulated panels are a worthy investment since they're easy to install during construction and they're the most energy-efficient type of insulation.  

Additionally, if you live in a very warm climate, then you can potentially save quite a bit of money by opting for reflective insulation, which is more affordable than comprehensive insulation but doesn't work well for cool climates. Blown-in, loose-fill, and spray foam insulation are all great options for attics in moderate climates.

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.

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.