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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.

Ductwork should be wrapped in insulation in certain locations. This includes unconditioned interior parts of the home, such as attics, crawl spaces, basement ceilings, and between floors. Duct joints should also be insulated since they're more likely to leak air than other parts of your system. Building codes in some areas also require ductwork to be insulated.

Insulating windows without plastic is usually the better method. Outside of simply replacing your windows, you could buy cellular shades, which trap air between the window and the home, add a window film that blocks UV rays and traps heat, or cover your windows with a low-emissivity coating called “low-E.”

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.