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For standard 2x4 walls, the ideal insulation has an R-value between R-13 and R-15. This level of insulation, typically around 3.5 inches thick, fits well within the wall cavity. Common options include fiberglass batts and spray foam. The best choice for your home can depend on your climate; use R-value maps, like those from Energy Star, to determine the recommended level for your region. For colder climates, you might also consider adding exterior insulation for greater efficiency. Always check with your local building codes to ensure you meet any required standards.

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.

It’s best not to install a radiant barrier directly under insulation because it requires an air gap on at least one side to work properly. Instead, you should install the radiant barrier over your existing insulation to create an air space to reflect the radiant heat.

Vapor barriers in attics are a bit outdated and most areas no longer recommend them for attic spaces. For vented attics, the best method is usually to install drywall and cover it with latex paint before attaching insulation fiberglass, cellulose, or foam insulation. When renovating an attic to use as a living space, a contractor can guide you on the best way to cover insulation.