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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.

In addition to adding insulation to your attic, you may want to add a radiant barrier. A radiant barrier works by using reflective material, such as foil, on the attic floor. As heat penetrates the roof and enters the attic, the radiant barrier reflects the heat upward and back through the roof. If you receive a lot of direct sunlight on the roof, using both a radiant barrier and insulation in your attic can provide significant benefits.

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.

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.