If you need to insulate behind finished walls, the best option will likely be to have an insulation professional install blown-in insulation. The installation requires small holes cut in your wall, after which the installer can blow loose-fill insulation into the void. After the installation, you’ll have minor drywall patching, which shouldn’t cost more than $100. Compared to ripping down the existing drywall, using a different type of insulation, and replacing and finishing the drywall, this should save you hundreds or even thousands, depending on the size of the room you’re insulating.

Radiant barriers and insulation serve different purposes, so one is not inherently better than the other. Radiant barriers reflect radiant heat, helping keep spaces cool, especially in hot climates, while insulation slows heat transfer through conduction. For optimal energy efficiency, it’s best to use both together. 

Radiant barriers can enhance the performance of insulation by preventing heat gain in the summer and heat loss in the winter. The choice between them depends on specific climate conditions and the building’s design requirements.

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.

Insulation can be easy to install, depending on the condition of your walls, but there are many safety considerations and potential challenges involved. For example, most types of insulation are made from materials that can cause irritation if inhaled or if they come into contact with your skin. Additionally, installing insulation incorrectly can actually make moisture issues in your home worse.