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.

Improves water efficiency, reduces oversight, and enhances landscape health.

Spreading tin foil over your windows works similarly to radiant barrier insulation, but not as well. It will reflect some of the sunlight that would otherwise come through the glass and warm up the inside of your home, so it might help a little to keep your home cool in the summer. However, the appearance isn’t preferable to blinds or drapes that can offer the same benefits. You might also consider tinting your windows instead, which is a more aesthetically pleasing change that can still block a good portion of the solar heat coming through your windows.