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Yes, usually you will need some kind of vapor barrier on the exterior walls when the framing is attached to concrete and masonry surfaces.

Yes, basement insulation needs a vapor barrier due to its susceptibility to moisture seeping in from the outside of the house. You should install a moisture barrier before you install insulation or purchase insulation that comes prebuilt with a moisture barrier to avoid water damage in your insulation and costly mold and water remediation expenses. 

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.

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.