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.

The cost to install or upgrade insulation generally ranges from $1.50 to $4.00 per square foot. The final price depends on factors such as the type of insulation chosen, the total area to be covered (home size), and local labor costs.

If you plan on heating your garage, you must install a vapor barrier when drywalling the garage. A vapor barrier protects your garage from frost and mold that can occur through condensation. If your garage is unheated, you don't need to install a vapor barrier. The only exception is if you regularly crank the heat in your car before getting in it. In this case, you may still want to install a vapor barrier, as your car can heat your garage.

Doubling the insulation in your home also doubles the R-value of said insulation. In other words, by adding more insulation, you can reduce the heat loss by half. Most homes benefit from increasing insulation. Older homes, in particular tend to have less insulation as is, so doubling your insulation could drastically reduce your energy bills, keep your home more comfortable, and take the strain off your HVAC system.