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2x4 walls can hold R-13 or R-15 batts of insulation, which is usually about 3.5 inches thick. You should also check with your local building codes to see if there’s a required thickness you need to adhere to. 

You can also use R-value maps to determine the best thickness and ratings for where you live. For example, someone in Texas doesn’t need as thick of insulation as someone in Montana. Energy Star has a good one you can reference.

If you’re a savvy DIYer around the house, own the basic tools, and have enough time on your hands, you can definitely install fiberglass insulation yourself. Doing the work yourself will certainly keep overall insulation costs down. However, if you’re not confident in your handy skills around the house, call in a professional.

R-value stands for resistance value and is a number assigned to insulation that states how good the insulation is at heat transference (insulation). The R-value of injection foam is around 4 which homeowners will be happy to know is actually quite good. Another benefit of injection foam versus fiberglass foam is that the colder it gets, the more injection foam retains its resistance properties, whereas fiberglass tends to lose its efficiency in colder temperatures.

You don’t need to remove old insulation before adding new insulation. As long as the old insulation is still in good shape—not wet, physically damaged, or moldy—then you can stack new insulation on top of old insulation to get a compounding effect. You can also mix different types of insulation, like adding blown-in insulation around existing fiberglass batt insulation or installing rigid insulation boards over rafters in your attic that already have batt insulation installed. If your existing insulation is very old or damaged, though, you’ll likely see more benefit from removing it and replacing it instead.