My Recommendation to you is to build a 2" wall base out of compacted screenings (crusher run), this will "lift" your walls to accomodate the 2" of styrofoam and put everything back on the same level. The compactor will likely run you about 30 bucks for the day, you'll probably need 2-3 yds of screenings since you'll want them to go well beyond the edge of your bottom rail.