My son is building a house with a cinder block wall on the up-slope side. There will be about five feet of wall contacting the soil (primarily sand with a little clay-- he is on a river in south Louisiana). The exterior of the wall will face the street and the interior will be the finished wall for the workshop/storage area (possibly subject to flooding every several years) with the living area directly above. What is the best method/materials for waterproofing before backfilling? There will be 2-3 feet of wall exposed above ground. He wants to do the project himself.