U20's were built primarily with balsa core - however Ron Moore built the first 30 hulls and I know that mine had balsa water line down and foam water line up in the hull. And the deck had balsa core.
I know that there are a few other random U20's that have foam coreing at some level.
Jim Antrim has been consulted pretty much by every builder to ensure they have the right cook book plan for the layup and I am a firm believer that this has created solid consistant boats built by all the different builders over the years. Jim gets excited every time the U20 demand pulls another builder into the process.