The walls should be properly wrapped in a house wrap. Use a quality product like Tyvek or Typar, and use proper installation techniques and sealing tapes. I'm sure there is plenty of info. to be found on either manufacturer's websites. Good housewrap isn't cheap, but it can make a stunning difference in this case. It's all about the details. If the stuff is installed right, it works well. If it looks like two drunk monkies with a slam stapler nailed it up, it just a waste of time and money.