Moisture

• Moisture can be ruinous to a wood-framed home. Moisture inside the walls encourages mold to grow, creating a potential health hazard for the people who live in the home. Moisture also promotes wood rot, causes studs and joists to warp, attracts bugs such as centipedes, and reduces the effectiveness of insulation. Unfortunately, no exterior covering material is impervious to water; some moisture is always going to get behind the siding.
  
  

Tar Paper

• Builders used to waterproof homes mostly by wrapping the outside of the structure in tar paper, also known as building felt. Heavy-duty paper treated with tar, this product repels water effectively, which is why it's still commonly used in roofing. The problem is that while tar paper can keep water out, it also traps water in. Water vapor inside the walls can't escape to the outside, leading to moisture damage. Some builders still use tar paper on the exterior walls, mainly because it's cheaper than house wrap.
  
  

House Wrap

• House wrap solves the moisture problems associated with tar paper. House-wrapping membranes are designed to repel water while allowing water vapor to pass through. Water that penetrates the house's exterior covering will collect on the wrap and run down to the base of the wall, where it can escape to the outside. Meanwhile, humidity inside the walls can escape through the wrap. Water vapor can travel the other way, too, of course, going from the outside to the inside. But that's OK. What's important is that the humidity doesn't get trapped in the walls, where the moisture can condense and do damage.
  
  

Air

• A secondary benefit of house wrap is as an insulating material. Though the wrap is not airtight, it limits air flow. That reduces heat exchange between the inside and outside of a home, which, in turn, can lead to lower utility bills.
  
  

Installation

• In a fact sheet prepared for people building homes in coastal areas, the Federal Emergency Management Agency and the U.S. Department of Housing and Urban Development warn that installing house wrap improperly can be worse than not using it at all. House wrap is properly installed in horizontal rows, from the foundation up, with the bottom of each row overlapping the top of the row below it by at least 6 inches. All seams need to be covered with protective tape. Installers must take special care around windows and doors to make sure they don't leave gaps where water could get behind the wrap.