We will use 15 pound roofing paper, Tyvek® DrainWrap™ or a product similar to it.

Tyvek® DrainWrap™ helps provide enhanced drainage behind claddings such as primed wood (all six sides), fiber cement siding, and foam board applied over flat substrates.  Vertical grooves on the surface of Tyvek® DrainWrap™ make it a superior moisture barrier, engineered to channel bulk water away from wall systems and drain safely to the outside. The   unique material structure of Tyvek® DrainWrap™ helps hold out bulk water, while allowing water vapor to pass through it, promoting drying in the wall system, which can help prevent mold and water damage. The unique non-woven fiber structure of Tyvek® DrainWrap™ also helps prevent air movement through the walls, contributing to a more energy efficient home. [Source: Dupont].

However, a common practice encouraged by the International Building Code is to install a vapor barrier, in the form of polyethylene (visqueen) sheeting between the wall framing and the drywall. The problem occurs as moisture vapor migrates from the inside of the home to the exterior. This moisture and/or vapor will become blocked by the waterproof visqueen, potentially trapping water or condensation behind the drywall. This trapped moisture will feed any available mold spores, causing mold to grow. This is why we use Class I Vapor Retarder paint, which is also encouraged by the International Building Code and is optional to the standard use of visqueen inside the walls. By placing the vapor barrier on the inside face of the drywall via paint, the potential for condensation buildup behind the drywall is eliminated along with the food source for molds and mildews.

Another problem with the use of visqueen vapor barriers in walls behind the drywall: When masonry sidings, including brick, block and cement board claddings get wet, they are porous and collect moisture. In a perfect world, they would dry out when the sun warms them and no problems would occur. However, in warm weather, many houses have air conditioning – cooling for the warm weather. Air conditioning creates a lower pressure inside the home than the air outside. So, when it is warm outside and the air conditioner is running inside, the moisture stored in the masonry and cement sidings will move to the inside of the house and, once again, get trapped by the visqueen on the walls. In this situation, there are usually generous buildups of condensation and it does not take long for mold and mildew to buildup on the framing members – out of sight and out of mind.

For this reason, it is strongly advised NOT to install visqueen on the walls of houses with air-conditioning.