As you build your home, one of the most important concerns is moisture control. Water can infiltrate your walls from rain and snow, while moisture can also build up inside wall cavities due to condensation forming where cool outside air meets warm indoor air. Water-resistant barriers and house wraps play a key role in maintaining moisture by resisting outside penetration from water while also allowing water vapor to escape.
The walls of residential structures are made up of many different layers to keep the building weathertight, structurally sound, and energy efficient. The diagram to the left shows where each of these layers is and how they work together to protect the interior of your home.
1. Drywall panels are used to create interior walls and ceilings.
2. Wall framing and insulation provide structural support and prevent heating and cooling loss.
3. Exterior sheathing provides a physical barrier, shear strength, and a surface to install the house wrap.
4. House wrap is designed to keep water from infiltrating the walls.
5. Siding or cladding covers and protects the exterior walls of a building.
Water-Resistant Barriers vs. House Wraps
House wraps, rain screens, and water-resistant barriers are terms that are often used interchangeably, which can cause some confusion, as they have similar but different functions. All house wraps are water resistant barriers, but not all water-resistant barriers are house wraps.Water-Resistant Barriers (WRBs) are materials that repel water installed behind siding or cladding as a secondary barrier. There are many different types of WRBs, but they all have the primary function of resisting water infiltration into a building assembly; yet they are porous enough to allow excessive moisture to escape from the stud wall cavity.
Organic Asphalt-Saturated 2-Ply Felt and Kraft Paper were the most common WRBs prior to the introduction of the plastic house wraps in the 1970s. These rolled materials consist of cellulose fibers saturated with asphalt.
Synthetic (woven) plastic house wraps are typically constructed similarly to a coarse fabric or high-strength perforated film that will allow moist air to escape from the stud wall cavity as needed.
Nonwoven house wraps are nonperforated and microporous, allowing water vapor to diffuse through the fabric itself.
Liquid- or Fluid-Applied WRBs are permanent coatings that can be applied by spraying, rolling, or troweling onto sheathing.
Preweatherized Semi-Rigid Air and Water Barrier Panels are structural sheathing laminated with an air and water barrier to the face of the panel that, when properly applied, can eliminate the need for house wrap. Examples: ForceField, Eclipse
House Wrap Materials
House Wrap Materials
An effective house wrap needs to be able to keep water from rain or melting snow from getting inside, while allowing water vapor that forms inside to escape. Woven and non-woven house wraps were developed to address this issue.
Synthetic (Woven) vs. Nonwoven
Synthetic (woven) plastic house wraps, which have tiny holes poked into them to allow moist air to escape, are available in two types:
• Inorganic 2-ply coated strand polypropylene plastic fabric is made of high-density polyethylene woven fabric that is coated on both sides with UV-treated, low-density polyethylene coating.
• Inorganic cross-laminated polypropylene microperforated plastic is made of three or more layers of cross-laminated (plywood-like) film for exceptional tear strength and increased holding power.
Nonwoven house wraps are nonperforated and microporous, allowing water vapor to diffuse through the fabric itself. They are available in two options:
• Inorganic nonwoven polypropylene fabric is made of a unique material that helps prevent the infiltration of air and water, but allows water vapor to escape through the material to prevent rot and mold.
• Drainage nonwoven polypropylene fabric with capillary is also made of material that allows water vapor to escape through it, but it also incorporates a drainage gap. The continuous drainage gap allows water to escape from the wall system quickly.
Rainscreen Systems
Rainscreen systems are composed of 3 parts: a weather-resistant barrier (house wrap), Aqua-Vent™ (which creates 1/4 inch or more of airspace), and the exterior veneer. The airspace allows the wall to dry out quickly, reducing the likelihood of moisture-related issues. It typically comes in a rolled product that is fastened to the wall. Rainscreen walls are commonly regarded as the best type of wall system and are rapidly being incorporated into building codes.
Factors at the Jobsite
Now that you have a better idea of the differences between house wrap and other types of water-resistant barriers, you’ll need to consider other factors when managing moisture in wall cavities.
Siding materials can have a major impact on water buildup and penetration. Surfactants found in materials like cedar and stucco can allow water to penetrate the wall systems. But when it comes to moisture control, surfactants aren’t the only concern that siding and cladding can pose. If siding such as wood and fiber cement is tightly installed against a wall, it has the potential to trap moisture. There should be a drainage plane behind any kind of siding or cladding on a building to prevent moisture buildup.
Climate can have a huge impact on the products you’ll need to control moisture in your walls. For example, if you live in an area that receives 40 inches or more of rainfall annually, you should install a rainscreen. Even if you live in a dry climate, your walls are not immune to moisture buildup. Consult local building codes to ensure that you’re providing adequate drainage for your home.
Exposure, including the orientation of all the walls, overhangs, nearby trees, and buildings, can determine the types of materials you need to keep your home weathertight. Areas that experience high winds or hurricanes may require house wrap products that are designed to handle those particular conditions. Contact your local building authority to see which products they recommend for your building and location.