Liquid Water vs. Vapor

To understand how weather-resistant barriers work to impede water intrusion, it’s important to distinguish between liquid water and vapor.

Moisture vapor, composed of water droplets in a vaporous form, is very small and invisible. Water molecules, in liquid form, have a stronger attraction to one another, making movement through space different than that of gases. WRBs are water resistant because the pores (spaces) in the sheet material are too small for liquid water to pass, but with pores that are large enough to allow moisture as a vapor to pass through. Thus they are said to “breathe.” WRBs need to be breathable to allow interior moisture to escape and to allow for drying if the building assembly gets wet. The advantage of a wrap product over building paper is that it is more breathable, more durable, and the seams can be easily taped to create a continuous water and air barrier system.

Building codes require WRBs under all cladding systems to prevent water penetration into building assemblies. The type of WRB is not specified in the codes. WRBs can be as simple as building paper or as be as multifaceted as a high-performance drainage wrap.

The function of a WRB or building wrap is to:

1. Create a weather barrier behind exterior cladding to protect the sheathing and aid in reducing water intrusion into the wall cavities.
2. Promote fast and efficient drainage of water out of the assembly.
3. Provide a vapor-permeable membrane that allows moisture trapped in sheathing to escape.
4. Provide a vapor-impermeable membrane that stops moisture ingress to sheathing.
5. Be an energy-efficient air barrier to stop air infiltration and exfiltration through walls.

Building Standards Require Water Tight Performance

Codes and building standards have progressively gotten stricter as a result of the increase in water-related damage in homes.

The 2009 & 2012 International Residential Codes (IRC) state:

The building code is very clear in its requirements to not only use a weather-resistive barrier but also to provide a drainage path for water to drain from the wall assembly.

Water management is also a significant component of green building programs because longer lasting, more durable homes and buildings equates to lower greenhouse gas emissions because less resources and energy are used to repair and replace them. “The greenest building you can build is one you don’t have to rebuild,” says building science consultant Steve Easley.

A Word about Air Barriers

Air barriers are used to control air infiltration and air exfiltration. Air barriers save energy and reduce the potential for moisture problems. Air leakage in buildings pushes out conditioned air requiring additional energy to reheat or cool the air. A typical home has more than a half-mile of cracks and gaps. Air barriers help save energy by reducing the buildings air exchange rate. Properly installed wraps can save between 15 and 20 percent on space conditioning costs.

Air barriers are also an important element in managing moisture. Air contains moisture, and when moisture vapor comes in contact with cold building materials it can be absorbed by those materials and even condense. This increases the moisture content of building materials, or local condensation, and can lead to mold, corrosion of steel structure or accessories and fungal decay in cellulotic materials. Air barriers reduce the flow of moisture-laden air in and out of structures thus reducing the potential for moisture accumulation.

Vapor Barriers and Retarders

It’s important not to confuse building wraps with vapor barriers/retarders. Vapor barriers and vapor retarders, are required by codes in the colder climates zones. They are predominantly installed on the warm side of the insulation. The original intent was to prevent moisture-laden air from entering conditioned living spaces from diffusing through the wall finishes and insulation and then condensing in wall cavities on cold surfaces.

The reality is that under normal circumstances, there is not much moisture flow by diffusion. Since vapor barriers and retarders are typically not sealed, and are installed with many gaps and voids, they are often not very effective at preventing moisture vapor from entering wall cavities.

The primary mover of moisture into wall cavities is air exfiltration/infiltration. Many building science experts believe that vapor barriers cause more problems than they solve because they reduce drying potential to the inside and trap moisture. Unlike vapor retarders, some WRBs are more vapor permeable, so that moisture can readily diffuse through the wrap and allow wet building assemblies to dry.