We all know that moisture is the enemy of wall systems, but understanding how it moves and changes—and how to manage that movement—is a bit more complicated. Digging into the science of how water and water vapor move is a helpful first step to designing and building wall envelopes that best manage that challenge.
Here’s a look how water vapor moves through wall systems and why.
What Is the Problem with Vapor Diffusion?
Vapor diffusion is the net movement of vapor molecules (which are gas-phase molecules formed through evaporation or sublimation) from an area of higher concentration to an area of lower concentration or from a warm area to a cooler area.
Though moisture vapor isn’t as big of a problem as rain or other bulk water entering the envelope, it’s still something that should be addressed. When water vapor passes through an assembly and comes in contact with a surface that has a temperature below the dew point (the temperature at which water vapor condenses), it becomes liquid, which on wood sheathing can lead to rot, mold, and decay.
For example, in cold climates, when there is hot, humid air inside and cold, dry air outside, moisture can build up on the inside of the wall system’s exterior sheathing and condense into water droplets. The amount of moisture inside a home can increase due to normal, everyday tasks and items such as cooking, bathing, talking, plants, and other factors.
Unventilated attics can also be a problem area: warm, humid air rises into the attic and has nowhere to go; if there is a vapor-closed membrane in the roof system, such as a self-adhered ice and water shield, synthetic underlayment, or closed-cell spray foam insulation, it will trap that moisture.
How Do You Manage Water Vapor in the Building Envelope?
The various materials that make up the building envelope each manage vapor differently. Those that are “vapor permeable,” let vapor pass through and dry out. Those considered “vapor retarders” or “barriers” slow or stop vapor movement.
Drywall, for example, is semi-permeable; vapor can pass, but does so slowly. Polyethylene sheeting is a vapor barrier—vapor can’t pass through at all.
What Is a Vapor Retarder?
A vapor retarder is a material that regulates moisture in a gas state through limiting water vapor movement. A vapor retarder helps prevent water vapor from moving into building assemblies, like the exterior wall cavity, where it can condense into liquid water within the structure.
- Class I vapor barriers (0.1 perms or less): sheet metal, glass, polyethylene sheet, and rubber membranes.
- Class II vapor retarders (more than 0.1 perms and equal to or less than 1.0 perms): extruded polystyrene or unfaced expanded, 30-pound asphalt-coated paper, bitumen-coated kraft paper, and plywood.
- Class III vapor retarders (more than 1.0 perms and equal to or less than ten perms): concrete block, gypsum board, board lumber, brick, 15-pound asphalt coated paper.
The 2021 IRC and IBC mandate Class I or II vapor retarders inside framed walls in Climate Zones 4, 5, 6, 7, and 8; Climate Zones 1, 2, and 3 do not need vapor retarders.
A vapor retarder should be installed on the inner side of the wall or roof near the warm side in winter.
VaporWise™, new from Benjamin Obdyke, offers a smart solution to interior vapor control: It’s a vapor-variable interior membrane that during winter acts as a Class 2 vapor retarder, helping to prevent interior moisture from entering the wall cavity; in the summer, the membrane’s permeability increases, allowing moisture to escape and ensuring a dry, healthy wall cavity year-round. (Learn more about VaporWise here.)
Think About the Vapor Profile of the Building Envelope
Building and design pros have to choose the right materials based on the climate zone of the build and where drying is expected to happen. Understanding the full wall assembly is vital to the performance and longevity of all materials as the vapor pressures change between winter and summer in all U.S. climates.
Preventing vapor diffusion is more than just a single component. We must think about the “vapor profile”—the vapor permeability of each component in the wall or roof assembly. Green Building Advisor recommends examining the vapor profile in four steps:
- Determine the vapor permeability of each component in the assembly
- Identify the least vapor permeable component(s) (also known as the “vapor throttle”)
- Assess the extent and direction of the vapor drive
- Assess the moisture storage capacity and drying potential of the assembly
This method is not exact, so more definitive answers may be found by consulting a building scientist or using a quantitative analysis tool.
A Systems Approach to Vapor Science
Managing water vapor is no different than controlling bulk water and air infiltration—it requires approaching the wall and roof assemblies as a system, where each component must be specified and installed with the others in mind. Benjamin Obdyke takes a systems approach with all of its products and offers system warranties along with individual product warranties.
If you’re looking for guidance on how to get started, sign up for a virtual jobsite visit today.