Vapor Barriers vs Vapor Retarders | Everything you need to know!

Vapor barriers and vapor retarders can be a very tricky subject matter. Using the wrong product in the wrong location can lead to mold and mildew inside your walls. That’s why vapor control is an important part of building construction and wall assemblies.

Vapor Movement

Vapor diffusion refers to the movement of water vapor molecules through porous materials, typically from areas with high vapor pressure (or warm air) to those with low vapor pressure (or cold air). In cold climates, water vapor moves from the warmer interior to the cooler exterior, while in hot climates, it moves from the warm exterior to the cooler interior. Vapor barriers, such as plastic sheets, prevent rain and vapor diffusion, while vapor retarders slow down vapor movement without completely stopping it.

Understanding Perm Ratings

Perm ratings measure a material’s ability to allow water vapor to pass through. In the American system, a perm rating of 1 means that, in 1 hour, 1 grain of water vapor will pass through 1 square foot of the material when the vapor pressure difference is 1 inch of mercury between the cold and warm sides. In the metric system, a perm rating of 1 means that, in 1 day, 1 gram of water vapor will pass through 1 square meter of the material when the vapor pressure difference is 1 mm of mercury. One metric perm is approximately equal to 1.5 US perms.

Different building materials have varying perm ratings, classifying them into four categories:

1. Class 1 materials: Vapor impermeable or vapor barriers with a perm rating of 0.1 or less. Examples include glass, aluminum foil, cork, rubber or vinyl flooring, and 6mil polyethylene.
2. Class 2 materials: Semi-impermeable with a perm rating of 0.1 to 1. Examples include vapor retarder latex paint, oil-based paint, and foil-faced fiberglass insulation.
3. Class 3 materials: Semi-permeable with a perm rating of 1 to 10. Examples include 3” EPS, OSB or Oriented Strand Board, and fluid-applied vapor retarders.
4. Class 4 materials: Vapor permeable with a perm rating of over 10. Examples include wood siding, Rockwool, and Tyvek.

To summarize, Class 1 materials with a perm rating of 0.1 or less are vapor barriers, while Class 2, 3 and 4 materials with a perm rating of over 0.1 are vapor retarders. 

Applications

In the past, plastic sheets were used behind drywalls, leading to mold issues due to trapped vapor. However, vapor barriers are still used under slab-on-grade foundations, basement concrete slabs, and crawl spaces, protecting against ground moisture.

Vapor retarders that also act as air and water resistive barriers are now widely used on walls. The code requirements for vapor barriers and vapor retarders vary based on climate zones. Warm climate zones may not require or prohibit vapor retarders, while cold climate zones and Marine 4 require either Class 1 or 2 vapor retarders on the interior side to limit vapor movement.

Choosing the Right Approach

It is advisable to avoid plastic sheets and prioritize vapor retarders to allow walls to dry out while limiting vapor movement. Air barriers also play a significant role in moisture control, preventing air infiltration and reducing thermal bridging. Continuous exterior insulation like Rockwool Comfortboard can help achieve these objectives effectively.

Conclusion

Understanding the distinction between vapor barriers and vapor retarders is essential for maintaining a dry and mold-free environment in building constructions. By making informed choices and prioritizing vapor control strategies, construction professionals can create more resilient and sustainable structures, contributing to a healthier indoor environment. Let us simplify this complex and highly debated topic to ensure successful moisture management in your construction projects!