When Is a Vapor Retarder (Barrier) for Insulation Needed?

It’s well-known that when it comes to buildings, even a little water can create big problems when it gets into the building materials. While there’s virtually no way to create a totally impermeable building envelope, keeping water out is maybe the most important aspect of construction (that and keeping the building structurally sound). While the impact of water can be seen immediately, water vapor (the gaseous form) becomes water when it meets a condensing surface. This is also really bad when it happens inside walls, ceilings, flooring, or even on insulation. Vapor barriers, when properly placed, help protect insulation and other building materials, as well as support better Interior Air Quality (IAQ).

What Is a Vapor Barrier, or Vapor Retarder for Insulation?

The purpose of vapor barriers and retarders is to keep water vapor from entering the materials by creating a mostly impermeable barrier between the outside of a building and the interior. While a true vapor barrier is completely impermeable (rated 0.0 perms), what most people are actually looking for and getting installed is a vapor retarder, which retards the transmission of water vapor under specific conditions.

When Is a Vapor Retarder for Insulation Needed?

Think of your vapor retarder as a control layer. The purpose of this control layer or vapor retarder is to keep moisture in the air from passing through the insulation or an assembly by creating a mostly impermeable barrier. Since moisture, or water vapor, generally goes from hot to cold, or from high concentrations to low concentrations, your climate zone typically determines the direction you are trying to stop the moisture from going.

One thing to note is the type of insulation also determines if a vapor retarder or vapor barrier is needed. Most closed cell spray foams become a vapor retarder at around 1-1 ½”. Fiberglass batts and rolled basement or building insulations are often made with a facer designed to be a vapor retarder. Materials used for blown in walls, are generally covered with poly or a smart membrane when installed in areas requiring a vapor retarder.

How and Where Vapor Retarders are Placed is Important.

Due to thermodynamics, warm air is always trying to move to colder spaces. With that warm air comes water vapor. Since cold air cannot hold as much moisture, that water needs somewhere to go. Without getting into dew points and how all this happens, the easiest explanation is that the water vapor in the air, condenses or collects on the coldest surface available. This leaves two choices.

  1. Keep the moisture from getting in.
  2. Make sure there is no cold surface in the wall or ceiling cavity.

For choice number one, you have several options. In the south, the idea is to have walls dry to the inside. In these climates, vapor retarders are often used on the exterior of the building. Once water vapor or moisture is inside the building, the mechanical or air conditioning is where water in the air gets removed from the home. This is why we don’t want a retarder on the inside preventing these assemblies from drying inward. This means no vapor retarder on the inside. That said… if there is a crawl space, having a liner or retarder underneath the building is critical. Left uncontrolled, these spaces can generate a lot of moisture resulting in buckled floors and many other problems.

In the North, the warm moist air is inside and trying to escape through the walls & ceilings, or any path it can find. While most projects in the past have simply used faced fiberglass batts or poly sheeting over the walls and ceilings, today there are many more options that offer even more control over the wall drying and how you control moisture.

Choice number two requires a lot more explanation and comes with a higher price tag as well as requiring that the surface of your air/vapor impermeable insulation stays above the dew point. Think closed cell spray foam, foam board, sealed panels, etc. For more information on this, see IDI video on Hybrid walls.

Vapor Barriers from IDI

Beyond providing the best insulation for contractors across the U.S., IDI has a variety of wholesale polyethylene vapor barriers or vapor retarders available for any type of climate you may be working with. Learn more about our polyethylene vapor barriers or contact your local representative to find the best product for your needs.

For more information on vapor retarders and how they relate to our industry you can also read this blog from the ICAA.


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