Vapor Barriers and Vapor Retarders — What’s the Difference?

No matter how solid any form of building material may be, a simple fact remains: Water vapor, albeit minimal, will be allowed to pass through the material.  Now, this may occur quickly or slowly, but this is a situation that is present, nonetheless.   Ultimately, this is referred to as permeability—or the material’s permeance—and both vapor barriers and vapor retarders are scored through a rating system, known as a “perm,” that is centered on permeance and permeability. Low perm ratings mean that any water vapor is going to be faced with difficulty when trying to get through the material. On the flip side, a higher perm rating means the opposite.   Perm is assigned both to vapor barriers and vapor retarders, which probably also lends to the confusion regarding just what the actual difference is between these two things amongst members of the building, architectural, and design community.

In this article, we will explain the differences and also provide you with some insight regarding usage in order to help you better understand.

VAPOR BARRIER vs. VAPOR RETARDER

Let’s first look at the terms used in the industry. Is it a vapor retarder or is it a vapor barrier? This is a difficult question to answer, of course, as these terms are oftentimes used interchangeably in the construction industry—which is not correct.

For example, within ASTM E1745: Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs, a vapor barrier is defined as, “A material or construction that impedes the transmission of water vapor under specified conditions.”

This, unfortunately, does not define the term vapor barrier—and doesn’t clarify much of anything either.

So, according to the Building Science Corporation, “A vapor barrier is defined as a layer with a permeance rating of 0.1 perm or less.”

And ACI approaches this a little differently still.  ACI 302.1R: Guide for Concrete Floor and Slab Construction states, “Several vapor retarder materials have been incorrectly referred to and used by designers as vapor barriers. True vapor barriers are products that have a permeance (water-vapor transmission rating) of 0.00 perms when tested per ASTM E 96.”

Then, in ACI 302.2R: Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials, it is stated, “Historically, the construction industry used the term vapor barrier to describe a polyethylene-based material below a concrete slab. Polyethylene, however, does not completely stop the transmission of water vapor. These products only reduce or retard water vapor transmission. It was therefore considered more appropriate to call these products vapor retarders instead of vapor barriers.”

The same ACI committee has made a recommendation that, “If it is determined that a vapor retarder meeting the ASTM E1745 for vapor permeance requirements is not sufficient to protect the flooring material to be installed, then a vapor barrier with a perm rating of .01 or less should be specified.”

While this statement implies that a material with a 0.01 or less perm rating may be considered a vapor barrier, it does not define a vapor barrier to be a product that has a permeance of less than 0.01 perms.

Are you Confused Yet?

We get it, this is all a lot to process. So, therefore, think about this. When we start to look at categorizing vapor retarders, materials can be separated into three general classes based on their permeance and are defined in the International Building Code (IBC).

• Class I vapor barrier/retarder: 0.1 perm or less

This covers most materials that are usually referred to as vapor barriers. When there is a 0.1 perm rating or less, the membrane is considered impermeable.  Some examples to consider here are polyethylene film, glass, sheet metal, foil-faced insulated sheathing and nonperforated aluminum foil.

• Class II vapor retarder: 1.0 perm or less and greater than 0.1 perm

These membranes have a permeance level that equals between 0.1 perm and 1 perm.  As such, they are considered as semi-impermeable products.  Some examples here include unfaced expanded polystyrene, fiber faced polyisocyanurate, and asphalt-backed kraft paper facing on fiber glass batt insulation.

• Class III vapor retarder 10 perms or less and greater than 1.0 perm

This particular class has a permeance rating between 1 perm and 10 perms; as such they are also considered semi-permeable.  Some examples include latex paints over gypsum board and #30 building paper and plywood. The 2006 International Energy Conservation Code (IECC) states specific conditions where Class III vapor retarders happen to be allowed. The IECC notes, “When design conditions exist that promote drying through the use of ventilated claddings or reduce closed cavity condensation potential through the use of exterior insulating sheathings.”

• Anything above 10 perms is defined as a vapor-permeable membrane.

Ultimately, what is a vapor barrier? Many people in the industry typically define it as being a Class I vapor retarder. However, when looking at the current International Building Code (and its derivative codes), there are no specific performance requirements when it comes to under slab vapor protection. The term vapor retarder is still used. Section 1907.1 of the 2012 IBC calls for 6-mil polyethylene or “other approved equivalent methods or materials shall be used to retard vapor transmission through the floor slab.”

Classifications like this make it easier for a building and design professional to assess and determine what type of vapor barrier or vapor retarded to be used in the project—and much in this regard, has to do with regional climate factors, which we will discuss shortly.

So, what should be done?

We need to get away from using the term vapor barrier and vapor retarder and actually define what performance characteristics the material is required to meet based on the actual project requirements. This is worded nicely within ACI 302.1R, “The committee recommends that each proposed installation be independently evaluated as to the moisture sensitivity of subsequent floor finishes, anticipated project conditions, and the potential effects of slab curling, crusting, and cracking. The anticipated benefits and risks associated with the specified location of the vapor retarder should be reviewed with all appropriate parties before construction.”

Regardless of the term, it is the function of the material that needs to be evaluated – to control the entry of water vapor into the building by vapor diffusion.

What is the Job of a Vapor Barrier—and a Vapor Retarder?

The primary job of a vapor barrier is centered on preventing and minimizing the chance of vapor diffusion. It is a thin layer of impermeable material—usually polyethylene sheeting.  A vapor barrier is used during the construction of a building to prevent moisture damage to the building’s fabric.

Now, a vapor retarder is defined by the International Residential Code (IRC) as “a vapor-resistant material, membrane, or covering with a perm rating of 1 or less.”

That definition might not make things any clearer to you, frankly. So, let’s talk about their individual uses—because this is pertinent to the way a structure is built.  Assembly that calls for a vapor barrier is focused on stopping moisture on one surface—for instance under a concrete slab.  And this is what is key. Vapor barriers bar the transmission of water vapor through walls, ceilings, and floors.

Vapor retarders, while they typically also do have low perm ratings, are still not as low as what is required in a vapor barrier. Vapor retarders can allow some moisture movement, and the last thing anyone wants is to use the wrong vapor-retarding products on walls, roofs, or floors.  Vapor retarders are designed to impede moisture flow through the wall assembly and protect the building envelope from damage done by condensation. So, when one of these is properly installed, it also acts like an interior air barrier and will minimize the flow of moisture-laden air during cold weather months into insulated cavities within the building.

How Climate Impacts the Decision-Making Process

Climate is a key consideration when deciding on the selection and positioning of vapor barriers or vapor retarders in the exterior wall assembly.  Think about the following points:

• When a builder is faced with a colder climate, a vapor retarder should be placed on the building envelope’s interior.
• A Class I vapor barrier or vapor retarder should not be used in climates with high summertime moisture loads. Additionally, these products shouldn’t be used on building envelopes with moisture storage claddings, like concrete or brick.  They also shouldn’t be used in building envelopes that have low-permeability exterior sheathings, like extruded polystyrene.
• When dealing with a marine or mixed-humid climate, one must first determine whether the climate is dominated by heat or cold. If the construction is in a hot climate, the vapor retarder should be placed on the interior. If it is cold, it should be placed on the exterior or completely left out.
• Also, in a mixed-humid climate, low-permeance poly film or aluminum foil should not be used.
• In a mixed-dry climate, a vapor retarder is usually not even required most of the time. Rainfall is light and humidity is also usually low. Of course, it’s key to check the local building code, regardless. A vapor retarder might be required to be installed on the interior.
• In a hot, humid climate, it’s recommended that a vapor retarder be placed at the exterior.
• In hot, dry climates, a vapor retarder is not required. Again, it is still necessary to check on local building codes to ensure you remain compliant in your building processes.

At the end of the day, when a vapor barrier or vapor retarder is not used correctly, there will be an increase in moisture-related problems that are experienced by a building or structure.  It is absolutely critical to control and manage the movement of moisture in buildings and structures.   And in turn, vapor barriers and retarders can serve to be an incredibly important part of a comprehensive moisture management solution.  Remember this is about air movement and moisture flow—these two factors are intertwined and connected.  They must be managed effectively, no matter what.

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W. R. MEADOWS is widely recognized as an innovator in the vapor barrier and vapor retarder field. We have created some of the toughest products in the industry and would love to help you understand what vapor barrier or vapor retarder is best for your construction project.  We invite you to reach out to us today to learn more about our product line and to arrange an individualized consultation. Our team of experts cannot wait to hear from you! Contact us now.

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Vapor Barriers: Leading-Edge Critical Building Protection
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