Any type of porous building material has the potential to let water vapor pass through it. This might happen very quickly or very slowly. Either way, trapped water is a major problem for home construction.

Moisture from water vapor doesn’t only damage the integrity of building materials, it can cause and exacerbate health problems as well.

The best way to slow the diffusion of moisture through building materials is to use a vapor retarder or vapor barrier, although these terms often get confused with one another.

How do you know when to use a vapor retarder vs vapor barrier? What is the difference between the two? Does it even matter?

You’ll find the answers to all of these questions and more in this vapor retarder and vapor barrier guide.

Vapor Retarder vs Vapor Barrier: Why Does the Difference Matter?

In the construction industry, the terms vapor retarder and vapor barrier get used interchangeably. While they are related, the two terms don’t have the same meaning, and that can make more than a little bit of difference.

Accuracy and attention to detail are essential when designing and constructing a new home. It is critical that teams of builders understand the difference between a vapor barrier and a vapor retarder.

Lack of specificity causes accidents and mistakes to happen. A wall, roof, or floor that gets built with the wrong type of vapor retarder or barrier will trap moisture instead of releasing it. This can lead to many building problems.

Here are the main differences between a vapor retarder vs vapor barrier.

What Is a Vapor Retarder?

A vapor retarder is a general term. It describes a variety of different types and classifications of materials that prevent water vapor from diffusing into a structure.

Vapor barriers are a type of vapor retarder. It completely stops moisture from permeating one surface. Other types of vapor retarders are more permeable, meaning they allow moisture to pass through building materials to some extent.

What Is a Vapor Barrier?

This type of vapor retarder is frequently considered by members of the industry to be a class I vapor retarder. However, according to International Building Code, there is no specific requirement to be considered a vapor barrier.

The most common use for a vapor barrier is to prevent moisture from diffusing through a concrete slab. Vapor barriers often provide an under-slab vapor retarder.

It is essential to use a vapor barrier when dealing with any type of moisture-sensitive floor building material.

An under-slab vapor barrier must have a permeance rating of 0.01 perms. That means that the permeability of the barrier is so low that only one grain of water vapor can pass through it per hour, per square foot, and per inch.

Understanding Permeance

No matter how solid a building material may be, all types of building materials allow some amount of moisture to pass through them. This process is called permeability, or permeance.

The Environmental Protection Agency strongly encourages builders to consider permeance when determining how to control moisture during the home building process.

A vapor barrier or vapor retarder designed to deter moisture from passing through the materials gets a permeance rating. This rating is called a “perm.” The perm rating tells builders how effective the vapor retarder is.

Low perm ratings indicate that water vapor has a very hard time getting through a material. Therefore, a vapor retarder with a low perm rating is effective at keeping moisture out. Like a vapor barrier.

As perm ratings grow higher, it indicates that water vapor is capable of passing through the vapor retarder at a faster and faster rate. 

Vapor Barrier Guide

With the two terms being so similar, it’s easy to see why they so often get confused among builders and architects.

Vapor barriers are the least permeable class of vapor retarders. They have permeance ratings of 0.1 perm or less.

When To Use a Vapor Barrier

A vapor barrier’s job is to prevent and minimize the chance of water vapor diffusing into your building materials. Vapor barriers are important to any type of construction.

It is especially necessary to use vapor barriers when working with highly sensitive buildings. In these types of structures, moisture prevention is integral to the strength and integrity of the building.

You should use vapor barriers when you are working with a structure that requires humidity control or has moisture-sensitive floors. If the building’s purpose is to store goods that are moisture-sensitive, vapor barriers are essential.

In these cases, a vapor retarder would be too permeable. It would not provide adequate protection against moisture diffusion.

A vapor barrier completely prevents moisture from transmitting through the ceiling, walls, and floors of a structure.

Even a project that does not involve a highly sensitive building can benefit from use of vapor barriers. In fact, the additional cost for a vapor barrier vs a vapor retarder is so minimal that vapor barriers have become the go-to choice for builders rather than the exception to the rule.

Types of Vapor Barriers

The most common materials for vapor barriers include polyethylene, high-quality plastic sheeting, and metal film such as aluminum. These materials are all very good at preventing water vapor from diffusing into a surface.

They are also resistant to tears and punctures heavy machinery might inflict during the building process.

Vapor Retarder Guide

Vapor retarders have a permeance rating of more than 0.1 perm, but less than or equal to 1 perm. They are more permeable than vapor barriers, but still offer excellent protection against vapor diffusion when used properly.

When To Use a Vapor Retarder

Since they allow some movement of moisture, it is important to understand when to use a vapor retarder vs vapor barrier during home construction. Vapor retarders work to prevent moisture from moving through walls, floors, and ceilings.

When installed properly, a vapor retarder behaves like an internal barrier of air. It minimizes the amount of moist air that can flow through the home structure.

Types of Vapor Retarders

The types of vapor retarders get classified into three levels. These levels are based on the vapor retarder’s permeance rating. The International Building Code defines the classes of vapor retarders.

Class I Vapor Barrier/Retarder

This class of vapor retarders is what the industry considers vapor barriers. They have a permeance rating of 0.1 perm or less. Therefore, they are considered to be impermeable materials.

Glass, aluminum foil, insulated sheathing, sheet metal, and polyethylene film are all examples of Class I vapor retarders.

Class II Vapor Retarder

This class of vapor retarders has a permeance rating of between 0.1 perm and 1.0 perm. They are semi-impermeable materials.

Some examples of Class II vapor retarders include fiber-faced polyisocyanurate and unfaced expanded polystyrene. Asphalt-backed kraft paper on a fiberglass batt insulation is also a semi-impermeable vapor retarder.

Class III Vapor Retarder

This class of vapor retarders has a permeance rating that is between 1.0 perm and 10 perm. Like Class II vapor retarders, Class III vapor retarders are considered to be semi-impermeable materials.

Latex painted over a piece of gypsum board is one example of a Class III vapor retarder. Other examples of this class include plywood and #30 building paper.

How To Install Vapor Barriers and Vapor Retarders

Once you determine the right class of vapor retarder for the project at hand, you need to know how to install it. The mode of installation varies depending on what type of foundation you’re working with.

Here’s what you need to know about how to install a vapor barrier or vapor retarder in basements, crawl spaces, and slab-on-grade foundations.

Slab-On-Grade Foundations

When working with a slab foundation, builders need to use a vapor barrier to stop any moisture from getting through the slab. They also need to protect against potential gasses from the soil infiltrating the concrete.

The best locations to place the vapor barriers are directly underneath the concrete slab and over the top of the capillary break.

Basements

For homes with basements, it is necessary to install a vapor barrier beneath the concrete slab. That way the basement remains dry and the moisture escapes by draining through the capillary break.

Basement walls also require vapor barriers as well as drainage and waterproofing systems. This is especially important for conditioned basements with insulation.

Crawl Spaces

Because most crawl spaces get built directly above the ground, it is very easy for moisture from the soil to accumulate. Installing a vapor barrier in crawl spaces is essential to keep moisture out of the materials and the home.

The best way to prevent moisture accumulation in a crawl space is to cover the ground with something other than dirt and include a vapor retardant layer as well.

Vapor Retarder vs Vapor Barrier: Takeaways

When do you use a vapor retarder vs vapor barrier? It depends on what type of material you want to protect. A vapor retarder is a semi-impermeable material that protects building materials from accumulating a significant amount of moisture.

A vapor barrier is an impermeable material that protects materials from letting any moisture pass through them. They are similar, but it is important to know the difference so you use the right retarder for your project.

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