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Your Hardwood Floors Need a Vapor Barrier. (Here’s Why)

Hardwood flooring, whether it’s solid or engineered, is an investment that will add both beauty and value to your home. The keyword there is “investment” because it’s not cheap. Both the flooring and the labor to remove the old floor and install the new one will cost you plenty of money and/or time.

One of the most common questions that come up when someone is considering hardwoods is whether or not a vapor barrier (or moisture barrier) is needed with hardwood flooring. Ths short answer is yes, and in this article I’m going to tell you why.

Do Hardwood Floors Require a Vapor Barrier?

Yes, you should always use a vapor barrier when installing hardwood floors, even when installing on the second floor of a house. However, it is important to understand the difference between a moisture barrier, a vapor barrier (retarder), and regular underlayment. Each has its own purpose and use.

Moisture Barrier vs Vapor Retarder. What’s the Difference?

The terms “moisture barrier”, “vapor barrier” and “vapor retarder” are often used interchangeably, but they’re actually different things with different purposes.

When most people say moisture barrier, they’re probably imaging an underlayment with a plastic or polyurethane backing that is truly waterproof. And they say vapor barrier, I believe they are likely imagining something like tar paper or roofing felt.

So what’s the difference between a moisture barrier and a vapor barrier or retarder? The short answer is that they are all classified as vapor retarders but they have different perm ratings.

Based on the perm rating, all vapor retarders are placed into 1 of 3 classes.

What is a Perm Rating?

A “perm” is simply the standard unit of measure used to determine the water vapor permeability of a given material. Materials with a very low rating are considered impermeable to water vapor, while those with high ratings are considered easily permeable.

What are the Different Classes of Vapor Retarders?

The ICC International Residential Code (IRC) defines Vapor Retarder Class1 as:

A measure of the ability of a material or assembly to limit the amount of moisture that passes through that material or assembly. Vapor retarder class shall be defined using the desiccant method with Procedure A of ASTM E96 as follows: Class I: 0.1 perm or less Class II: 0.1 < perm < 1.0 perm Class III: 1.0 < perm < 10 perm

Now, that’s fairly dense and not all that easy to understand, so here’s a table to make this simpler:

ClassPerm RatingDescriptionExample(s)
Class I<0.10Impermeable (barrier)6 mil polyurethane sheeting
Class II0.10 < 1.0Semi-Permeable (retarder)Aquabar B
Class III1.0 < 10.0Permeable#30 building paper, plywood, latex paint

What Vapor Barrier Should You Use for Hardwood Flooring?

The correct vapor barrier depends on the subfloor. If installing hardwoods over a wood subfloor, then a Class II vapor retarder is typical. If the subfloor is concrete, you will use a Class I vapor barrier. It is important to verify your floor’s specific requirements in the installation manual.

Vapor Barriers for Plywood Subfloor

As I mentioned above, when installing hardwoods on a wood subfloor, then you typically use a class II vapor retarder (see table above).

An exception to this may be if you are gluing or nailing with a glue assist since you would typically glue directly to the subfloor. You will need to consult with your flooring’s installation guide for guidance.

You may be wondering why, if moisture is the enemy of hardwood flooring, we wouldn’t use a full moisture barrier when installing over a wood subfloor. The answer is really quite simple.

A moisture barrier will trap moisture that rises up through the subfloor between the barrier and the subfloor. Over time, as enough moisture accumulates, the subfloor could rot and mold could develop.

So even though moisture is the enemy of hardwoods, and they should not get wet, you do not want to completely block moisture rising up through the wooden subfloor. Rather, the purpose of the vapor retarder is to slow the moisture penetration down. This allows the hardwoods time to adjust to changes in moisture.

Because a vapor retarder won’t completely stop moisture from rising through the subfloor, you need to address any issues with excess moisture before you install your hardwood flooring. A vapor retarder will not prevent damage if there’s too much moisture under the subfloor, such as from a wet crawl space.

If you do have excessive moisture in your crawl space, I’ve got some tips on how to remediate that in this article.

Make sure you test the moisture content of your plywood subfloor prior to hardwood installation to make sure it’s within the flooring manufacturer’s guidelines. This is important to not only protect your flooring from damage but also to make sure you don’t void the warranty.

Besides slowing the absorption of moisture, vapor retarders also have the added benefit of trapping dust on the subfloor below them and they allow you to avoid wood on wood contact, allowing for easier installation.

Common Vapor Barriers for Hardwoods on Plywood Subfloors

By far the most common vapor retarder you will find recommended for plywood subfloors is #15 Asphalt Saturated Roofing Felt. Most installation guides recommend it and the NWFA recommends it.

However, I’m not a fan of using #15 roofing felt as an underlayment for hardwoods for a couple of reasons:

  • It has a strong smell that lasts a long time. You’ll be installing tar-covered paper that doesn’t have any sort of barrier over it under your floors. You’re going to smell it, and you’re going to smell it for a long time.
  • #15 roofing felt typically has a perm rating of around 5. This makes it a Class III, or permeable, vapor retarder. If you have moisture under your subfloor, this may not cut it.

The vapor retarder I recommend and personally use is a product called Aquabar B. It’s a Class II vapor barrier that’s easy to use and much easier on the nose. If you want more information, I’ve posted an in-depth review of Aquabar B here.

Vapor Barriers for Concrete Subfloors

Most people think that moisture wouldn’t be a concern when installing hardwood flooring over a concrete subfloor. I mean a concrete subfloor is a big impermeable slab that’s bone dry, right? Wrong.

If you’ve ever seen someone mix concrete then you know that one of the main ingredients is water. And even when the concrete is set and dry, it will easily absorb moisture, either from below it or from humid conditions.

In fact, concrete absorbs moisture from underneath so readily, that both the IRC and IBC require a Class I moisture barrier to be placed under the concrete slab2.

Additionally, once the slabs are poured, they’re often exposed to the elements for weeks before the rest of the house is built. This means that the concrete foundation can get rained on, soaking in more moisture and it’s possible that it will never fully dry out.

So now that you know that concrete can itself be a source of moisture, what should you do to protect your hardwood floors? You probably know the answer – use a moisture barrier.

The National Wood Floors Association (NWFA) strongly recommends the use of a vapor barrier with a perm rating of less than .133, essentially a Class I vapor retarder or vapor barrier.

It is also important to check with your flooring manufacturer to find out if they support a concrete subfloor, above or below grade, with your product and what the moisture requirements are. For example, Shaw supports concrete subfloor with their engineered products and requires that the moisture content after a calcium chloride test be 3 pounds per 1000 square feet or less4.

Once you’ve determined that concrete subfloors are acceptable for your hardwoods and the moisture content is in compliance with your hardwoods’ requirements, you need to determine how you will install them and what to use for a moisture barrier.

While you can nail hardwoods to a concrete subfloor, it’s not the method I would recommend. One issue is that it’s difficult. The other is that you’ll be continually puncturing the vapor barrier, and I don’t recommend that when dealing with concrete.

I recommend that you either use a floating floor (preferred) or glue down installation method. Each has it’s own vapor barrier requirements.

Vapor Barriers for Floating Hardwoods on a Concrete Subfloor

When floating hardwoods over concrete, you have quite a few options for moisture barriers. The key is to use a product that has a polyurethane film to actually block vapor barriers and act as a barrier. Below are some common options.

6 mil or better polyurethane sheeting

Pros:

  • Cheap class I vapor barrier
  • Easy to use
  • Allows for immediate floor installation

Cons:

  • Does not offer any padding or sound control for floating floors

Two-in-One Type Underlayment

When I say “two-in-one”, I’m referring to an underlayment that provides both a moisture barrier as well as sound control for floating engineered hardwood flooring over concrete. These have the added benefit of providing sound cushioning as well as protecting your engineered hardwoods from moisture.

The key consideration is where you’re installing your hardwoods. Some of these are rated for below grade (in the basement), while others are not.

I recommend making a trip down to your local home improvement superstore as they tend to carry quite a few of these.

Pros:

  • Sound muffling and moisture barrier in one underlayment

Cons:

  • More expensive

Vapor Barriers for Gluing Hardwoods to a Concrete Subfloor

The most common vapor barrier used when gluing hardwood flooring to concrete is a 2 component epoxy system. One example of this is Titebond 531 Moisture Control System.

These products are spread over the concrete subfloor and help seal them to prevent moisture from rising through the concrete and penetrating the hardwood floors. It’s very important to follow the directions carefully to ensure proper protection.

It’s also critically important to make sure you’re using a product that will maintain your floor’s warranty.

Do You Need a Vapor Barrier on the Second Floor?

You should always use a vapor retarder when installing hardwoods, even on the second floor of the house. Many manufacturers require it in order to maintain the warranty. Using a vapor retarder also eliminates wood on wood contact making the flooring easier to install and reducing potential squeaks.

The bottom line is, why wouldn’t you? If you’re going to invest in hardwoods, spending a couple of hundred dollars on a vapor retarder will probably not be a deal killer. The second floor is also a good place to consider a padded underlayment to help muffle the sound of people walking on the hardwoods.

Sources:

  1. http://media.iccsafe.org/Annual/2016/Vapor-Permeability-of-Materials-and-Assemblies-Determining-the-When-and-Where-of-Vapor-Retarders.pdf
  2. https://hardwoodfloorsmag.com/2017/03/28/subfloor-focus-minimizing-moisture-part-2-use-vapor-retarders-concrete-subfloors/
  3. https://nwfa.org/wp-content/uploads/2020/03/Concrete-Subfloors_Updated.pdf
  4. https://pdmsview.shawinc.com/viewer/doc/3776