How to Fix and Prevent Subfloor Moisture Problems

Parquet wood flooring that doesn't have any subfloor moisture problemsImagine this: you install a gorgeous hardwood floor. Your client is so happy with the new flooring, and so are you.

Sounds great, right?

But imagine that a few months later, the homeowners call you and tell you that the finished floor is cupping. You took note of the equilibrium moisture content (EMC) of the job site, and you were careful to purchase wood at the proper moisture content.

What could’ve caused this outcome? One possibility is your subfloor.

A subfloor is usually made up of wood or a concrete slab, and it’s what goes underneath the floor of your home.

If the subfloor, both wood and concrete, is too wet, the excess moisture could seep into your floor and cause all kinds of issues, like cupping, warping, and more.

This isn’t the outcome anyone wants. It causes dissatisfaction for your client, and it causes you to put more time and work into a project.

But the good news is with the right knowledge and the proper steps, you can avoid subfloor moisture issues and, in turn, ensure that the floors you installed stay beautiful and functional for years to come.

Keep reading, then, to learn all about

What can cause subfloor moisture issues?

An air vent that is helping to prevent moisture problems in the subfloorSubfloor moisture problems, like cupping or warping of floorboards, can occur for a variety of reasons, and all of them have to do with a lack of moisture control.

In this post, we’ll be highlighting some common ones:

  • Building not enclosed
  • Lack of a vapor barrier
  • Untested subfloor material
  • Unidentified moisture source

Building not enclosed

During construction, if the building isn’t enclosed or the HVAC isn’t installed, the subfloor doesn’t have a chance to dry properly because it’s exposed to the elements.

This may have kept the subfloor from releasing moisture and reaching the proper moisture level.

Lack of a vapor barrier

A vapor barrier is an underlayment, such as plastic sheeting, that goes beneath the subfloor prior to installation.

If a subfloor is installed without a vapor retarder, then moisture from the ground can be absorbed by the subfloor. Even if the subfloor has been properly tested moisture could move from the ground, through the subfloor, and into the wood floor.

Untested subfloor material

Not testing a subfloor’s moisture condition puts you at risk of having subfloor moisture issues. Without testing, there is no way of knowing if the moisture condition of the subfloor is appropriate for a flooring installation.

Thus, one cannot reliably determine if the subfloor has been allowed adequate drying time, or if it is too wet for safely proceeding with the flooring installation.

Unidentified moisture source

Sometimes, the source of moisture might not be obvious, such as a leaky pipe, improper plumbing, or water vapor coming up from the ground. Any of these moisture sources can cause subfloor moisture problems.

Now that we’ve discussed some common subfloor moisture issues, let’s talk about how to fix them.

How to fix subfloor moisture issues

While preventing a moisture issue is always easier than solving it—we’ll get to prevention in a second—there are ways that you can deal with moisture issues in your subfloor once they happen:

  • Identify the source of moisture
  • Allow the subfloor to dry
  • Add a vapor barrier
  • Use a moisture mitigation system

Identify the source of moisture

The first thing you should do when attempting to fix subfloor moisture issues is to identify the source of moisture. This is because, depending on the source, the solution to the problem might be different.

For example, if the subfloor is absorbing ground moisture, a vapor barrier should be installed. But if there is a leak, the best thing to do is find the leak and stop it.

Allow the subfloor to dry

This sounds like the obvious solution, and that’s because it is. If the subfloor is wet and causing problems, drying it out may stop those problems.

However, this often requires pulling up the current flooring and letting the subfloor dry to match the ambient conditions. So, while it certainly makes sense, this fix can also be time-consuming and expensive.

You can speed up this process by placing several fans in the room as well as using a dehumidifier. Also, ensure that the air conditioning is working well and keep it running while the subfloor dries.

Add a vapor barrier

Adding a vapor barrier entails adding an underlayment below the subfloor in order to keep ground moisture from seeping into the subfloor.

If you suspect that the moisture issues are caused by water vapor coming from the ground, this can be an excellent solution.

How do you tell? This will manifest in similar ways as other moisture problems would: with cupping, crowning, or warping of your floorboards. If no other sources of moisture are present, then it’s likely coming from the ground.

Keep in mind that adding a vapor barrier is mostly a preventative measure.

In other words, it’s best done prior to subfloor installation—especially if the subfloor is concrete—unless there is a crawl space beneath the subfloor.

If there is a crawl space, then you may be able to install a vapor barrier.

If not, then a moisture mitigation system is another option.

Use a moisture mitigation system

Using a moisture mitigation system requires that the flooring be removed. Once the existing flooring is pulled up, a barrier can be installed between the subfloor and the flooring to eliminate the possibility of moisture traveling from the subfloor to the floor.

Repair or replace your subfloor

An old, damaged floor with wood that is chipping offIt’s possible that long-term exposure to moisture can damage the subfloor so much that it
may be moldy or rotted and might even have to be replaced.

If the subfloor ever shows mold growth, or if it’s a wood subfloor that’s beginning to flake, it’s time to replace the subfloor.

If only one part of the subfloor was damaged, replacing that section alone might also be a good solution.

While all of these are possible solutions to a pre-existing problem, the best way to treat subfloor moisture issues is to totally prevent them.

Here’s how.

How to prevent subfloor moisture issues

All of the methods of treating subfloor moisture issues that we’ve discussed—identifying sources of moisture, allowing the subfloor to dry, adding a moisture barrier (either under or above the subfloor, depending on the material), and using a moisture mitigation system—are also acceptable methods for prevention.

However, the best way that you can prevent a subfloor moisture issue is to test the moisture condition of the subfloor prior to flooring installation, during installation, and after installation.

Moisture testing will look different depending on the type of subfloor that is present. In general, the subfloor will usually be made out of either wood or concrete. Let’s look at how moisture testing for prevention works for both materials.

Wood subfloor

For a wood subfloor, moisture testing should be done using a wood moisture meter, such as the DS500.

Because you’ll want the flooring’s MC (moisture content) to match the EMC of the installation location, you should require the same MC from the subfloor. This is usually 8% or 9%, but again, the EMC will help you determine your target MC.

Concrete subfloor

A basement with a concrete floorFor a concrete subfloor, the most reliable method you can use to test the concrete moisture condition is an in-situ relative humidity test.

Per the ASTM (American Standards of Testing Materials) F2170 standard, the relative humidity should be below 75% or whatever relative humidity level the flooring manufacturer specifies. Different flooring products can handle different relative humidity levels, usually varying between 75% and 90%.

Take the right steps, and subfloor problems are a thing of the past

Regardless of whether you have a wood subfloor or a concrete subfloor, moisture testing is absolutely essential!

In the end, proper moisture testing, watching out for signs of moisture, and using other preventative measures can ensure that your subfloors are dry enough and that the floors placed over them stay beautiful for many years to come.

You can learn more about preventing other moisture-related issues by checking out our article about measuring moisture in wood floors!