How to Prevent Subfloor Moisture Problems

Preventing subfloor moisture problems

You already know that you have to measure the moisture content of a wood floor and let it acclimate in order to prevent moisture problems. But paying attention to the subfloor is also an important step in preventing moisture problems in wood floors.

If you don’t make sure that your subfloor is guarded against moisture, you may be putting the floor at risk of extreme cupping, like in the case of this unlucky homeowner. Moisture can seep into the floor from the bottom up if you don’t take steps to guard against this sneaky problem.

So, how do you prevent moisture problems in your subfloor?

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What’s the acceptable range of moisture content in a subfloor?

The first step is to understand what you’re looking for in terms of moisture content.

Solid strip flooring material less than 3” wide should be within 4 percentage points of the moisture content of a wood subfloor.

For solid plank flooring material 3” or wider, the difference between the two should be no more than 2 percentage points.

Taking steps to prevent moisture problems in a subfloor

#1: Take moisture measurements before, during, and after installation

You already know that it’s imperative to measure the moisture content of the wood flooring itself and to allow it to acclimate, but you also have to measure the moisture content of the subfloor — if you forget this step, you’re asking for problems!

Test for moisture at several locations in the room — a minimum of 20 per 1,000 square feet — and average the results.

Use the proper tools to ensure that you’re measuring subfloor moisture content correctly. If the subfloor is made of wood, we recommend using the Wagner MMC 220 pinless moisture meter. If you’re measuring the moisture of a concrete slab, use a concrete moisture meter like the Wagner Rapid RH.

#2: Use a flooring underlayment

Installing flooring underlayment that contains a vapor retarder is a great way to prevent problems from popping up. When installed correctly, a flooring underlayment can help you reduce the amount of moisture that migrates from the subfloor up into the wood floor.

It’s an especially good idea to use an underlayment when you’re installing a floor in a below-grade environment like a basement or an attic, as these areas are usually more prone to wide fluctuations in moisture levels.

The manufacturer of the flooring and/or subfloor you are using for a project may suggest a specific underlayment to use. In this case, you will want to follow the manufacturer’s recommendations.

Rolling out a flooring underlayment

An underlayment that provides extra moisture protection, like FloorMuffler brand flooring underlayment, can significantly prolong the life of your floor by protecting against common moisture-related problems like cupping in hardwood floors.

When installing a solid or engineered nailed or stapled down floor, use a membrane (saturated or laminated, felt or paper.) For engineered, floating floors, use a roll or sheet membrane like FloorMuffler.

In certain situations, you can also use Bostik GreenForce adhesive as a flooring underlayment with superior moisture protection.

#3: Help homeowners take preventative measures during wetter months

You should instruct your clients to prevent moisture problems moving forward.

To guard against moisture problems in the subfloor, you can suggest that they place a dehumidifier in the room during the humid summer months if there is a risk of excessive moisture, especially if the floor was installed in a below-grade environment.

Preventing subfloor moisture problems in hardwood floors

Of course, this will not solve a serious moisture problem. But it can keep some amount of moisture at bay, especially when paired with a flooring underlayment that contains a vapor retardant.

If you’d like to go above and beyond, you can also give your clients a wood floor moisture meter and train them to take moisture readings every several months to keep moisture problems in check and tackle issues before they start. That way, you won’t have to worry as much about callbacks moving forward.


Excessive moisture in a hardwood floor is a scary problem to have and once cupping or other problems pop up, it’s hard to reverse the damage.

But if you make sure make sure to measure the moisture content of your subfloor and train your clients to prevent excessive moisture from developing in their homes, you’ll be pretty well guarded against this problem.

For more information on preventing subfloor moisture problems, call us at (800) 737-1786 or email info@cityfloorsupply.com to speak with a wood floor expert.

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