[Flooring Installation] Moisture in Wood Floors

moisture in wood floors

In this two-part series, we're examining moisture's relationship with flooring and how to prevent major issues caused by it.

Check out our other installment: "[Flooring Installation] Concrete Floors and Moisture"


Be it a home, an upscale office, or a restaurant, there's something about the aesthetic of a hardwood floor.

Boasting a classic look that many are now turning to in contemporary design, wood floors are a timeless addition to any space that can last for years when installed properly.

When it comes to moisture, however, hardwood floors are not unique when compared to other building materials. Just like their subflooring or nearby walls of plaster or sheetrock, excessive moisture can pose serious problems – even during installation.

But unlike other materials, moisture in wood floors can cause enough damage that the only solution is completing a major replacement.

How do you avoid this?

Investing the time to conduct thorough moisture tests before the first board is installed.

Wood Floors: A Welcome Environment for Moisture

Hardwood floors are far from impervious to moisture. In fact, how the material interacts with moisture has served as the basis of a type of moisture meter reading – Wood Moisture Equivalent (WME) – a comparable quantifier of moisture saturation in other building materials.

Wood is hygroscopic, meaning it will absorb or release moisture until it reaches equilibrium with the ambient environment (we'll talk about measuring Equilibrium Moisture Content [EMC] later). 

It's why you'll see wooden doors swell and stick to their frames on humid days or wood floors over unsealed concrete slabs tent or become uneven.

As a hygroscopic material, wood will always have some level of moisture in it unless it's been completely dried or spent a long enough time in an arid environment.

Compounding matters more, different species of wood have different levels of reaction to excessive moisture. Even between hardwoods, there are nuances between species in their density or how porous they are. And when moisture does enter the equation, it's nearly impossible to take accurate & precise readings with a moisture meter without making wood species calibrations.

[How To] Measuring Moisture in Wood Floors Pre-Installation

For those installing wood flooring, the material's relationship with moisture presents two potentially very expensive problems:

  1. The flooring not fitting the space it's installed in – being either too snug or too loose.
  2. Damage to the flooring, such as buckling, cracking, warping, or mold.

In either case, failure to properly check for moisture in wood floors can lead to costly repairs, not to mention a dissatisfied customer.

To be sure, this is the best time to check the moisture levels of the hardwood flooring you'll be installing.

Pin Moisture on woodWhy?

Because it's not yet installed.

During the pre-installation phase, gathering the moisture readings you'll need will take some time, but not because taking the readings are hard to do. Rather, the materials being tested need time.

According to National Wood Flooring Association Guidelines (NWFA), first and foremost, you'll need to take an initial relative humidity (Rh) measurement with a hygrometer of the space in which the flooring will be installed. Readings should be within the required range set by the flooring's manufacturer. Humidity levels beyond that range can cause the flooring to later fail or become damaged.

In addition, you'll also need to take moisture content readings with a pin-type meter of the hardwood flooring immediately after arriving on-site. This is where species calibration comes into play to take an accurate reading. NWFA standards require testing "... a minimum of 40 boards for up to the first 1,000 square feet and an additional four readings per 100 square feet thereafter."

Resource: Unsure about your moisture meter? Check out our article on what to look for in the best wood moisture meters.

After the first measurements, it's time to wait. The hardwood flooring will need time to equilibrate with its new environment. To that end, the hardwood should be left unwrapped or unboxed in the space it will be installed in for several days (at least four, but ideally one week).

After giving the wood time to acclimate, take readings again.

What's an acceptable moisture level in hardwood flooring?

This is where EMC comes into play. In general, an ideal moisture content level for wood flooring is between roughly 6-9% when ambient temperatures are between 60-80 F and Rh is between 30-50%.

Measuring Moisture in Subflooring

It's not just the wood flooring you have to worry about. The subflooring it will be sitting on should also be moisture tested.

For wood subfloors (which is what the vast majority of subfloors are), simply use a pin-type meter to take a reading. An acceptable subfloor moisture level is the same as what applies to the hardwood flooring. There should be no more than a 3% difference in moisture levels between the two.

For concrete subfloors, checking moisture levels is more involved, and you'll use a different test altogether. Rather than using a traditional moisture meter, instead you'll use a hygrometer and the in situ testing method to the ASTM F-2170 standard to check the Rh from inside the slab. In situ testing takes at least 24 hours to complete. Moisture readings should be between 35-40% humidity or 2-3% moisture content before laying flooring overtop.

For both subflooring types, moisture readings outside of acceptable ranges will require taking some sort of corrective action, for instance installing a moisture barrier for wood floors. Without addressing the moisture issue first, you're essentially guaranteeing a flooring failure.


Pro Tip: Because wood is hygroscopic, it will always react to moisture in the ambient environment. In other words, hardwood flooring will shift – albeit, nominally – as environmental moisture levels change. To ensure this movement is unnoticeable, leave about ¼" of space between baseboards & the flooring.0


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Taking Post-installation Measurements for Moisture  in Wood Floors

Post-installation moisture measurements are the last kind of moisture measurements you'll want to take. Chances are, if you're being asked to conduct a post-installation moisture survey, there's likely a moisture issue affecting the hardwood flooring.

At this point, surveying for moisture in wood floors is no different than it is for home inspections or in flood damage restoration.

After conducting an initial visual inspection, you'll need two tools:

  1. A pinless moisture meter to locate moisture presence beneath the surface of materials within its depth of measurement.
  2. A pin-type meter to quantify moisture levels in materials and confirm the pinless meter's findings.

While the circumstance of the inspection isn't ideal, a thorough and accurate survey is necessary to not only locate the source of the moisture intrusion but also take corrective action to ensure it's not a problem in the future.


Hardwood Floor Moisture Testing | Protecting an Investment

Hardwood flooring can be a great addition to any space when installed properly.

To ensure the wood floors are protected from moisture damage during installation, it is important for installers to invest time in conducting thorough moisture tests beforehand.

Taking these steps will save you and your clients money in the long run by avoiding costly replacement projects due to excessive moisture levels or other water-related issues.

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