Wood Subfloor Moisture

In many of my weekly emails over the past few years, I have talked about concrete moisture, but rarely about moisture in wood subfloors and what can happen. This is becoming a big issue in the field.

We are in the time of the year when humidity levels are extremely high. In homes without climate control (new or existing homes), these high humidity levels will elevate the moisture level in the wood subfloors that can impact newly installed flooring. 

Every installer regardless of what type of flooring he is installing should have a meter for checking wood subfloors. Moisture meters have become the most crucial tool for the flooring installer, but it is probably the most neglected tool in the industry. Continuing to work without a moisture meter just increases your chance for failure. Moisture Meters can be used by all flooring trades, not just the hardwood installers. Moisture Meters can be used to check subfloors for water damage, moisture content of wood subfloors prior to installing any flooring, and if the hardwood is at the correct level for the subfloor.

There are two types of Moisture Meters for wood subfloors;

  • Pin or Invasive – This is the older of the two. The pins measures electrical resistance across opposite sides of the pins which have been inserted into the wooden product. Generally pin meters will measure from 6% – 30%. 
Moisture Meters for Wood Subfloors
  • Pinless or Non-Invasive – This meter can easily be moved across the floor to located wet areas or a wet subfloor and are not affected by temperature and rough surfaces. Pinless or Non-Invasive meters will also measure from 6 – 30%. This method of testing was developed for measuring the moisture content of timber, but is arguably problematic for large areas of walls and floors where pin holes may be unsightly, especially in materials such as drywall and in solid surfaces where the pins cannot penetrate.

Taking a reading from the surface of such materials can be misleading, as the surface may be dry due to low humidity. However, below the surface the material or substrate may be wet. Non-invasive moisture meters that work on the principle of impedance measurement using direct contact electrodes and have been adapted to the needs of various industries over the years. Direct contact electrodes offer more accurate results, versatility, depth of signal penetration and reproducibility of readings than other Non-invasive moisture meters.

how to use moisture meter

Whatever type of Moisture Meter you have or will buy, make sure it can read OSB subfloors.

Why should I test Wood Subfloors?

When wood subfloor takes on excess moisture it expands. With this expansion, the flooring will move, buckle, or cause joint show through. Loose-Lay flooring installed in new construction during the hot, humid summer months generally has some type of failure during the first heating season. The flooring is installed when the subfloor has picked up the excessive moisture and swelled. The flooring is fit with some expansion space and looks fine for a few months. The heat is turned on in October/early November and the drying process of the subfloor begins. The subfloor dries out; shrinking in the process and the once good expansion zone is now getting tighter. The subfloor continues to dry and the flooring starts to buckle because the fullness created by the subfloor shrinkage now has nowhere to go.

Flooring over crawl spaces is a nightmare to begin with. Every wood subfloor over a crawl space should be tested for moisture. The crawl space should also be checked for a minimum 6 mil black poly covered in stone overlapped and run up the walls. The minimum height for a crawl space is 18″ high and should have vents or a dehumidification system. If the space is not vented, the excess moisture is trapped and the dry subfloor will pick up this moisture and cause flooring issues. If the moisture is not controlled in the crawl space, this will cause the subfloor to go through a lot of up and down changes leading to a flooring failure.

How many Readings should I take?

Hardwood suppliers will recommend a minimum of 20 readings per 1000 sq. ft. and average the results. This is an excellent rule to follow with all types of flooring.

How Does the Wet Subfloor Affect Flooring and Flooring Products?

Take a damp sponge and lay a paper towel on it. What happens? The dry paper towel picks up the excess moisture in the damp sponge and expands. This is what is happening as you install conditioned wood underlayments, hardwood flooring and other moisture sensitive products on a wet subfloor.

Plywood Underlayment Joint Show Through – this can happen when a dry underlayment is taken to a jobsite an immediately installed on top of a wet wooden subfloor. The underlayment is stapled in to place and then it begins to pick up the excess moisture from the wet subfloor and begins to expand, pushing the seams to complete fullness resulting in a slight peaking of the underlayment seams. The flooring is installed with a wet adhesive, adding more moisture to the underlayment. A few days later, you can see every underlayment joint up through the vinyl.

Cupped Hardwood – this can happen when the bottom of the hardwood picks up more moisture than the face of the hardwood. Again, the dry hardwood is installed on top of a wet subfloor and the bottom of the hardwood picks up the extra moisture from the subfloor and expands leading to cupping or even buckling the hardwood off the subfloor.

What are my Subfloor Moisture Limits?

This will vary from product to product.

Hardwood Floors – the maximum subfloor moisture level will be either 12% or 13% depending upon the manufacturer. Then, the hardwood flooring moisture content must be within 2% – 4% of the subfloor moisture. 2% for Plank (3¼” wide or more) Hardwood and 4% for Strip (2¼” wide) Hardwood. 

Floating Floors (LVT/Cork/Laminate/Hardwood/Ceramic/Vinyl) – I personally feel subfloors over 13% moisture content will have to much movement for the floating floor to accommodate. As the subfloor shrinks, it will negate the expansion zone of the flooring product and the flooring will buckle at some point. Very few vinyl manufacturers will give you a number for wood subfloor moisture.

Glue Down Floors Again, I personally feel subfloors over 13% moisture content will have to much movement for the underlayment and the flooring to accommodate and the chance of subfloor show through is very high.

How do we cure this? Two things no one wants to hear;

  1. Acclimation of the flooring products and sundries
  2. Get the HVAC System up and running, starting the drying out process.

We talked about taking moisture readings; Step 2 is documenting your readings. By documenting your moisture readings, you now have a reference point to go back to should a problem happen. For instance, you take your readings prior to installing a hardwood floor and the subfloor moisture content is 10% and the strip Hardwood was at 7%. You get a call 9 months later the hardwood floor is cupped.

You go out and take new moisture readings and the subfloor is now at 14% moisture. You reference back to your original readings and this shows that somehow this subfloor is picking up moisture from another source (flood/leak, etc.) since the time of installation. Moisture readings only tell you what the moisture content at the time of the readings. They do not predict what can happen over time. When I say document, document the following; 

  • Customer
  • Address of installation
  • Date and Time when readings were taken
  • Readings
  • Location of the readings
  • Pictures of readings 

Moisture Meters





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