Why Moisture Meters and Thermo-Hygrometers are Essential for Successful Wood Floor Installation

Moisture can have a large impact on the beauty and sustainability of anything made out of wood, from cabinets and furnishings to wood flooring. It also affects other hygroscopic, water-absorbing materials in the building envelope such as drywall, ceiling panels, carpets, etc. Before the installation of a wood floor, the team will need to examine building conditions. If there is too much moisture in the sub floor in a newly erected building, or after water intrusions in older buildings, the excess moisture will be released into the building—and the last place you want to see water seeping is onto your new wood floor.

To avoid that risk, the first step before proper floor installation is making sure all materials surrounding the floor are dry from the concrete substrate to the roof. Two instruments are helpful with accomplishing that task: a moisture meter for materials and a thermo-hygrometer for air, relative humidity, and ambient temperature. If concrete slabs are to be measured, the Lignomat RH system follows the ASTM F2170 procedure for measuring moisture in concrete slabs before installing resilient floor coverings. The floor manufacturers also list conditions for a successful floor installation in their installation manual. To keep the floor manufacturer’s warranty, the recommendations should be followed.

Thermo-Hygrometers

When choosing a hygrometer, accuracy rating and humidity range are important. Standard off-the-shelf hygrometers usually have an accuracy rating of +/- 5% for relative humidity and +/-20F for temperature. In general, the lower the moisture content (overly dry, below 30%) or higher (too moist, above 65%) the less accurate standard hygrometers become. Lignomat makes an RH thermo-hygrometer probe which is +/-2% accuracy for relative humidity between 10% to 90% and +/-10F for temperature and can be used as an add-on instrument.

Moisture Meters

With or without pins: There are two choices when using a moisture meter. Both technologies have some merit, and depending on the task at hand, one or the other will better suited. Of course, the best course of action would be acquiring a moisture meter which can do both.

Pin or Pinless?

Pinless moisture meters: In theory, pinless moisture meters use electromagnetic wave technology, which measures the density in a three-dimensional field underneath the measuring pad. The density of the same block of wood changes if there is more or less water present. The meters indicate the average moisture content in the measuring field, with the section closer to the surface having a larger impact. If the moisture varies, the pinless or non-invasive meters will show the average. For more accurate moisture percentages, the measurements have to be corrected for different wood species.
- Pinless moisture meters are placed on top of a flat surface and indicate the average moisture in the measuring field below the measuring pads. The extent of the measuring field is determined by the maximum measuring depth and the size of the measuring pads.
For wood floors: Measurements are fast and allow for a large number of floor planks to be moisture tested n a short time, leaving no visible marks (including no pin holes). Pinless moisture meters work best on flat surfaces. Floor planks are always flat and the meter sits perfectly on any smooth or hand-scraped surface. The thickness of floor planks also fits the geometry of most dual-depth pinless moisture meters. Dual-depth moisture meters such as the Ligno-Scanner SDM measure ¼” and ¾” deep. Keep in mind that indicated moisture values may be incorrect when measuring engineered floor planks composed of a hardwood wear layer and core layers of different materials. When measuring moisture in materials other than solid hardwood, the best results are delivered when you have a sample with a known moisture content. Then, after finding a value for dry, all other values can be compared to the dry sample.
- A pinless moisture meter is the ideal choice to check moisture when the floor is delivered, throughout the acclimation and installation process, and later on when the floor is in use. There may be some changes in moisture content during acclimation. However, after the floor is acclimated to the ambient conditions, there should be no more changes. If the moisture remains unchanged, the floor is dimensionally stable. The floor planks will not move, and no warping or cupping occurs. Continuous moisture checks even after installation can prove no changes or warn that moisture is absorbed or lost by the floor planks.

Note: Most floor manufacturers indicate that the acclimation and installation should take place under the same ambient conditions that are normal for the space when in use. Doors and windows should be installed and closed. The air conditioner or heat should be running. Acclimation and installation only make sense if these conditions are warranted. If you are crisscrossing the conditions below, a wood floor is not happy, even when the temperature is a comfortable 720F. The moisture meter and thermo-hygrometer will indicate if the floor is stable:

25% relative humidity stable moisture content is 5.5%
35% relative humidity stable moisture content is 7%
45% relative humidity stable moisture content is 8.5%
55% relative humidity stable moisture content is 10%
65% relative humidity stable moisture content is 12%

With pins: In theory, pin moisture meters measure the electrical resistance between two metal pins kept a certain distance apart. The electrical resistance changes with the moisture content of the wood. If moisture varies within the small segment of the non-insulated part of the pins, the highest value is indicated. That means if the uninsulated part of the pin touches a wet surface, the indicated moisture values will be high.

Wood species corrections are necessary to obtain accurate moisture percentages. Most pin moisture meters have built-in corrections for different wood species. If lumber is hot or cold, the electrical resistance in the wood changes, so temperature correction is necessary for hot or cold lumber.
In practice, the pins of the moisture meter have to be pushed or pounded into the wood, and the moisture in the area between the pins is measured. Pin moisture meters allow for the testing of small areas. This is important when the moisture content within the material to be measured is changing, specifically between core and surface moisture. For the floor industry, that becomes important when the floor planks have absorbed moisture from the subfloor at the bottom of the planks or from the air above the floor at the top of the planks.

For an inspector, a pin moisture meter with longer insulated pins allows checking the subfloor without removing the floor planks. The pins need to be hammered through the floor planks and into the subfloor below to indicate the moisture percentage of the subfloor. Pinless meters are not able to indicate different moisture levels within their measuring field and the measuring field is limited to ¾” unless mentioned otherwise by the moisture meter manufacturer.

The ideal is to have access to both a pin and pinless meter such as the Ligno-VersaTec from Lignomat, which has the added benefit of different attachments available to tackle any moisture problems that may occur during maintenance. A slim electrode can be added to the pin-pinless-RH moisture meter Ligno-VersaTec to fit into tight spaces. Long pins will measure through insulation in search of moisture infiltration. Pinless mode allows scanning large areas quickly, floors, walls or ceilings, but when a problem is detected, the pin electrode helps with further investigation.

Flooring decisions and strategies for installation can be made a lot easier when you understand the products and processes that will produce results that will last beautifully for a lifetime.