Testing Sub-floors, and Wood Flooring Products for Moisture

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The determination of moisture content is an essential part of quality control within the flooring installation process. Flooring installers must know the dryness of not only the wood flooring, but its subfloor and the concrete slab beneath it, if one exists. Hand-held electrical tools, called moisture meters, should be part of the toolbox of every flooring contractor, for measuring moisture in wooden or concrete subfloors and in the wood floor materials. It is equally important to check % of relative humidity of the area of the floor installation as well as the storage space.

Working with moisture meters
Moisture meters have become one of the most critical tools of the trade, yet they are often neglected by those who need them most - flooring contractors. The installer who continues to work without one is simply jeopardizing his jobs. A single moisture-problem installation should be enough to convince him or her to invest in a meter and make use of one of the most cost-effective tools in the business.

Moisture meters have many purposes. They can determine if floor boards are dry enough for an installation to proceed. They can check subfloors and concrete for high moisture levels; they can decide when a second coat of finish can be applied; they can assess water damage.

There are two main types of meters--Probe and Pinless.

The probe type, which is the older method, measures electrical resistance across opposed sets of pins, which are pushed into the wood. The higher the moisture content, the lower the resistance.

 

TESTING WOOD SUBFLOOR - WOOD FLOORING

Probe-type meters are fast and easy to use. They come with different measurement indicators. The lower-cost units have L.E.D. display lights indicating different moisture levels; the better-quality units have analog or digital displays and provision for different species and wood temperatures.

The pinless, dielectfic or independence types, which are also referred to as "non-destructive" because they don't leave any small holes in the wood, are quite different.

Signal penetration for pinless meters is up to 1 inch for both hardwood and softwood. The meter can be moved across the surface to identify pockets of moisture in a wood block or plank. It is relatively unaffected by temperature. Rough surfaces have very little effect on the reading. Measurements can also be taken through coating, varnish or paint without damage to the surface. See Manufacturers of Moisture Meters

One advantage of probe type meters is that those with insulated pins can measure moisture content at varying depths--you can tell whether the moisture content near the bottom of a board is higher than near the top, for example.

Deciding which kind of moisture meter to buy is a matter of sorting through the features you think you will need, and how often you may use it, and then understanding the various features and benefits. It is important that the meter you choose offers the following:

• A wide moisture content range from at least 6 percent to 30 percent. (The accuracy of readings outside of these limits are generally considered questionable.)
• A clear analog or digital dial.
• The necessary adjustment tables for various species; (some meters have a built in adjustment for this function)
• For a probe meter, the ability to take external probes, and a selection of pin sizes.
  

Testing Wood Subfloors ~ Wood subfloors are actually easy to check for moisture content. Just test for moisture at several locations in the room and average the results. In most regions, a "dry" wood subfloor that is ready to work on has a moisture content of 12 percent or less( down to 7% in low R/H states ). If excessively high readings are obtained, installation should not proceed until the origin of the moisture is identified and moisture problems are remedied. During the winter, an overly moist subfloor can be dried out by running the heat for a few weeks. Air conditioning during the summer will do the same thing. Before flooring can be installed, the moisture content of the subfloor should be within 4 percentage points of the flooring that will be laid on it.. If the moisture content between the flooring and subflooring varies more than 4 percentage points, then the flooring should not be installed.

Testing Concrete Subfloors ~ As concrete moves through its initial drying period, regular checking of moisture content can start after 30 days. In most cases it will take 60 days or more before the slab is dry enough for wood flooring installation to proceed. Excess moisture in the concrete will cause problems such as condensation or failure of the adhesive under the flooring.

Concrete is made up by mixing an aggregate of varying sizes (sand, gravel, or crushed stone) Portland cement and water.  The water and Portland cement mix together to form a new chemical compound that we refer to as paste.  This paste is a liquid when it is first mixed together which flows into the spaces that occur between all of the aggregate.  A chemical reaction also starts to occur when the Portland cement and water are combined that causes the paste to harden.  Any excess water that is originally mixed into the concrete that is not used in the chemical reaction remains as water droplets distributed throughout the paste portion of the concrete.  Since this chemical reaction occurs over time some of the residual water trapped within the concrete becomes part of the cement paste as it continues to react or cure.  In fact some of this additional water is important to the curing process that gradually increases the strength of the concrete.  During the initial curing period (7 to 10 days), additional water must be added to the surface to replace the water that evaporates from the surface or a sheet of plastic is placed over the surface to inhibit surface evaporation.  Curing compounds are also sometimes applied to the surface to inhibit surface evaporation.

 Any water that remains after curing is substantially completed will continue to evaporate through the exposed surfaces of the concrete.  In the case of a floor slab that is poured on an unvented steel pan or a 6 mil polyethylene vapor barrier, only the exposed upper surface can accommodate the evaporation of this excessive moisture.  During the period concrete is exposed to an exterior environment this drying is retarded by high exterior relative humidity; rain and snow.  It is not unusual to have very high interior moisture levels in a concrete floor slab that is many months or possibly several years old.  The water to cement ratio that occurs during the initial mixing is the primary factor that influences the amount of residual moisture contained in the concrete.  The amount of water required to chemically react all of the Portland cement is fixed by the quantity of the Portland cement in the mix.  As more excess water is trapped in the concrete, the resultant concrete structure is more porous due to the spaces that remain after the water has dried out.  It is common practice to add extra water making the concrete “soupy” so that it flows and levels more easily and makes it easier to place the freshly mixed concrete when pouring a floor.  Or in the case of high rise construction extra water is added to make it easier to pump the concrete to the upper floors.

Moisture conditions in concrete slabs that ultimately create moisture problems in flooring may not the flooring contractor's responsibility, but it is the flooring contractor's responsibility to ensure that potential moisture problems are resolved before installation begins. Unless the flooring contractor takes the initiative to determine the potential problems, through testing for moisture content, he is the one who will get called by the unhappy homeowner-because the buckling wood is the only result the homeowner sees.

A flooring contractor can begin his determination with some subjective and logical questions: What is the history of other homes in the area, as well as the history of the building, the quality of the building and the quality of the slab?

Also, what is the age of the concrete? (An installer should not accept a slab as "ready" on age alone.) What is the concrete's visual appearance? Color of concrete surfaces: The "proper" color of concrete should be a light yellowish-grey (similar to limestone) not sugar white. In fact, a sugar-white surface usually means the concrete was improperly cured and can create a severe moisture problem later on (usually months later). Any concrete that is blotchy, with dark spots and martyred some whitish freckling within the darkened areas should be suspected of having contaminants, admixtures or other problems unsuitable for flooring.

Flooring contractors should make sure to rely on flooring manufacturers' recommendations for your definition of what qualifies as "acceptable moisture content," as well as for which type of moisture testing each manufacturer prefers and care should be taken to insure there is no confusion on the units of measurement.

Testing for moisture in concrete can be accomplished using specially designed and calibrated moisture meters, and there are also several types of physical tests that can be used. The most common types of tests are discussed later on.

Electrical Moisture Meter testing of concrete
Some meters are designed and calibrated specifically for concrete use. Regular checking of moisture content of the concrete slab during the drying out period is required to ensure it has reached sufficient dryness to accept the floor covering.

Electrical testing works on the principle of impedance measurement in the concrete slab which is translated to a percent moisture content reading.

When testing concrete slabs, particularly if they are on-grade or below-grade, the moisture condition should be tested not only on the surface, but also in the body of the slab ( some moisture meters do this in a non-destructively). The reason for testing both the surface and the mid-section is to ascertain if there is continuous moisture movement toward the surface. If the flooring is installed while the slab is in this condition, upward movement of moisture will continue and the moisture will move into the floor. It is important the insure the moisture meter has the capability to measure into the slab, not just the surface. The results are costly and damaging.

As always, tests in multiple locations throughout the slab. Moisture meters that give meaningful readings and that unit of measurement can be related to manufacturers recommendation, should only be used. See: Moisture Detection Equipment

 

Recommended relative humidity levels: The recommended relative humidity levels in concrete for wood floor installations are 1) 75% or less for Engineered wood flooring, and 2) 60% or less for Solid wood flooring which are adhered directly to a concrete surface.  You should note that when solid wood is installed over concrete, wood flooring industry standards require that a very low permeability vapor barrier should be installed over the surface of the concrete.  Although adhesives are often designed to be “water proof” this does not necessarily mean that the adhesive is a proper vapor barrier.  In fact most adhesives do not effectively function as proper vapor barriers.  The exception to this is a heavy application (at a spread rate of no more than 30 square feet per gallon) of Asphalt adhesive combined with 6 mil thick polyethylene. 

Various physical tests of concrete~ Here are some other tests that installers employ to check the moisture content of the concrete before starting the installation. All tests should be done at several different locations in a room--typically along exterior walls and walls with plumbing enclosures, as well as over mechanical chases.

EMISSION TEST

THE CALCIUM CHLORIDE TEST: The calcium chloride test is becoming one of the oldest used concrete moisture tests. The calcium chloride test has been used most often by sheet vinyl installers, but a growing number of wood flooring installers now employ the test as well. Costs can run about $50 or $60 per test. As always, refer to flooring manufacturer recommendations, since some believe other tests are more accurate.

The calcium chloride test works by measuring changes in weight of anhydrous calcium chloride crystals.

A small plastic dish of crystals is sealed with a plastic tape. The entire dish is weighed on a gram scale prior to exposure and the weight, date and time the test was started must be recorded. The lid is then opened, and the dish of crystals is carefully set down on the concrete for 60 to 72 hours. The dish is enclosed within a 7-by-10-inch cover, which is sealed to the concrete. During this time, the only source of moisture being absorbed by the crystals is what can evaporate out of the covered concrete surface area.

At the end of the test, the dome is removed and the lid is placed back on the dish and sealed. Again the dish is weighed on the gram scale and the date and time are marked. The change in weight is multiplied by a constant and divided by hours to provide an estimated rate of evaporation, in pounds.

Pounds is the equivalent weight of the water that evaporates out of a 1,000-square foot surface area during 24 hours. Water weighs 8.3 pounds per gallon. If the test reports 8.3 pounds emission, then one-gallon of water is leaving a 1,000 square foot surface area in 24 hours.

A conservative but generally recommended allowable amount of moisture emission as expressed by the calcium chloride test is 3.0 pounds per 1,000 square feet per 24 hours at the time of the installation of the flooring. A note of caution: Use care in lid dealing and removal of the dish, and weighing as exposure to atmosphere will dramatically effect the results.

THE POLYFILM TEST: Pieces of 24-inch squares of polyfilm are placed at several points on the subfloor, sealed to the subfloor on all four sides with silver duct tape. After 24 hours, the patches are removed and inspected for signs of condensation. If beads of water are found on the subfloor or the concrete appears darker, further testing is necessary. If there is no indication of moisture under the polyfilm, the installation may proceed. The reading is valid at 24 hours, but it's even better if the test can stay in place until 72 hours have passed.

The polyflilm test can also be "accelerated" by using a heat source (such as a 40 to 60 watt lightbulb) 18 inches above the plastic.

THE PHENOLPHTHALEIN TEST: This uses a 3 percent phenolphthalein solution in water-free ethyl alcohol. Dime sized holes,1/4-inch deep, are drilled in various areas of the slab, particularly around walls. Then two drops of the solution are applied into each of the drilled areas. If there's no color change in the solution, there should not be enough moisture and the alkalinity is not high enough to affect the installation. But if the phenolphthalein turns pink or dark red within five minutes and the pH is 9.0 or higher, further testing must be done with a more precise method.
Color of concrete surfaces: The "proper" color of concrete should be a light yellowish-grey (similar to limestone) not sugar white. In fact, a sugar-white surface usually means the concrete was improperly cured and can create a severe moisture problem later on (usually months later). This is due to the poor hydration of cement within the surface, creating a disproportionately high water to cement ratio...which appears white. A note concerning this procedure: This should NOT be the only method used for testing moisture.