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Sources of Moisture concerns when Installing Wood Floors

With the introduction of engineered flooring, hardwood floors can be used in virtually every room in the house, even in basements. Water on or near the building site is still a consideration, however. It cannot be ignored or neglected.

Installing Below-grade and On-grade

Grade Level-Installation Type

Moisture comes from a variety of sources-from the earth itself, from surface water (i.e., rain water runoff), as well as from the interior atmosphere. Let's consider the various ways that moisture problems originate, and how they can be controlled and prevented.

A room that is exposed to excessive moisture would be prone to trouble if solid wood flooring were installed in it. Basement floors are particularly tricky installations. But some products, such as laminated flooring, may be suitable in many of these circumstances because of their dimensional stability.

The use of these alternative materials is necessary when the installation is below-grade. "Below grade" is defined as any part of the slab having four inches or more of earth above it. Additionally, walk-out basements are considered a below-grade application, even if a portion of the basement is above ground.

Most laminated (also called engineered) flooring consists of two or more layers of wood glued together, with the grain of each running at 90 degree angles to the layers adjacent This tends to result in increased dimensional stability.

Use of engineered wood flooring
Engineered wood flooring uses multiple layers of wood glued together, with the grain of each running at 90-degree angles to the layers adjacent. As the wood fibers absorb moisture and want to expand, each layer is restrained by the other and improved dimensional stability results.

Because it displays far less expansion and contraction with moisture changes, laminated flooring can be successfully installed in areas with wide humidity variations. It usually does not require full acclimation before installation. In on-grade or above-grade installations, various flooring types can be used: solid or laminated. Moisture is still a consideration, although it is less likely to cause problems than in below-grade installations. Once the grade limitations are determined, it's time to look at other potential sources of moisture.

Potential sources of moisture


During the building of a new home, moisture can enter the house in a number of ways. When the frame is going up, the 2-by-4s and subfloor materials often get rained on. Because builders usually work on tight, demanding schedules, homes are sometimes built trapping moisture that needs time to evaporate. The amount of moisture introduced by framing lumber and the wet trades can be significant. See Moisture Detection Equipment

GROUND WATER: Sometimes the level of the water in the ground is raised above the bottom surface of the foundation due to:

  • heavy or prolonged rains;
  • a spring that appears only during wet seasons and is not discovered when the house was built;
  • water flowing along an impervious layer within the soil.

Any of these may cause water to penetrate the foundation walls or rise through the ground surface into the foundation or crawl space.

SURFACE WATER: Rain water falling on the ground or from the roof can pass through or under the foundation walls. In some areas, heavy soils may retain surface drainage and cause water pressure against the foundation walls or slab.

Surface drainage problems may also occur because the crawl space is below the finish grade outside the house. 

CRAWL SPACE: Many houses are built over a crawl space--that is, the floor of the house is built over an open space that is deep enough to allow a person to gain access to the under floor area by crawling. The minimum depth of the crawl space should be 24 inches under the floor joists or 18 inches under the girder. An 18-inch clearance beneath any ductwork is also desirable. Moisture problems within a house that show up as condensation and/or frost on windows in the living area, can result from dampness in the crawl space, or from inadequately vented living space. Moisture problems in crawl spaces may be due to the construction of a crawl space in an area of high water table, improper grading of the lot for drainage or the omission of moisture control devices such as vapor barriers, ground cover and ventilation openings.

Obvious symptoms of excessive moisture passing upward through the floors may include any of the following: a musty odor; mold on the walls near the floor, in corners and in closets; moisture condensation on insulated windows or storm windows; and moisture condensation in the walls with resulting paint peeling. Some of the more common ways moisture gets into crawl spaces are:

  • ground water moving through the foundation walls or up through the earth floor;
  • surface water moving through the foundation walls or flowing in through ventilators;
  • capillary rise of ground moisture;
  • "green" slabs, or concrete foundations that weren't fully cured when the rest of the house was built above it;

Building codes in most areas requires cross-ventilation in a crawl space equal to at least 1.5 percent of the square foot area within the crawl space.

In addition, a ground cover of 6-mil polyethylene sheets (preferably black-clear will sometimes sweat) should be placed over the entire area of the crawl space soil, lapped at least 6 inches and held in place by bricks or other weights. (Black polyethylene works better than clear, because the black plastic slows the growth of plants, which can occur even in a relatively dark crawl space. In addition, some wood flooring manufacturers are now recommending 8-mil poly in lieu of 6-mil.)

In cases where concrete is poured to create a floor in the crawl space, the 6- or 8-mil polyethylene cover is still required.

Capillary action causes moisture to travel upward into the slab. Gravel beneath the slab can slow the movement of moisture, but 6-mil plastic below the slab will provide a more effective vapor retarder.

CAPILLARY RISE OF GROUND MOISTURE: Moisture travels upward by capillary action--as much as 14 to 18 gallons per day have been noted under a 1,000 square-foot house--and evaporates within the crawl space. Capillary rise occurs in nearly all areas where the soil is clay or silt.

Capillary action is the effect of surface tension that causes water to rise up a narrow tube, against the effect of gravity. In building construction, capillary action can occur between two surfaces placed together, or within porous materials. This relates to the installation of wood flooring in that moisture can be drawn through both the subfloor and the concrete below it.

It is best not to build below the highest expected water table, for to do so is to have water under pressure trying to enter through any crack or weakness in the construction. Porous granular filling material around and under the building, and connected to drainage lines, can be used to divert drainage water away from the structure.

Making these adaptations is the responsibility of the general contractor. However, the flooring installer who has been hired by the general contractor to do the floors in a new home or building should be aware of these details.

MOISTURE FROM THE HOUSE: There are many sources of moisture from within the house. Mopping the floor in a 150-square-foot kitchen can release the equivalent of 4 1/2 pints of water into the air a shower or bath about 1/2 pint; washing the dinner dishes about 1/2 pint. Also, a family of four gives off about 1/2 pint of water per hour just breathing (this is why bedrooms are unexpected moisture sources). As moisture is released in a house, it moves to all rooms by natural air movement or by forced air movement from furnace or air conditioning.

MOISTURE FROM MECHANICAL SYSTEMS: Moisture is sometimes introduced into the crawl space from the mechanical systems within the house. To avoid this, make sure the clothes dryer is vented to the outside. Also, condensated water from cooling systems and water from automatic ice makers should be discharged away from the building.

RELATIVE HUMIDITY: When humidity increases, the effect on the wood floor can be damaging. This occurs most frequently in homes in which occupants are there for a short period of time, such as a weekend home or vacation cabin, or in rooms that are closed off (not heated) to save energy.

If air conditioning or heating is not used or is shut off, ventilation is a must even when the home is not occupied. Otherwise, the floor will expand in the high humidity, and cupping and buckling will occur. This "greenhouse effect" will be exaggerated even more when a plank floor has been installed, because wider boards react to moisture with more movement.

Minimizing moisture from wood subfloors

A heavy moisture invasion can seep up through a wood subfloor. It may occur slowly, but its effects are damaging. Proper installation of flooring calls for checking subfloors for moisture.

To protect against moisture rising through subfloors, the installer needs to make sure there is a proper moisture-vapor retarder.

In any case, the moisture content of solid strip flooring should be within 4 percentage points of the subfloor. (That is, if the subfloor is measured at 10 percent moisture content, the strip flooring should have no less than 6 percent moisture content and no more than 14 percent.) For solid plank flooring, the difference should be no more than 2 percentage points. Solid strip or plank flooring requires a felt paper moisture retarder between the floor and subfloor. If a wood subfloor is laid over an existing slab, the moisture retarder can be cemented to smooth, clean-swept concrete.

In joist construction, a vapor retarder of 15-pound saturated felt paper should be laid between the wood flooring and the wood subfloor. If the wood subfloor is laid over a concrete slab, the felt paper moisture retarder can be cemented to smooth, clean-swept concrete. Other vapor barriers or retarders may also be appropriate over concrete slabs.

Minimizing moisture from concrete: Traditional construction of homes tend to have regional variations. While many of the older homes in New England, the Mid-Atlantic and Midwest regions have basements, most Southern homebuilding is done on concrete slabs. The slab rests on a carefully prepared base and is usually surrounded by a footing that runs below the frostline, providing the actual foundation for the house.

The no-basement design eliminates many of the moisture problems associated with basements and below-grade features, obviously. But the installation of flooring over concrete is not without its own intricacies. Concrete appears to the untrained eye to be a solid, impermeable foundation, yet it's actually a source in itself for moisture. Concrete expands when it absorbs moisture in humid weather or by exposure to rain, and it contracts again when the moisture evaporates.

ALL concrete surfaces regardless of age or grade level will emit or conduct some degree of moisture, usually in the form of a vapor. This is a very natural and necessary function of healthy concrete-it's like continual "breathing." However, too much moisture emission without a proper moisture barrier has resulted in flooring failures. Many times the blame for this is placed on a faulty product, improper specification or faulty workmanship, when the real reason lies with slab vapor emission conditions.

Moisture conditions are not the flooring contractor's responsibility. Yet, the contractor should take the initiative to determine potential problems and advise the customer of available remedies before the start of installation.

Concrete is composed of crushed rock, gravel, sand, cement and water. The introduction of water into a cement batch will begin a series of perpetual chemical interactions. The basic recipe for building concrete is completely dependent on water being present in the slab.

Water in concrete is necessary to continue the process of cement curing well past the first few critical weeks. Therefore, all slabs should be tested for moisture before the floor's installation.

Both on-grade or below-grade slabs need time to both cure and dry out enough to handle flooring. General guidelines suggest that 60 days is sufficient for the curing to occur, and floors can be installed after this. These figures, however, are influenced by a number of variables, and should not be used as the sole criterion as to whether or not it is safe to install a floor. Above-grade slabs poured in metal pans take significantly longer to dry and have been known to require several months to well over a year to be safe to install upon. Since drying times are influenced by many factors, it is necessary to conduct proper moisture tests regardless of the slab age or grade level. Testing concrete for Moisture or see Moisture Detection Equipment

Because moisture can rise through concrete by capillary action, moisture-vapor barriers and moisture retarders need to be part of the installation process. Laid between the base of gravel or crushed stone and the slab, this barrier is usually in the form of heavy plastic, uninterrupted film. This film prevents the penetration of moisture through the slab to the interior surface, where it can ultimately damage hardwood floors.

Additionally, another moisture barrier or retarder should be placed on top of the slab before installation. A few choices for the barrier are:

1.) Low end PVC vinyl applied over the slab in multipurpose adhesive with seams sealed and the surface prepared before spreading the wood adhesive. The use of this method requires dependence upon the water-soluble, multipurpose bond in the presence of moisture and wood adhesives that are accepted over vinyl. This may also require a vinyl blocker.

2.) Polyfilm-6-millimeter polyethylene film in 36-inch or 48-inch wide rolls-applied over a "skim" coat of asphalt mastic. The mastic should be troweled with a straight-edge trowel to skim coat the slab, with coverage of about 80-100 square feet per gallon. After about 30 minutes, during which the solvents flash or evaporate, the polyfilm can be rolled over the mastic by walking over the film, embedding it in the mastic. Air bubbles that form under the film can be forced out toward the seams with a push broom.

3.) 15-pound roofing felt in asphalt mastic applied with a notched trowel at the rate of 50 square feet per gallon. A second similar coating of mastic and asphalt felt should be added, with overlaps staggered to achieve a more even thickness.

There are several other moisture barrier systems available, including two-part epoxy, rubberized electrometric membrane, and other sealing liquids.

In all cases, installers must verify acceptability of a particular system with the flooring manufacturer, and also verify adhesive compatibility when planning to glue down over any moisture barrier. If you are not familiar with the system, check with the adhesive manufacturer, the flooring manufacturer and the manufacturer of the moisture barrier system before attempting an installation. Bottom line, if you even suspect a problem with moisture DO NOT start the installation.




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