With the introduction of engineered wood 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
Wood Floors Below-grade and On-grade
Which Floor What Room?
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 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
Engineered Wood Flooring Products
Engineered Wood Floors-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
Once the grade limitations are determined, it's time to look
at other potential sources of moisture.
Methods for Basement Installation of Wood Flooring Products
On top of the concrete slab and prior to
installation of the basement hardwood floor, another vapor
barrier should be installed. These vapor barriers can be:
- PVC applied over the slab with a
- 6 mil polyethylene sheets embedded
into a skim coat of asphalt mastic.
- 15 pound roofing felt embedded into a
skim coat of asphalt mastic.
NEW CONSTRUCTION:( see
JOB SITE CHECK LIST)
During the building of a new home, moisture can 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.
Moisture Detection Equipment
Sources of Moisture
US Moisture Map & EMC Chart)
Sometimes the level of the water
in the ground is raised above the bottom surface of the
foundation due to:
- heavy or
- 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
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.
problems may also occur because the crawl space is below the
finish grade outside the house.
Subfloors - Mositure Barriers
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.
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
- surface water moving
through the foundation walls or flowing in through
- capillary rise of ground
- "green" slabs, or
concrete foundations that weren't fully cured when the
rest of the house was built above it;
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. Clear many time will sweat,
creating an additional moisture concern. 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.
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
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.
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.
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.
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
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.
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.
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
moisture from wood subfloors
Moisture Barrier on Subfloors
& On the Ground (Graphic)
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.
against moisture rising through subfloors, the installer
needs to make sure there is a proper moisture-vapor
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.
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.
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
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.
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.
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.
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.
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
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
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
another moisture barrier or retarder should be placed on top
of the slab before installation. A few choices for the
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.
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.
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
several other moisture barrier systems available, including
two-part epoxy, rubberized electrometric membrane, and other
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 allow the
installation to be started.
See JOB SITE CHECK LIST- A MUST DO BEFORE STARTING