Early in my trade career, I worked as an installer for a
flooring company. Now that I'm a contractor, I no longer
install flooring every day — but I still do the
installation myself on some jobs.
It always pays to be extra careful when doing this kind of
work, because flooring failures are among the leading causes of
insurance claims filed by contractors. Many of those failures
are due to installing floors over damp substrates. One of the
main lessons I learned during my days as an installer was how
important it is to check the substrate for moisture before
laying the floor, especially when it's a concrete slab.
Just about every type of floor covering is negatively affected
by excess moisture. A damp substrate can cause wood and
laminate floors to cup or buckle, carpets to grow mold or
mildew, and sheet materials like vinyl and linoleum to blister
or come loose (see Figure 1). Also, many of the adhesives,
patching materials, and leveling compounds used under flooring
will fail if installed on overly damp substrates.
Figure 1. Certain types
of hardwood flooring — including parquet, bamboo, and
some engineered products — can be successfully installed
on above-grade slabs, but only if the concrete is sufficiently
dry. Slab moisture can cause flooring to cup (A) or swell so
badly it buckles and no longer lays flat (B). In extreme cases
the wood flooring tears completely free from the substrate (C).
Excessive moisture can also increase the pH (alkalinity) level
of concrete, causing flooring adhesives to fail, as it did
under this vinyl (D).
I was taught to inspect the site to make sure that the
conditions were right for a successful installation. Most
installers will check to see that there is proper drainage
outside the building and a suitable temperature and relative
humidity inside, and that new wood flooring is at the correct
moisture content. But many fail to properly check the moisture
condition of the substrate.
An old rule of thumb is to wait at least 60 days after concrete
is placed before installing an impermeable floor covering. But
that recommendation may not be conservative enough. Only under
ideal conditions — a mix with a low water-to-cement
ratio; low humidity; warm air temperature; protection from the
weather — is a slab likely to get dry enough in just 60
Familiar Test Methods
The only way to really know if a slab is dry enough for
flooring is to test it.
Polyethylene sheet test. Many
contractors and flooring installers still use the old
polyethylene sheet test, which involves taping a sheet of
plastic to the slab, waiting 16 hours, and then checking to see
if condensation has formed under the sheet, in which case the
concrete beneath will have turned dark (Figure 2).
Figure 2. The
traditional way to test for moisture is to tape a sheet of
plastic to the slab, wait overnight, and then check whether
condensation has formed underneath. If it has, the slab is too
wet for flooring. But if it hasn't, the tester can deduce only
that the slab might be dry enough for flooring.
If condensation has formed, the slab is definitely too damp for
flooring. But even if it hasn't, the slab may not be dry
enough. The only thing the test has established is that it
might be dry enough.
Electronic meters. There are much
better methods than the polyethylene sheet test for determining
the amount of moisture in slabs, including some that rely on
electronic meters. These devices — which cost $400 and up
— measure the relative humidity or moisture content of
the concrete. Some are placed on the surface and measure the
moisture from there. Others must be connected to probes placed
in holes drilled in the slab. In new construction, special
humidity probes can be cast into the concrete.
Meters allow you to quickly test moisture content, but
interpreting the results can be difficult because flooring
specs don't always refer to moisture content.
Calcium Chloride Test
I prefer to use the calcium chloride test, which is referred to
in flooring specs as ASTM F 1869-04, "Standard Test Method for
Measuring Moisture Vapor Emission Rate of Concrete Subfloor
Using Anhydrous Calcium Chloride." This test measures the
moisture vapor emission rate (MVER), or the rate at which water
vapor is coming out through the surface of the slab.
I like the calcium chloride test because it works with both new
and existing slabs and doesn't damage the surface or require
How it works. The calcium chloride
test is performed by placing an open container of dry anhydrous
calcium chloride (a white granular material) on the slab,
covering it with an airtight dome, and then, after a specified
period of time, comparing the calcium chloride's before and
If moisture was present in the slab, some of it will have
escaped into the dome and been absorbed by the calcium
chloride, making it heavier. The MVER is calculated by plugging
this weight gain into a formula. Once I have the MVER, I can
determine whether I need to take corrective action before
installing the floor.
What it costs. If you were a chemist
you could put the test together on your own, but for the rest
of us, purchasing a commercially made testing kit makes more
sense. A kit consists of a shallow dish (like a petri dish), a
premeasured quantity of calcium chloride, and a dome that can
be sealed to the slab. Individual kits cost about $12 apiece
and can be purchased online or at any flooring distributor. A
number of companies make these test kits (see list), but I'm
most familiar with the ones made by Vaprecision; its kits are
in the photos used in this story.
In addition to the test kits, you will need a scale that reads
accurately down to one-tenth of a gram. Vaprecision sells one
for $125, but less expensive models are available
Performing the Test
To comply with ASTM F 1869-04, you must perform a minimum of
three tests for the first 1,000 square feet of slab and an
additional one for every 1,000 square feet after that. Be sure
when doing multiple tests that you hit the areas you suspect of
harboring excess moisture.
Before any testing is done, the surrounding room must be
brought to the temperature and relative humidity that will
exist when the building is occupied. These conditions must be
achieved at least 48 hours before testing and must be
maintained while the test is under way. Otherwise the results
will not be accurate.
The areas of the slab that are to be tested must be clean, so
if adhesives, sealers, or patching materials are present,
scrape or grind them away.
When the slab is clean, weigh the prepackaged container of
calcium chloride and record this "starting weight" on the form
provided by the manufacturer. Then unseal the container by
peeling off the tape; open it; and place the container, its
top, and the tape on the prepared section of slab. Cover them
with the provided plastic dome (Figure 3).
Figure 3. The calcium
chloride test measures the amount of water vapor being emitted
by the slab. To perform this test, the author weighs a
prepackaged container of calcium chloride (top), opens it
(bottom left), and places it under an airtight plastic dome
that seals to the slab with a peel-and-stick gasket (bottom
The dome must be tightly sealed to the surface — nothing
should be able to enter it except through the concrete. The
dome (or domes, if you're doing more than one test) should be
left in place for the specified period of time; Vaprecision
recommends 60 to 72 hours.
When it's time, remove the dome and reweigh all three items:
the container, its top, and the original piece of sealing tape
(Figure 4). This will be the "ending weight." Be sure to use
the same scale that you used to measure the starting
Figure 4. Some 60 to 72
hours after sealing the dome, the author slices it open and
removes the container of calcium chloride (top). He then
reweighs the container plus the tape that once sealed it
(bottom left) and uses a formula provided by the manufacturer
to calculate the moisture vapor emission rate — or MVER
— of the slab (bottom right).
The ending weight will be slightly higher than the starting
weight because the calcium chloride will have absorbed any
moisture that was present in the dome and any moisture vapor
that entered through the slab. To calculate the MVER, you take
the difference between the starting and ending weight and
insert it into the formula provided by the manufacturer of the
test. The result will be the number of pounds of vapor emitted
per 1,000 square feet of slab per 24 hours.
Maximum Allowable MVER
Variations of the calcium chloride test have been around since
the 1950s, so specifications for flooring products nearly
always include a reference to the maximum allowable MVER
(Figure 5). If the specified level of vapor emission is
exceeded and the flooring product fails, the warranty will be
void and the installer or the GC will be on the hook for the
necessary repairs. This applies to all types of flooring as
well as to the adhesives and patching materials used in the
Figure 5. Contractors
who ignore the installation specs do so at their own peril.
Most flooring manufacturers specify the maximum allowable
moisture content for wood and concrete substrates.
In most cases the goal is for the slab to emit 3 pounds or less
of moisture vapor per 1,000 square feet (of slab area) per 24
hours. Always check the specifications for the particular
product you plan to install. Most engineered and laminate
flooring can be installed over dry concrete — as can most
solid-wood flooring, if there is a plywood substrate between it
and the slab. However, solid-wood flooring should never be
installed below grade — even if the MVER of the slab is
Whenever I install flooring over a slab, I perform calcium
chloride tests and give a copy of the results and of the
installation specifications to the client. I keep copies for
myself just in case.
Future problems. Since the calcium chloride test measures the
MVER at a particular point in time, a test result below 3
pounds per 1,000 square feet per 24 hours does not guarantee
that there won't be problems at some later date.
If the conditions change — a water pipe breaks next to
the building, a perimeter drain clogs, or heavy rains saturate
the soil — vapor transmission could increase to the point
where the installation fails.
A properly poured and cured slab kept at normal living
conditions shouldn't have an excessively high rate of vapor
transmission. Certain provisions (such as subslab drainage and
vapor retarders) can be made only before the concrete is
placed; once it's in place there are a limited number of ways a
contractor or flooring installer can reduce moisture vapor
The best method is to keep water away by sloping the soil away
from the building and installing gutters and perimeter drains.
In areas where people irrigate their lawns, homeowners should
be told how important it is that they not overwater or spray
the foundation with sprinklers.
Sealing the surface. After conditions
outside the house have been corrected, there are a few steps
the contractor can take inside to lower the MVER. An older
method is to use a cheap sheet vinyl as a vapor retarder: The
vinyl is glued to the concrete and then the real floor covering
is installed over it. However, if enough moisture is coming out
of the slab, it could cause the vinyl adhesive to fail.
A newer method is to seal the surface of the slab with a
product designed to reduce vapor emission. Many such products
are available, but I'm most familiar with Bostik's MVP4
(978/777-0100, www.bostik-us.com), a moisture-cured urethane
membrane applied by trowel. Once the membrane has cured,
certain types of wood flooring can be applied over it with a
compatible Bostik adhesive (Figure 6). Bostik doesn't specify
by how much MVP4 will reduce vapor transmission, so after it's
been applied the slab should be retested.
Figure 6. The slab in
this room had an MVER of more than 3 pounds, so the author
sealed it by troweling on a layer of Bostik's MVP4 barrier
membrane (top). This reduced vapor emission enough that he
could glue down bamboo flooring with a compatible Bostik
Another option is to use an epoxy barrier coating like Taylor
1410 Platinum MAC System (800/397-4583, www.wftaylor.
com). This material is applied to the surface by brushing or
rolling; according to the maker it can reduce the MVER from 10
pounds to 3 pounds per 1,000 square feet per 24 hours.
I prefer using these sealing products to laying plastic over
the slab (as many laminate-flooring companies suggest doing).
On several occasions I've pulled up a laminate floor installed
in this manner and found mold and moisture between the slab and
Contractors will sometimes install plywood subflooring over
sleepers attached to the slab. Any water vapor that comes out
of the slab will make its way into the plywood — and
eventually into the flooring — so it's a good idea to
install a continuous vapor retarder (a coating or lapped
plastic sheeting) over the concrete before installing sleepers.
But it's best not to do this until the slab and sleepers have
had time to dry.
Dave Northup is a contractor in Homer, Alaska.