Moisture vapor transmission
rate (MVTR), also water vapor transmission rate
(WVTR), is a measure of the passage of water vapor
through a substance. There are many industries where
moisture control is critical. Moisture sensitive
foods and pharmeceuticals are put in packaging with
controlled MVTR to achieve the required quality,
safety, and shelf life. In clothing, MVTR as a measure
of breathability has contributed to greater comfort
for wearers of clothing for outdoor activity. The
building materials industry also manages the moisture
barrier properties in architectural components to
ensure the correct moisture levels in the internal
spaces of buildings.
Courtesy of National Science Foundation
Report: Water Vapor
Transmission of Plain Concrete (click
ASTM states that a moisture vapor transmission rate of
4 pounds per 1,000 square feet over 24 hours is the maximum
allowable for concrete to be covered with a coating, sealer,
or adhesive. You can determine the MVT rate by performing
a calcium chloride test (ASTM F 1869). This simple test
involves placing a preweighed tablet of unhydrated calcium
chloride on the concrete surface and then covering it
with a sealed, domed plastic lid. After 72 hours, the
tablet is weighed again to determine how much moisture
left the concrete and was absorbed by the calcium chloride.
A simple math calculation provides an extrapolation of
the MVT rate in a 1,000 square foot area over 24 hours.
Testing concrete for moisture vapor transmission ensures
proper bonding of overlays and toppings. Test before applying
any 100% solid epoxies or high-solid polyurethanes. One
inexpensive moisture test is the Tape-Down Test (ASTM
approved). Tape a 16" x 16" piece of 6-millimeter polyethylene
down to the concrete surface with duct tape. Leave the
test down for a minimum of 16 hours. If any condensation
is present, you have a moisture problem. The second test
is the Calcium Chloride Test (ASTM approved). This is
a kit (click here)
that measures the amount of moisture calcium chloride
absorbs from the concrete over a 16-72 hours. The calcium
chloride is mailed in for results. This kit also includes
a pH test. pH is important in preventing delamination
of the floor. Concrete has a natural pH of 8 to 10. The
concrete should be around a pH of 7 before applying your
There are various techniques
to measure MVTR, ranging from gravimetric techniques
that measure the gain or loss of moisture by mass,
to highly sophisticated instrumental techniques that
in some designs can measure extremely low transmission
rates. Note that special care has to be taken in measuring
porous substances such as fabrics as some techniques
are not appropriate. Likewise for very low levels,
many techniques would not have the resolution to provide
a reliable result. Numerous standard methods are described
in ISO, ASTM, BS, DIN etc, -- these are quite often
industry-specific. Instrument manufacturers will often
be able to provide test methods developed to fully
exploit the specific design which they are selling.
The conditions under which
the measurement is made has a considerable influence
on the result. Both the temperature of and humidity
gradient across the sample need to be measured, controlled
and recorded with the result. An MVTR result without
specifying these conditions is almost meaningless.
Certainly no two results should be compared unless
the conditions are known. The most common international
unit for the MVTR is g/m²/day. In the USA, g/100in²/day
is also in use, which is approximately 1/15 of the
value of g/m²/day units. (More precisely, the
ratio is 1/15.500031, or very close to 2/31.) Typical
rates in aluminium foil laminates may be as low as
0.001 g/m²/day, whereas the rate in fabrics can
measure up to several thousand g/m²/day.
Often, testing is conducted
on a sheet of material. Calculations based on that
can be useful when designing completed structures
(packages, clothing, etc). Seams and seals are also
very important to end-use performance; performance
verification and validation of complete containers
or irregular objects is often recommended.
F 1869-10: Standard Test Method for Measuring Moisture
Vapor Emission Rate of Concrete Subfloor Using Anhydrous
Calcium Chloride .
F 2170-09: Standard method for determining relative
humidity in concrete floor slabs, (using In-Situ probes).
F 710-08: Standard Practice for Preparing Concrete
Floors to Receive Resilient Flooring.
E 96 (wet): Test method to determine reduction
in moisture vapor emissions measured in grains/hour.ft.
ASTM E 1745: (Class C) requirement,
Division 3 of project specifications for under slab
ASTM E 1907: Determining moisture
related acceptability of concrete floors to receive
moisture sensitive finishes.
ASTM D-7234: Standard Test
Method for Pull-Off Adhesion Strength of Coatings
on Concrete Using Portable Pull-Off tester.
ASTM 1308: Test method for
effect of household chemicals on clear and pigmented
ASTM C 856: Standard testing
method for Petrographic Examination of Hardened Concrete.
ASTM C 33: Specification for
concrete aggregates for concrete blenders, ready-mix
Water vapor moves
into building cavities by two mechanisms: diffusion through
building materials and by air transport (leakage), which
is usually far more significant and problematic. A vapor
retarder and an air barrier serve to reduce this problem,
but are not necessarily interchangeable.
Vapor retarders slow
the rate of vapor diffusion into the thermal envelope
of a structure. Other wetting mechanisms, such as wind-borne
rain, capillary wicking of ground moisture, air transport
(infiltration), are equally important.
In modern construction,
vapor barriers have become controversial. But their use
is legislated within the building code of some countries
(such as the U.S., Canada, Ireland, England, Scotland
& Wales). Current building science recommendations are
to vary the location of the vapor retarder in the thermal
envelope (exterior walls and ceiling/roof) depending on
the climate zone.
Some building codes
require an interior vapor retarder in heating-dominated
climates or an exterior vapor retarder in cooling-dominated
climates. In most climates, however, it is often better
to have a vapor-open building assembly, meaning that walls
and roofs should be designed to dry: either to the inside,
the outside, or both.
In areas below foundation
level (or, subgrade areas), particularly those formed
in concrete, vapor retarder placement can be problematic,
as moisture infiltration from capillary action can exceed
water vapor movement outward through framed and insulated
A slab-on-grade or
basement floor should be poured over a cross-laminated
polyethylene vapor barrier over 4 inches (10 cm) of granular
fill to prevent wicking of moisture from the ground and
radon gas incursion.
What happens if you apply a sealer, coating,
or adhesive to a slab with an MVT rate exceeding 4 pounds
per 1,000 square feet or 75% relative humidity? Chances
are good that the application will fail, turn white, or
peel. You can find more information on moisture testing
and moisture vapor transmission on the Internet. Simply
conduct a search using the terms "concrete moisture vapor
transmission" or "concrete moisture testing."
A brief description
of alternative methods for dealing with slab moisture
problems by Peter A. Craig. Moisture-related problems
with floor coverings and coatings applied over concrete
slabs have become a serious and costly issue for designers,
constructors, manufacturers, installers, and owners. Problems
are being experienced on both new and renovation projects.
The long-awaited day
has come, the project is complete, and everyone involved
is eager to show off the new look of the facility. But
wait a minute. Something is terribly wrong. Adhesive is
bleeding through joints between the floor tiles, and there
are bubbles in the sheet goods and epoxy flooring. Worse
yet, there is a noticeable odor in the air. Sabotage?
No, unfortunately what is occurring on this project has
become all too common across the country. Such problems
are being caused by moisture and moisture-induced high
pH levels beneath flooring materials.
& pH Stabilization System Application
Review (JD Technical can schedule an on-site visit if necessary)
A key to getting good
results with epoxy coatings is surface
preparation. Overkill is much better. You should clean/degrease
followed by at least one thorough clean water rinse. Allow
the surface to dry completely (at least 12 hours) before
coating. Work quickly, since the coating has a limited life
span once the two components are mixed together.
There are plenty of instances when
a vapor barrier is going to make a tremendous difference
in the moisture content, the damage from mildew
and mold, or humidity in a building. If you are not
sure of which waterproofing products should be used, call,
email or fill
out our contact form. Get
your ACT® together!
These definitions are intended to promote general familiarity
with common terms of the concrete industry.
Admixture: a natural or manufactured
chemical which is added to concrete before, or during,
its mixing. Admixtures are often deployed to add special
properties to concrete.
Ambient RH: the relative humidity
surrounding a concrete slab. The RH of air in a room
or in the open qualifies as ambient.
ASTM: American Society for Testing
and Materials: a global body which develops voluntary
consensus standards in construction.
Calcium Chloride testing: a moisture
content assessment procedure which measures the water
evaporation rate at the surface of concrete.
Cement: a substance which binds other
materials together, such as aggregate and water, to
Concrete: a composite mixture of cement,
aggregate and water.
Concrete moisture meter: a measurement
device which assesses the relative humidity level of
Curing: the process by which concrete
hardens due to the chemical process of hydration.
Equilibration: the process by which
the relative humidity of a building material, such as
concrete, is balanced with the RH of its surrounding
environment; also used to refer to the process of an
in situ probe reaching similar or balanced conditions
(temperature, moisture levels, etc) to the surrounding
Hydrostatic pressure: units of pressure
which are created by a liquid when it is at rest.
In Situ probe: a sensor which must
be embedded in a concrete slab (typically by drilling
a hole into the slab and installing a sensor within
a sleeve) in order to assess the slab's moisture content
Leapfrogging: the act of moving concrete
RH probes to several test places in a concrete slab
without allowing sufficient time for the probes to equilibrate.
Moisture content (MC): the term used
to describe the amount of water vapor within a concrete
Multi-use probe: a relative humidity(RH)
measurement device which contains a sensor and a separate
plastic, cylindrical sleeve. Builders place the sleeve
inside a hole drilled in concrete, then insert the sensor
into to cylinder to measure concrete RH. ┬áMulti-use
probes can be re-calibrated in order to be re-used on
another job site.
MVER: Moisture Vapor Emissions Rate
which indicates the rate of moisture evaporation from
the surface of concrete. MVER is a result of Calcium
Portland cement: a widely-used form
of cement which hardens in contact with water to produce
a water-resistant product.
Power troweling: the process of smoothing
a concrete slab surface using motorized equipment.
Relative humidity (RH): the term used
to describe the ratio of water vapor in the combined
mix of air and water; relative humidity is also used
to express the ratio of moisture content in concrete.
RH testing: a procedure which determines
the amount of water vapor (or relative humidity) in
Service conditions: the point at which
a building's concrete slab is fully enclosed and the
heating, ventilation and air conditioning (HVAC) systems
are up and running.
Single-use probe: a relative humidity(RH)
measurement device designed for use on a single job
site. A single-use probe is typically skim-coated into
the slab once testing is complete.
Slab temperature: the temperature
of concrete below the surface of the slab.
Substrate: a structural reinforcement
beneath a concrete slab; or, the concrete slab or subsurface
directly beneath a flooring finish.
Vapor barrier: a building material,
such as plastic or foil sheeting, which serve to slow
(or retard) moistures permeation from external sources
into a concrete slab.