WHY Are Vapor Retarders Used?
Vapor retarders are frequently specified for interior concrete
slabs on grade where moisture protection is desired.
Protection from moisture is required when floors will be
covered with carpet, tile, wood, resilient, and seamless
polymeric flooring, or when moisture-sensitive equipment
or products will be placed on the floor. Permeation of
water vapor through concrete slabs can cause failure of
moisture-sensitive adhesives or coatings resulting in
delamination, distortion or discoloration of flooring products,
trip-and-fall hazards, and possibly fungal growth
and odors.
Low-permeability membranes below floor slabs on grade,
in conjunction with sealed joints, also provide a barrier
to radon penetration into enclosed spaces when such conditions
exist.
WHAT Conditions Require Vapor Retarders?
A floor is part of the building envelope and should be constructed
to eliminate moisture infiltration into the slab and
into the occupied building space. For many years, vapor
retarders were specified only for floor slabs intended to receive
floor coverings. However, even floors intended for
“bare” use in service, such as warehouses, mechanical rooms,
and unfinished expansion areas, often are converted to other
uses and then moisture-sensitive flooring is installed. Such
“adaptive re-use” cannot be predicted during design and construction
of a new building. Therefore, it is sensible planning
to include a vapor retarder under every interior floor
slab in every building. Vapor retarders are generally not
necessary when placing exterior slabs on grade.
Vapor retarders do not prevent migration of residual moisture
from within the concrete slab to the surface. It is important
to use a concrete mixture with the lowest water
content that will afford adequate workability. Chemical and
mineral admixtures are generally used to minimize the water
content in a concrete mixture and provide adequate
workability for placement. After proper curing, the concrete
slab should be allowed to dry out and tested to ensure
that moisture is not being transmitted through the slab
prior to installing flooring materials.
HOW to Place Concrete on VApor Retarders?
Current recommendation of ACI Committee 302 is to place
a concrete slab directly on top of a vapor retarder when the
concrete slab surface will receive a vapor sensitive floor
covering. If environmental conditions exist for increased
possibility of plastic shrinkage cracking, placing concrete
directly on the vapor retarder can help alleviate the plastic
shrinkage cracking somewhat by enhancing bleed water.
Placing concrete directly on the vapor retarder can also
create potential problems. If environmental conditions do
not permit rapid drying of bleed water from the slab surface
then the excess bleeding can delay finishing operations.
Bleed water trapped below a finished surface can
cause delaminations (CIP 20) or blisters (CIP 13) if finishing
operations are not performed at the correct time after
bleedwater has disappeared from the surface. Concrete may
stiffen slower, which means that trowel finishing operations
must be delayed; thus increasing the susceptibility of plastic
shrinkage cracking. Curling (CIP 19) can occur due to
differential drying and related shrinkage at different levels
in the slab. Most of these problems can be alleviated by
using a concrete with a low water content, moderate cement
factor, and well-graded aggregate with the largest
possible size. With the increased occurrence of moisture
related floor covering failures, minor cracking of floors
placed on a vapor retarder and other problems discussed
here are considered a more acceptable risk than failure of
floor coverings.
The sub-grade and base should be adequately compacted.
The base should be well draining and stable to support construction
traffic. A clean fine-graded, preferably crushed,
material with about 10 to 30 percent passing the No. 100
[150-mm] sieve and free of clay or organic material is generally
recommended. Concrete sand should not be used as
it is easily displaced during construction.
The sub-grade and base should be adequately compacted.
The base should be well draining and stable to support construction
traffic. A clean fine-graded, preferably crushed,
material with about 10 to 30 percent passing the No. 100
[150-mm] sieve and free of clay or organic material is generally
recommended. Concrete sand should not be used as
it is easily displaced during construction.
If recommended in the geotechnical evaluation of the
jobsite, install a 6 to 8 inch [150 to 200 mm] layer of coarse
gravel or crushed stone as a capillary break. Note that a
coarse stone capillary break will not reduce moisture vapor
transmission from the subgrade. A vapor retarder is still
required above a capillary break.
If a capillary break layer of coarse stone is used, choke the
top surface with 2-in. of graded, fine-grained compactable
fill to prevent damage to the vapor retarder from sharp corners
of the coarse stone. Place the vapor retarder on top of
the smooth, compacted fill.
Vapor retarder sheets should be overlapped by 6 inches [150
mm] at the seams and taped and sealed around utility or
column openings, grade beams, footings, and foundation
walls.
If an interior concrete slab will not have a vapor-sensitive
floor covering but will be located in a humidity controlled
area it may be placed over the granular fill/blotter layer
provided the slab and base material is placed with waterproof
roof membrane in place. Further the granular material
should not be subject to future moisture infiltration.
When the choice is made to place the concrete over a granular
blotter layer, a minimum 4 inch [100 mm] layer of
compactable, easy-to-trim, granular fill material should be
used. A “crusher-run” material graded from 1½ in. [37.5
mm] to dust size works well. If this is not practical, cover
the vapor retarder with at least 3 inches [75 mm] of crushed
stone sand. Do not use concrete sand. To reduce slab friction,
top off the crusher-run layer with a layer of fine-graded
material. The granular layer should ideally be placed under
cover and should be dry prior to concrete placement to function
as a blotter and remove water from the fresh concrete. |