1. There is a difference between vapor retarders, vapor barriers, and waterproofing materials and when each should be used under a concrete slab on grade. 2. Vapor retarders are used to slow the transmission of water vapor from the soil into the building through the slab, while vapor barriers completely block vapor transmission. 3. Proper installation of a vapor retarder under the slab is important to prevent moisture-related problems with flooring materials and to restrict soil gases from entering the building.

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Providing:
Specifications, Quality Assurance,
The Where, When, and Why of
Inspections & Investigations
for your Construction Projects
Slab on Grade Vapor Retarders
By Randal J. Reifsnider, AIA, CSI, CCS, LEED AP, SCIP
ABSTRACT:
There is a difference between Vapor VR vs. VB? In most cases, only a VR is
Retarder, Vapor Barrier, and
Waterproofing and why they are used Many use the terms vapor retarder required. However, where a soils
under a slab on grade. This article
(VR) and vapor barrier (VB) report indicates there is liquid
addresses those differences and subgrade water present or if it is
focuses on the use of vapor retarders. interchangeably. However, there is
a difference. A vapor retarder reasonable to anticipate liquid
FILING: RETARDS or slows (but does not subgrade water may be present at
prevent) the transmission of water some time during the life of the
UniFormat™
vapor. A vapor barrier BLOCKS the building, use waterproofing under
A1030 Slab On Grade
transmission of water vapor. the slab. A separate VR may be
MasterFormat™ required, too, depending on the
VR is a material that has a selected waterproofing product.
07 26 00 Vapor Retarders (primary)
permeance of 0.3 perms or less
03 30 00 Cast-in-Place Concrete
when tested in accordance with VR's Purpose
KEYWORDS: ASTM E 96. VB is a material that
VRs are used under a concrete
Vapor Retarder, Vapor Barrier, Slab on
has a permeance of 0.00 perms.
slab-on-grade to:
Grade, Waterproofing, Cast-in-Place
VRs are made from a variety of
Concrete, permeance. 1. Restrict water vapor from
materials including polyethylene
entering a building.
REFERENCES: sheets, asphalt/polyethylene
composite sheets or polymer 2. Minimizes shrinkage
ACI 302.1R - Guide for Concrete Floor stresses and cracking.
and Slab Construction modified bitumen sheets.
3. Prevent slab curling.
Polyethylene sheets are often
ACI 302.2R - Guide for Concrete Slabs 4. To keep soil gases, such as
that Receive Moisture-Sensitive referred to informally as
radon, from entering a
Flooring Materials "Visqueen." However, "Visqueen" is
building.
a proprietary brand name and
ASTM E 1745 - Standard Specification should not be used as a generic It is particularly important to
for Water Vapor Retarders Used in
term. restrict moisture entering the
Contact with Soil or Granular Fill under
Concrete Slabs building through the slab where
Many sheet plastic VR products are
the floor finish will be a moisture-
high density polyethylene (HDPE).
ASTM E 1643 - Standard Practice for sensitive product such as:
Selection, Design, Installation, and These products resist punctures
Inspection of Water Vapor Retarders during construction and help 1. Vinyl, linoleum, rubber, and
Used in Contact with Earth or Granular ensure the VR remains effective. other resilient flooring
Fill Under Concrete Slabs
materials.
VR vs. WP? 2. Wood.
National Institute of Building
Sciences (NIBS), Whole Building The primary purpose of the VR 3. Carpeting.
Design Guide, article "Building under the slab is to prevent water
Envelope Design Guide - Floor Slabs" 4. Sealers, epoxies, and other
by Mark Postma, PE of Carl Walker, Inc. vapor migrating from the soil, impermeable coatings.
http://www.wbdg.org/design/env_bg_ through the slab, and into the 5. Adhesives used to install
slab.php building. finished flooring materials.
The primary purpose of It is also important where there
waterproofing (WP) is to prevent will be moisture-sensitive
liquid water from entering the equipment, products, or
building. environments.

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Water vapor entering a building ACI 302.2R provides additional Care should be taken during the
through a slab-on -grade can guidance on the materials placement of reinforcement to
cause floor tile to dislodge, properties and locations of VRs prevent the VR from being
resilient sheet flooring to bubble, under concrete slabs that receive punctured. Reinforcement
floor coatings to blister, and wood moisture-sensitive finish flooring. supports should be bricks or
to warp. According to ACI302.2R, should rest on protective pads. VR
specifying a more stringent perm damaged during placement of
Standards rating than the 0.3 standard reinforcement should be
ASTM E 1745 sets minimum maximum has advantages. immediately repaired.
product criteria for sheet
membrane VRs for use under
VR Installation Cautions
slabs-on-grade. It limits the VRs should be placed directly over A VR or VB under a slab-on-grade
maximum water-vapor permeance a compacted, well-drained helps the concrete hydration
to 0.3 perms and establishes three granular fill (typically, gravel) process by limiting the surface
durability classes based on tensile which has been placed over area available for evaporation of
strength and puncture-resistance: unexcavated or undisturbed earth water. However, if the evaporation
Class A (most durable), Class B, with the longest dimension rate on top of the slab is too high,
and Class C (least durable). ASTM parallel to with direction of slab the slab could curl and the surface
E 1745 does not govern material pour. In addition to providing can become dry and crusty while
type nor thickness. Most products support for the slab, granular fill the concrete below the surface
offered as VRs will have perm provides an area for moisture remains relatively soft. It is
ratings significantly less than the below the slab to accumulate and important to control the hydration
standard's maximum. be dispersed. process to assure the slab's proper
appearance and performance.
Not all polyethylene sheets have The National Institute of Building
been tested according to Sciences (NIBS) Whole Building Measures need to be taken
ASTM E 1745. When specified, be Design Guide www.wbdg.org immediately after concrete
sure to check submittals for this recommends using 2 layers of 6 placement to assure the hydration
requirement before approving. mils (0.15 mm) polyethylene process does not occur too
sheets to reduce puncturing and rapidly.
ASTM E 1643 sets minimum
provide redundancy at seams. This
installation criteria for VRs for use Refer to ACI 308R "Guide to
is a different system than the
under slabs-on-grade and includes Curing Concrete" for
single layer of 10 mils (0.25 mm)
details and checklists for the field, recommendations of controlling
VR recommended by ACI 302.1.
and pre-design and design concrete hydration.
However, NIBS recommendation
considerations. VR installation
may actually achieve better results
recommendations below are based Add Your Comments
since it addresses the same
on this standard. ASTM E 1643
performance criteria as ACI 302.2R We invite your comments. Visit our
also recommends that slabs
and includes built in redundancy. blog and add your comments to this
should be tested for moisture
article's announcement.
emissions prior to placement of VRs should lap over footings and
www.conspectusinc.com/blog
finish flooring. foundation walls. Seams should
overlap a minimum of 6 inches
ACI 302.1R provides guidance The information contained in this
(150 mm) per ASTM E 1643.
for the materials and installation document is offered for educational
Seams, laps, and penetrations
of concrete slabs in general. It purposes, only, and not as technical
should be sealed with
requires VRs to comply with advice suitable for any particular
manufacturer’s recommended project or specific condition. Technical
ASTM E 1745 and to have a
adhesive, pressure-sensitive tape, consulting is unique to the facts of a
minimum thickness of 10 mils
or both. Attention to sealing particular condition, and Conspectus
(0.25 mm). This is thicker than the
seams and penetrations is critical recommends that a specialist be
6 mils (0.15 mm) traditionally
to preventing soil gases, such as consulted to determine solutions for
specified for polyethylene VRs.
radon, from entering the building. each specific condition.