Standards and how to specify under-slab moisture control.

We look to ASTM to help define what an underslab vapor retarder system should look like. The starting point is ASTM E1643 for selection, design, installation, and inspection which in turn references ASTM E1745 as the specification of the underslab material itself as well as ACI 302.2R for additional location recommendations. So, let's dive into the starting point: ASTM E1643...

ASTM E1643 -

Standard Practice for Selection, Design, Installation, and Inspection of Water Vapor Retarders Used in Contact with Earth or Granular Fill Under Concrete Slabs.

Traditionally thought of as "just the installation standard", ASTM E1643's title points to it being THE defacto starting point for underslab vapor retarders for not just installation but also SELECTION, DESIGN, and INSPECTION of the installation.

Sections 5.1 & 5.2

ASTM E1643 5.1

See ACI 302.2R–06 for material, design, and construc tion recommendations.

ACI 302's major contribution to the design of underslab vapor retarder systems is centered around the use of granular layers between the vapor retarder and the concrete slab. See below for a helpful diagram.

ASTM E1643 5.2

"See Specifications E1745 and E1993/E1993M for vapor retarder specifications."

E1745 is for plastic vapor retarders and E1993 is for bitumenous ones. So, we'll look to ASTM E1745 for the specification of our polyolefin vapor retarder membranes. While E1745 does prescribe a general permeance of less than 0.1 U.S. Perms, E1643 gives additional advice on how to design the permeance requirement for any specific product. See the next section for design details...

Section X1.6

ASTM E1643 X1.6

"Water Vapor Permeance of Vapor Retarder—

In order to prevent moisture damage to the slab on grade, moisture sensitive goods stored in contact with the slab, floor covering systems, and floor coating systems, the water vapor permeance of the vapor retarder material shall be such that accumulation of moisture in the slab through the vapor retarder material does not occur. The vapor pressures of the below grade environment and the interior environment shall be calculated and analyzed. For humidity sensitive interior environments, calculate the effect of vapor diffusion through the vapor retarder, slab on grade and, if applicable, the floor covering or coating on the interior humidity levels. Select a vapor retarder material with a water vapor permeance rating that will maintain interior humidity levels within specified tolerances. The water vapor permeance of flooring material or coating shall be obtained, if available. Calculate the amount of moisture entering the slab through the vapor retarder material. Calculate the amount of moisture that can diffuse through the flooring material. Ensure that the water vapor permeance of the vapor retarder material does not allow accumulation of moisture within the slab due to water vapor permeance of the flooring material. Analyze soil temperatures with regard to heat flux through the slab on grade as well as interior temperature and RH levels. Determine if conditions exist for a dew point within the slab. If such conditions can potentially exist, analyze the amount of moisture accumulation within the slab versus the drying potential of the slab through its top surface, and if applicable, through the floor covering system to determine if prolonged and detrimental wetting of the slab will occur. If so, incorporate measures to eliminate conditions for a dew point to occur. One such measure is installing an insulation layer directly below the slab and vapor retarder."

Actually design the system? Yep. Just like any other building component. The good news is that we can do the work for you! Have our Hygrothermix consulting group turn your project details into a slab moisture content prediction up to any point in time of the life of your building. We'll help you design your system to perform as intended to prevent slab moisture issues like flooring failures, mold & mildew, and rebar corrosion. We'll help you design your subbase and vapor retarder specification based on your soils report, concrete mix design, flooring type, climate, indoor operating conditions, and more.

ASTM E1745 -

Standard Specification for Plastic Water Vapor Retarders Used in Contact with Soil or Granular Fill under Concrete Slabs.

This standard, called for by ASTM E1643, gives a membrane a Class A, B, or C designation based on the performance on a handful of ASTM tests. For all of the classes, the permeance threshold is <0.1 U.S. perms. For a more precise look at the specifying an appropriate permeance value, see ASTM E1643 X1.6 for design recommendations or simply contact Hygrothermix for design assistance.

In addition to baseline permeance (ASTM F1249), drop dart puncture (ASTM D1709 method b) and tensile (ASTM D882) there is also permeance before and after conditioning tests that simulate the below-slab environment, and optional fire rating, ultraviolet exposure, and petroleum poisoning testing. See the table below for the complete testing requirements of ASTM E1745.

There are different required puncture and tensile values for Classes A, B, and C, but we recommend you stick with Class A if you don't want to have to tip toe on your installation and/or have to do too much pre concrete placement repair of your membrane.

Class A requirements:

2200 gram puncture
45 lbf/in tensile
<0.1 perm permeance*

*See ASTM E1643 for design advice on permeance or contact Hygrothemix consulting for design assistance.

But wait! Don't I NEED a "vapor barrier" (<0.01 Perms) below my slab?

ACI 302.2R

Guide for Concrete Slabs that Receive Moisture-Sensitive Flooring Materials

This guide's most important, and still relevant, contribution to the underslab vapor retarder industry is it's work and guidance on the presence/location of a granular layer. See the below diagram for a visual explanation.

Putting it all together....

When we combine the recommendations from ASTM E1745, ASTM E1643, and ACI 302.2R, we can boil it down to:

Utilize a Class A (E1745) plastic membrane for puncture (D1709) and tensile strength (D882) installed directly under the concrete slab with a low-shrinkage mix design (ACI 302.2R) that accomodates one-sided drying of the slab. The underslab membrane's permeance (F1249) and stone capillary break depth should be further designed via hygrothermal analysis (WUFI) based on the soils report, concrete mix design, use or not of insulation, floor covering type, and the climate and operating conditions of the building (ASTM E1643). The hygrothermal analysis should be performed by a an experienced practicioner* utilizing a program capable of performing the iterative, coupled, and numeric heat and moisture transient calculations over the required life of the building. This will predict the future moisture content of your system protecting your buildings from slab moisture-related issues like flooring failures, mold & mildew growth, and rebar corrosion. All installed with quality bonding, splicing, repairing, and booting accessories per industry and manufacturer's recommendations (ASTM E1643).

DO NOT specify:

Membrane thickness
- This presuposes a linear relationship between plastic thickness and performance attributes. This is old thinking. ASTM E1745 replaced D4397 for this very reason. Resin technology has vastly improved. A 10-mil product is no longer necessarily twice as good as a 5-mil... in fact its not necessarily better at all. Allow manufacturers to innovate with better resins and thinner products. As long as your performance requirements are specified (ASTM E 7145 Class A, for example) you are covered.
Arbitrarly low permeance values
- This is lazy and expensive specification work. 9 out of 10 dentists agree you should design your slab system. Let us help you design and select the proper permeance for your specific projects.

*For further discussion or analysis of your upcoming project(s) see our Hygrothermix consulting services or contact us directly.