The installation of vapor barriers is common practice for commercial buildings with on-grade floor slabs, especially when impermeable floor coverings or coatings are specified. Without them, water vapor traveling from the ground through the concrete can cause loss of bond, or deterioration of the covering or coating.

Standard practice

All too often, when a decision is made to include a vapor retarder the choice becomes 6-mil polyethylene commonly found on jobsites. Often, it's the concrete contractor who spreads it out just before placing concrete. If mesh or rebar is part of the work, it is laid on top of the plastic, workers drive stakes through it to secure forms, and while placing concrete, further damage results from shovels, wheelbarrows, and other placement equipment. By the time the concrete is hard, there can be numerous holes in the plastic sheet and what little money was invested to moisture proof the floor is wasted. Because the value of membranes is misunderstood and the belief that concrete is waterproof, this problem continues.

Changing our thinking

As buildings are constructed to be more energy efficient, the problem of managing humidity increases. When there isn't much air exchange due to leakage around windows, doors, and drywall, or where roofs and walls join, a ventilation system brings air in from the outside, gradually replacing the air inside. It's common now to include air handling equipment to condition the fresh air supply. This leaves the problem of moisture vapor that comes into a building through floor slabs on grade. This moisture can exceed the ability of dehumidifying equipment, so limiting moisture becomes important.

While working under a waffle slab, the contractor doesn't tape the seam in the bottom of the interior footing. When concrete is placed, the membrane is pushed against the voids underneath it and the taped seam provides for expansion.
Stego Industries While working under a waffle slab, the contractor doesn't tape the seam in the bottom of the interior footing. When concrete is placed, the membrane is pushed against the voids underneath it and the taped seam provides for expansion.

Vapor barriers versus vapor retarders

The permeance (perm) of a membrane is defined as the vapor transmission rate of moisture through it. By definition in ASTM E 1745, vapor retarders must pass less than 0.3 perms. Although no standard specification that defines the term “vapor barrier” exists, the accepted definition of a vapor barrier is that it passes less than 0.01 perms. ASTM E 1745 defines three classes of membranes: A, B, and C by three physical characteristics. Notice the class of material you select really depends on the amount of abuse expected during construction. Class C products are the most frequently used but Bret Houck, national marketing manager for Stego Industries, San Juan Capistrano, Calif., says that the price difference between a 15-mil Class A vapor barrier and a 10-mil Class C vapor retarder products is minimal, so they recommend 15-mil Class A material.

Installation basics

Membranes are located either directly beneath the concrete or under a graded, compactable granular “blotter.” What's at stake is the amount of curling that can occur when mix water leaves the bottom of a slab slower than it does on the top surface. Protection of the membrane during construction is also a factor. Houck suggests the subbase stone be covered with a layer of fine, compactable material to protect the vapor barrier from punctures when concrete is placed directly on top of it. Membranes also should wrap under footings and up the outside wall to provide a sealed connection to the foundation waterproofing.

The focus so far has been more on water vapor transmission than liquid water issues. Installing drainage systems on both sides of footings is an important detail; and the practice of installing sump pumps has long been mandated. The first consideration is always to remove standing water before it rises to the level of the membrane. The membrane should prevent water vapor transmission, “brown gas” emissions, and radon gas accumulations.

The installation standard in ASTM E 1643 requires 6-inch seam overlaps between pieces of membrane for flat-slab installations. Seams should be sealed with manufacturer's tape with a perm rating less than the 0.3 perms maximum for a vapor retarder. Seams should be arranged either perpendicular to the direction of concrete placement or so the concrete flows in the same direction as the overlap.

Sealing around penetrations through membranes is also very important. Form stakes are the leading penetration. Contractors can avoid this by setting bulkheads and forms without stakes.

A closing thought

The concept of water vapor moving through a concrete slab is hard for some to comprehend. Soil tests might initially indicate the percentage of moisture at a building site is low and the water table is deep. It's difficult to understand why there should be a problem for structures built in desert regions. But within a year of construction, the relative humidity of the ground under a concrete slab is always 100%.

California Civil Code now outlines “actionable defects” and two of these relate to water and water vapor issues in foundations. These codes will be adopted by other states too, placing additional responsibility on specifiers, builders, and concrete contractors.