The installation of vapor barriers is common practice for commercial buildings with interior on-grade floor slabs, especially when impermeable floor coverings are specified. Without them, water vapor traveling from the ground through the concrete can cause loss of bond to the floor covering or destruction of the covering. But until recently vapor barriers have not been used much for residential construction. The primary reason for the move to vapor barriers for residential construction is the “tightening up” of residential buildings to improve their thermal efficiency. Mold and fungus litigation has also promoted increased use of vapor barriers because the movement of moisture from the ground through floor slabs is a primary delivery system for moisture into the house envelope.

STANDARD PRACTICE

Here is an all too often jobsite occurrence. When the decision is made to include a vapor barrier under a floor slab the choice is 6 mil polyethylene. Usually it's the concrete contractor who spreads it out just before placing the concrete. If mesh or rebar is part of the work, it is laid on top of the plastic. When the reinforcement pokes holes through the plastic, there is no effort to repair them. Workers drive holes through the plastic with stakes to secure form-work. Placing the concrete further damages the vapor barrier from shovels, wheelbarrows, and other placement equipment. Sharp edges on aggregate cause damage, too. By the time the concrete has set, the vapor barrier is like a sieve and what little money was invested to moisture proof the floor is wasted. A lack of understanding of the value of membranes is the root cause of the problem, plus the belief by many in the field that concrete is highly waterproof and the amount of water that gets through it is insignificant.

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.
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.

WHY WE NEED TO CHANGE OUR THINKING

As mentioned earlier, as homes have become tighter to make them more energy efficient the problem of managing humidity levels has also increased. Today, moisture is controlled with air conditioning in hot weather, furnaces in the winter (where the problem is often to increase the humidity), and increasingly with “air-handlers” or “air-to-air handlers.” When there isn't much air exchange due to leakage around windows or doors, around drywall, or where roofs and walls join, a ventilation system that brings in air from the outside gradually replaces the air inside. It's common today to include dehumidifiers to condition the fresh air supply. This leaves the problem of moisture vapor that comes into a home through floor slabs. Given the right conditions, this moisture can exceed the ability of dehumidifying equipment. Limiting the movement of this moisture then becomes critical.

VAPOR BARRIERS OR VAPOR RETARDERS

The permeance of a membrane is defined as the rate of water vapor transmission through it. By definition, vapor retarders must pass less than 0.3 perms; the generally 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. The class of material you select depends on the amount of abuse you expect 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 is small, so Stego recommends that contractors buy the less permeable 15-mil Class A material.

The contractor was told to install the membrane after the footing, so there was no opportunity to place membrane under the footing. In this case, workers stop the membrane on the footing at the direction of the engineer.
The contractor was told to install the membrane after the footing, so there was no opportunity to place membrane under the footing. In this case, workers stop the membrane on the footing at the direction of the engineer.

The decision of whether to install a vapor barrier or a vapor retarder rests with the degree of protection needed. Lee Quigley, assistant general manager for the Riverside, Calif. office of SelectBuild (formerly Campbell Concrete of California), says that in some areas where they construct house foundations and floors there are methane gas deposits that seep through the soil. This is especially the case where housing developments are built on old dairy farm pastures. SelectBuild then installs an expensive vapor barrier plus vent pipes in the subgrade soil. Quigley adds that in Southern California moisture problems and mold issues are common, so owners regularly specify vapor retarders ranging from polyethylene sheeting to more sophisticated membrane systems.

There is one other consideration worth noting as you decide on the membrane for an application. Contractors tend to use the least costly membrane for their work, which would most certainly be the common polyethylene sheeting. But Houck says you should consider one other factor before choosing a product. Membranes, like low density or generic polyethylene, incorporate a percentage of recycled material that loses its performance over time. There are several factors that cause this: contaminants and chemicals in the ground, concrete alkalinity, soil organisms, wetting, drying, heat, and other pollutants. So what starts out to be a material that meets specifications doesn't end up being one. And increasingly contractors find themselves in litigation after the warranty period for a home has expired. ASTM E 1745 requires that the vapor retarder pass accelerated conditioning tests to comply with the standard.