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Why Vapor Retarders Are Used Under Slabs

What Is A Vapor Retarder?

They’re materials which reduce the transfer of water vapor from the sub-slab support into the concrete slab. The specifications are generally outlined by ASTM E 1745 and most retarders possess permeance under 0.3 US perms.

A low-density polyethylene vapor retarder is the most common type used, with a minimum thickness of 0.25 mm or 10 mils to decrease vapor transfer. It’s also recommended for use due to its durability before and after installation. Any membrane material designed to be used as a true concrete slab vapor retarder has a permeance rating of 0.0 perms per square foot per hour, as calculated by ASTM E 96.

When Are Vapor Retarders Used?

The barrier is frequently installed when an interior concrete slab placed on a grade needs protection from moisture infiltration. This is common when:

  • The floor covering is carpet, resilient, seamless polymeric, tile, or wood; or
  • Moisture-sensitive equipment or products are placed on the floor surface.

If water vapor seeps through the slab, it’s possible for:

  • Surface delamination as moisture-sensitive adhesives and/or coatings fail;
  • Discolored or distorted flooring products;
  • Trip-and-fall hazards; and/or
  • Fungal growth and odors.

Another benefit of placing a low permeability membrane is creating a barrier for enclosed spaces against radon penetration when also sealing joints.

What Situations Require Vapor Retarders?

Because a floor is a critical part of the building envelope, it’s necessary to construct it with the end goal of eliminating moisture infiltration into the concrete slab and the occupied building space. Having this foresight is a good way to avoid future hassles associated with moisture intrusion when a previous bare use room is converted for everyday use, including the installation of moisture-sensitive flooring. However, a vapor retarder usually isn’t required when an exterior slab is placed on a grade.

Does A Concrete Floor Vapor Barrier Stop Moisture From Reaching The Surface?

No. All concrete slabs experience residual moisture from within the slab migrating to the surface, but a vapor barrier won’t stop this process. Instead, it’s strongly recommended to use a mixture with low water content but not too low as to sacrifice workability. The water content can be minimized and adequate workability retained through the use of chemical and/or mineral admixtures.

How Concrete Is Placed On A Vapor Retarder

First, make sure the compaction of the sub-grade and base is sufficient. The base needs to drain well and have proper stability to support construction traffic. It’s recommended to use a clean, fine-graded material free of clay or organic material. Ten to thirty percent of the material should be able to pass through a number 100 sieve. Don’t use concrete sand as it displaces easily during construction.

Job Site Geotechnical Evaluation

Depending on the results of a job site geotechnical evaluation, it might be necessary to place a six-inch to eight-inch capillary break layer made from coarse gravel or crushed stone. Yet, this wouldn’t negate the use of a vapor barrier, regardless of the material used. If a coarse stone is used, remember to cover the top surface with a two-inch layer of graded, fine-grained compactable fill. This prevents the sharp stone corners from damaging the barrier which should be placed on top of the compacted fill layer.

Secure Vapor Retarder Sheets

Make sure the sheets overlap by six inches at all seams and are taped and sealed around any:

  • Column or utility openings;
  • Grade beams;
  • Footings; and
  • Foundation walls.

Humidity Controlled Areas

If an interior concrete slab won’t have a type of floor covering sensitive to moisture but will be placed in a humidity-controlled area, it can be directly poured on the granular fill if the base material and slab are placed with a waterproof roof material.

Placing Concrete Over A Granular Blotter Layer

Use a minimum four-inch layer of compactable, granular fill that’s also easy to trim. In this situation, it’s common to use a crusher-run material that’s been graded from 1 ½ inch to dust-size. If this option isn’t practical, use at least three inches of crushed stone sand to cover the vapor retarder. A layer of fine-graded material placed on top of the crusher-run layer helps to reduce slab friction.

Ideally, the granular layer is placed under the cover and is dry before concrete placement.

Other Considerations For Placing Vapor Retarders

If a vapor-sensitive floor covering is to be placed on top of a concrete slab, the slab should be installed directly on top of the concrete floor vapor barrier. This can also alleviate the risk of plastic shrinkage cracking when favorable conditions are present by encouraging bleed water.

Potential Issues With Placing Concrete On Vapor Retarders

If the rapid drying of bleed water is impeded by environmental conditions, finishing operations may be delayed. Plus, bleed water unable to reach the surface is a frequent cause for blisters and/or delaminations as the time frame for finishing changes.

The risk of plastic shrinkage cracking increases if trowel finishing is postponed due to a slower rate of concrete stiffening. Another potential issue is curling caused by different drying and related shrinkage at various slab levels.

Most potential issues can be mitigated by using concrete with:

  • Low water content;
  • Moderate cement factor; and
  • Well-graded aggregate with the largest possible size.


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