Should concrete be cast on a granular base placed over the vapor retarder or directly on the vapor retarder? The answer to this question has been debated for years because there are benefits and risks associated with each option.
Finishers prefer to cast the concrete directly on a granular base placed over the vapor retarder because the base acts like a "blotter" and absorbs mixing water. Reducing mixing water shortens the bleeding period and may increase the stiffening rate of the fresh concrete which allows the finishing operations to start sooner. Additionally, casting concrete on a granular base instead of the vapor retarder may reduce the risk of: tearing and puncturing the vapor retarder, surface blisters and delaminations associated with a prolonged bleeding period, subsidence cracking over reinforcement, slab curling, and cracking caused by plastic and drying shrinkage.
The granular base should be a 3- to 4-inch-thick layer of a trimmable, compactable and self-draining material. Do not use concrete sand because the compacted surface will not hold up to normal construction activities. Irregular and rutted base surfaces cause irregular floor thicknesses and increase the risk of random slab cracking.
Casting concrete directly on the vapor retarder isolates the concrete from external moisture sources and reduces the potential of creating a water reservoir in the granular base layer. Concrete cast directly on the vapor retarder typically dries faster than concrete cast on a granular base enabling the flooring to be installed sooner. Therefore, the drying period to reach the often-specified MVER of 3.0 pounds per 1000 ft2 per 24 hours is shorter. More importantly, this option eliminates the risk of creating a moisture reservoir in the granular layer below the slab that can provide a constant source of moisture leading to moisture-related flooring problems for the life of the building.
When concrete is cast directly on the vapor retarder, all of the excess mixing water rises to the surface prolonging the bleeding period. Extending the bleeding period delays finishing and may increase the risk for the occurrence of other concrete problems including surface blistering and delaminations, subsidence cracking over reinforcement, slab curling, and random cracking.
Because the benefits and risks associated with each option are job dependent, designers should make the decision as to where to place the vapor retarder on a job-to-job basis. Using the flow chart in Figure 1 can help make this important decision. For on-ground floors that will receive moisture-sensitive flooring, choosing to cast the concrete directly on the vapor retarder may be the best option, especially if the granular base layer has the potential to be exposed to external sources of moisture during or after construction. When choosing to cast the concrete directly on the vapor retarder, consider using a reduced joint spacing, low shrinkage mix design or other measures to minimize slab curling.
Installing the vapor retarder
For the vapor retarder to work as intended, it must be vapor tight, not just water tight. All seams, penetrations, tears and punctures must be properly sealed or repaired. During the final inspection, prior to placing the granular base layer or concrete, inspect the vapor retarder as though it were an imaginary "bathtub" that should hold water if filled. Locations where water would leak out are actually locations that will allow moisture vapor from the ground to enter the granular layer or the concrete slab. Even 5/8-inch-diameter stake holes and small tears from reinforcement chair legs can allow enough moisture to enter the concrete to cause localized flooring failures.
Before rolling out the vapor retarder, ensure the finished surface of the granular base is dense, smooth, level and at the specified elevation. Remove wet spots and repair ruts or depressions caused by construction activities. Roll out sheets of the vapor retarder in the direction of the planned concrete placement. Overlap seams a minimum of 6 inches and completely seal with special below-slab sealant tape. Placing concrete in a direction parallel to the seams will help prevent fresh concrete from damaging the tape and getting under the overlaps.