Written by Brady Hillegas, EIT, Junior Structural Engineer, Allegheny Design Services

A few months ago, I was sitting in a project meeting that included the owner, general contractor, and the design team. The owner asked the question “Do we need to put a vapor barrier down below the slab?” The contractor immediately said he is not making that decision because he does not want to be liable for the outcome. The presence, or lack thereof, of a vapor barrier has both advantages and disadvantages.

As the conversation continued, multiple options were suggested regarding a vapor barrier. Below are some of the suggestions:

  • Place the vapor barrier directly below the slab.
  • Pour the slab without the use of a vapor barrier.
  • “Lay down the vapor barrier, have a thin layer of gravel, and pour the slab on top,” someone suggested.
  • Another suggestion was to lay down the vapor barrier and provide holes periodically which would provide a partial moisture barrier and in theory would minimize the chance of surface blisters on the slab.

Each suggestion has its pros and cons, and all of them have a direct effect on each team of the project. The type of floor covering or when it is sealed has a major impact on the results of each method.

Figure 1: Vapor barrier below welded wire fabric reinforcing. Photo Courtesy of the Columbus Dispatch.

Concrete directly on a vapor barrier

Concrete poured directly onto a vapor barrier is the best method to keep moisture from the soil from penetrating the slab. When I took a construction materials class in college, my professor engrained into my brain to know “water is evil!”  Everyone knows that water seeping into a building is never a good thing.

The downside is that it can affect the finish of the slab. Contractors say it takes longer and is harder to finish. Using a vapor barrier can lead to a slab curling, or blistering, on the surface. Blistering occurs when the excess water in the slab has not bled out completely before the slab is sealed. Small bubbles will appear on the surface and can be aesthetically unappealing. Most owners will not be happy with the finished product.


Pour the slab without the use of a vapor barrier

Some contractors may recommend this because it can save them money. It saves money in terms of material and labor, but the possible damage from water caused by omitting the barrier will come with a heftier price tag. Contractors may try to save time and money by arguing against a barrier, but it will nearly always be in the owner’s favor to use one if it is for an enclosed structure. If the slab on grade is for a pavilion or open structure without moisture sensitive floor coverings, then the vapor barrier can usually be omitted without consequence.

Pouring concrete on a gravel layer on top of a vapor barrier

This solution reduces the chance of having issues with finishing the concrete slab. A gravel layer reduces the chances of puncturing the vapor barrier. However, some negative consequences can come with this option as well. When a vapor barrier is placed below stone, the contractor may create a “bathtub”. A bathtub is created when an outside water source penetrates the gravel, i.e., when the roof has not been placed and the building is not enclosed fully. This leads to a higher presence of moisture in the gravel, and it can only evaporate through the air. This is a time-consuming process and should be done before the slab is poured or the water will want to penetrate the slab.

I must note, some people may request to use sand on top of the vapor barrier. This is never a good idea because sand can be displaced and become uneven when people walk on it. Concrete poured on an uneven surface can lead to an uneven floor thickness. Sand can absorb a higher amount of water as well, and as stated before, is not ideal between concrete and a vapor barrier.

Figure 2: Gravel placed on top of vapor barrier. Courtesy of BuildingScience.com

Concrete Slab directly on vapor barrier with holes in it

This solution was brought up during our meeting. The person supported it by saying it allows the slab to bleed moisture better and make it easier to finish. They also stated it will prevent most moisture in the soil from penetrating the slab. The logic makes sense; it provides the best of both worlds in terms of preventing most moisture and allowing an easier finish. However, holes in a vapor barrier still allow moisture to get through, which can cause localized issues. Even if floor coverings are damaged only in small spots, those spots must still be removed and replaced.

Occasionally, holes happen accidentally due to construction traffic. It is important to repair these holes prior to placing the slab so owners will be satisfied with 100% protection.

When to use each method

We have laid out multiple options, but still have not determined how and when to use each. ACI 360R-06, Design of Slabs-on-Ground, gives us a flow chart to help us make a decision:

Figure 1: A flow chart for specifiers to determine when and where a vapor barrier is used. (ACI 360-R06)

There are 3 different options, each of which will require project coordination between the contractor and the design team. Moisture sensitive floor coverings or humidity-controlled areas always require a vapor barrier under the slab. ACI 302.2R-06 is a guide specifically for slabs that have moisture-sensitive coverings.

Communication between the project team is vital

At the end of the day, both the design team and contractor’s goal is to deliver an exceptional finished product within budget and schedule to the owner. It is vital that the vapor barrier is discussed between the contractor and design team early in the project to keep the project moving forward without delay when the slab gets poured. In an early project meeting, make sure this gets discussed to confirm the project team is on the same page. The use and placement of a vapor barrier can have serious implications down the road.

Have questions? Feel free to contact us today or visit our site to learn more! 304-599-0771.
Thanks for reading.

Written by Brady Hillegas, EIT, Junior Structural Engineer, Allegheny Design Services

Brady works as a Junior Structural Engineer at Allegheny Design Services. His responsibilities include, but are not limited to, production of construction documents with Revit and structural engineering design. He attended West Virginia University where he graduated summa cum laude in Civil Engineering in 2020. He has prior experience in construction being in the field and in the office.