Do I Need a Vapor Barrier Under Concrete Slab?

Concrete can be used for many aspects of construction, including floors. When getting ready to pour a slab-on-grade or a floor for a home, garage, or other structure, there’s a lot of preplanning to do. One important decision is whether or not to use a vapor barrier under concrete slab construction.

Concrete is permeable, so ground moisture can penetrate upward to the surface. If the pad surface is sealed with an impermeable finish or protected and heated by a permanent structure, it should have a vapor barrier. Slabs exposed to the elements shouldn’t unless they will be sealed.

In this article, we’ll explain what a vapor barrier is, when it should be used, and how to install it. We’ll identify the differences between moisture and vapor barriers, and discuss different types of barriers, and make some recommendations. Our goal is to provide you with the information you need for your building project.

What is a Vapor Barrier?

A vapor barrier is a material that retards or restricts moisture vapor penetration and is also known as a moisture or vapor retarder. To be considered impermeable, a vapor barrier or retarder requires the permeance of 0.1 Perms IMP or less. True moisture or vapor barriers have a permeance of 0.00 Perms. A common practice is to use a thin layer of polyethylene (plastic) sheeting as a vapor barrier.

The permeable nature of concrete allows ground moisture to penetrate upward to flooring or anything in contact with the concrete. The result could be a damp basement, growth of mold or mildew, ground vapor off-gassing, or damage to flooring or the concrete pad. A vapor retarder or barrier minimizes the movement of moisture through the concrete.

Standard concrete has the permeance of 3.2 Perms IMP which will allow ground moisture to wick up through it. A layer of 6-mil poly has the permeability of 0.059 Perms IMP and 10-mil poly 0.3 Perms IMP. The thicker the poly, the better and more durable the barrier. It is interesting to note, though, that 1 mil aluminum foil has the permeance of 0.00 Perms IMP.

Pro Note: the unit ‘mil’ or mils refers to thousandths (1/1000) of an inch and is used to measure the thickness of various materials.

Do You Need a Vapor Barrier Under a Concrete Slab?

Although concrete will take up to a month to fully cure, it begins to harden within an hour of pouring and makes a finished usable solid slab within 24 hours. So, anything you want in or under the concrete needs to be there before the pour. Always check with the local building department or a professional before pouring.

Is a Vapor Barrier Required by Code?

Concrete slabs that will be within heated structures must have a vapor barrier. Subsection R506.2.3 of the 2018 IRC states that a minimum of 6-mil polyethylene vapor retarder be used between the concrete and the base course or prepared ground. It also requires seams or joints in the barrier to have a minimum 6” overlap sealed with compatible material.

The IRC further identifies that unheated structures such as garages and utility buildings don’t require a vapor retarder unless they will later be heated. Patios, walkways, carports, and driveways likewise don’t require a barrier unless they will be enclosed within a heated structure in the future.

The American Concrete Institute (ACI) recommends a minimum of 10-mil polyethylene or thicker for vapor retarders under concrete. Their Guide to Concrete Floor and Slab Construction ACI 302.1R-15 identifies that thicker poly provides a better barrier and is also more puncture resistant. ASTM E-1745 also advocates for thicker poly or other membrane material to be used to form more of a barrier against vapor.

Can I Pour Concrete Directly on Dirt?

Site preparation for slab-on-grade or concrete pads on the ground is important to the structural integrity and life span of the concrete. While it is acceptable to pour sidewalks, patios, driveways, and shed floors directly on undisturbed or compacted soil, it isn’t recommended.

Pouring concrete directly onto dirt can cause the pad to crack or shift out of level as the soil underneath settles. Excess ground moisture can also degrade the concrete causing it to fail or crumble, especially if there is any freeze-thaw action. Heavy loads placed on or moving across the concrete can also cause it to fail or crack. Proper site preparation is important when planning a concrete pour.

Site prep involves the removal of vegetation, roots, and foreign material. Well-draining Group 1 soils can be leveled and compacted, or use clean sand, gravel, or crushed stone or concrete to form a drainage base.

Compaction of the base materials is necessary to ensure it is stable. Section R506 of the IRC identifies how concrete floors should be built, as does the ACI.

What Happens if You Don’t Use a Vapor Barrier?

When planning a concrete pad or floor that will be inside a heated structure, a vapor barrier is required to be compliant with most building codes. A vapor barrier under the concrete decreases the amount of moisture that can wick through the concrete and into the living space.

Not using a vapor barrier under a concrete floor in a heated building can create a damp living environment susceptible to mold and mildew growth. Flooring laid over the pad can absorb the moisture and swell or rot, as can wall structures in contact with the floor. As excess moisture evaporates, it can also cause dampness and health issues in other levels of the structure too.

Exterior concrete slabs or those in unheated structures don’t require a vapor retarding barrier. However, if the surface will be sealed with an impermeable membrane or product, there should be a vapor barrier under the pad. Ground moisture moving through the slab will be trapped in the concrete by the impermeable surface, causing the concrete to swell, cup, or otherwise degrade, leading it to fail.

Although a vapor retarder or barrier isn’t required under slabs used for unheated garages and outbuildings, it is recommended. Moisture moving through a concrete slab into the building structure can cause dampness issues, and even cause metal sitting on the pad to corrode and stain the concrete. Additionally, the moisture can cause impermeable floor paints or membranes to bubble and lift.

What Is the Difference Between a Moisture Barrier and a Vapor Barrier?

What many people refer to as a vapor barrier and moisture barrier, is really a vapor diffuser or retarder. They minimize the amount of moisture vapor entering a structure or material and are part of a building’s moisture control strategy.

However, there are classes of barriers that are used to identify materials that stop more moisture movement. Also, within each Class, there are different products that are better than others at decreasing vapor movement and diffusion.

The minimum moisture or vapor barrier for use under concrete is a Class 1 Vapor Retarder, with a rating of 0.1 Perm IMP or less. The IRC identifies 6-mil polyethylene with a rating of 0.059 (or about 0.06) Perms IMP, as the minimally acceptable barrier material. The AIC recommends the permeance of 0.03 Perms as the minimum, which can usually be achieved using 10-mil polyethylene.

A true vapor barrier has a permeance value of 0.00 Perms IMP. It is impermeable to moisture and vapor. Most materials identified and used in the construction industry as vapor barriers are actually vapor or moisture diffusers or retarders. They prevent most, but not all, movement of moisture.

What Can I Use for Vapor Barrier Under Concrete?

A vapor retarder used under concrete to manage moisture movement must have a Class 1 rating and a permeance of 0.06 Perms IMP or less to be code compliant. The minimum acceptable thickness according to the 2018 IRC is 6 mil polyethylene. It has a rating of 0.059 Perms IMP or about 0.06 Perms IMP.

The American Concrete Institute considers 10-mil polyethylene with a 0.03 Perms IMP rating as the minimum barrier for under residential concrete surfaces. Thicker poly barriers such as 15-mil and 20-mil often have lower permeance values, and are more durable options. Select the material that has the lowest permeability and the best durability for under the pad.

Types of Vapor Barrier

There are different types of vapor retarders that are acceptable for use under concrete. To be IRC 2018 compliant they must be a minimum of 6-mil (6/1000”) polyethylene which has a permeability of 0.06 Perms IMP. The 6-mil poly is compliant for use under concrete as a vapor retarding barrier, however, 4-mil isn’t thick enough nor does its 0.08 Perm rating meet the minimum for concrete use.

Thicker polyethylene sheeting is not only compliant but is also more durable and impermeable to moisture. Different manufacturers offer vapor barriers with permeability and puncture resistance that are unique to their products. Depending on the manufacturer, 10-mil poly ranges from 0.0183 to 0.03 Perms, 15-mil from 0.0063 to 0.0158 Perms, and 20-mil between 0.0052 to 0.038 Perms.

Many of the thicker vapor diffusers or barriers have reinforcement cord or string within the polyethylene layers to give them greater tensile strength. The increased strength makes them easier to move around and more durable when placing concrete. The added thickness also makes the sheeting more resistant to punctures.

Where to Put the Vapor Barrier?

A code-compliant vapor membrane should be placed on top of the prepared ground and directly below the concrete of all pads that will be enclosed within a heated environment. Slabs that won’t ever be enclosed in a heated structure do not require a vapor retarder. However, if an impermeable paint or sealer is planned for the surface, a moisture barrier should go under the concrete.

When preparing for a concrete pour, the vapor retarding barrier should be placed directly below where the concrete will be poured. The membrane is commonly placed on top of the compacted gravel, sand, or soil layer, or on any insulating layer upon which the concrete will be poured. Rebar, mesh, radiant heat lines, or other materials to be encapsulated in the concrete go above the vapor barrier.

How to Install Vapor Barrier Under a Slab

Installing a vapor retarding barrier is almost the last step before pouring a concrete slab. Ensure the ground, aggregate, and/or sand layers have been compacted, tamped, or rolled level. Roll out the vapor membrane material on top of the last layer. It should be the last layer before the concrete – even on top of any rigid foam insulation.

The barrier should go 3” to 4” up any walls and be sealed to the foundation concrete with appropriate binders, or extend 4” to 6” beyond the formed framework. All seams in the barrier must have a minimum of a 6” overlap – some products require a 12” overlap – and also be taped or sealed together. Cut small openings for protrusions through the ground layer, such as plumbing or electrical work, and tape or seal around them to maintain the moisture barrier.

Vapor Barrier for Existing Concrete Slab

Existing concrete slabs that do not have a vapor retarder or barrier will permit moisture to move through them. The moisture will commonly pass through the slab and evaporate if located outside, which is fine. Unfortunately, that moisture going into heated structures can cause dampness, mold, mildew, and rot.

It is impossible to lift a concrete pad and slide a vapor barrier under it. Attempting to apply an impermeable sealer to the concrete surface without a vapor barrier frequently results in the sealer bubbling and delaminating. The trapped moisture will dissolve the bond between the concrete and sealer, resulting in an expensive mess.

Some professionals recommend laying a vapor barrier on top of the concrete and covering it with a subfloor or floating wood floor. Another alternative is filling all cracks with epoxy, grinding or roughing the concrete surface, applying an epoxy vapor block, and then adding more layers of epoxy topping. A process that can quickly run into the thousands.

Best Moisture Barrier for Protecting Concrete Slabs and Floors

A moisture retarding barrier needs to have a permeance of 0.06 Perms or less. It should also be at least 6 one-thousandths of an inch thick to withstand punctures and tears during the concrete pour. The IRC identifies 6-mil polyethylene as the minimum acceptable barrier for under concrete, and the ACI states 10-mil poly as their minimum recommended choice.

Different manufacturers produce a range of polyethylene vapor diffusers or retarders from 6-mil to 20-mil, with the common choices being 6, 10, 15, and 20-mil. The permeability, like the thickness, varies from product to product, as does the price. Look for a product with the lowest permeance and greatest thickness for the best vapor block and resistance to punctures.

Plastic Sheeting Roll 6 Mil

Rolls of 6-mil polyethylene with a permeability of 0.06 Perms are available in varying sizes and colors. They are commonly clear or black and in 10’x100’ rolls that cover up to 1000sqft and 20’x100’ rolls for 2000sqft.

The sheeting is durable and low-density for use as a vapor retarding barrier in hot or cold climates. The 6-mil is the minimum thickness accepted by most Building Codes for use as a vapor barrier under concrete.

Plastic Poly Sheeting 10 Mil

The ACI identifies 10-mil polyethylene as the minimum it recommends for use as a vapor retarder under concrete pads. Available in different sized rolls, with 10’x100’ and 20’x100’ being the more common choices.

The black, blue, green or translucent white poly sheeting is denser and thus more durable than thinner sheeting, so more resistant to punctures by stone edges. Plus, it has a permeability between 0.0183 to 0.03 Perms (50%+ better than 6-mil) depending on the manufacturer.

Plastic Poly Sheeting 15 Mil

Heavier vapor barrier material with greater puncture resistance and tensile strength, such as 15-mil polyethylene, has been worming its way under residential concrete slabs. The permeability of 15-mil ranges from 0.0063 to 0.0158 Perms (10x or more than 6-mil), depending on the manufacturer.

The green, blue, black, white, or green 15-mil poly is a more effective barrier and will greatly reduce moisture and vapor through the concrete into living areas, reducing the risk of mold, mildew, and dampness.

Plastic Poly Sheeting 20 Mil

Rolling 20-mil polyethylene sheeting out will provide the most puncture resistant and greatest tensile strength vapor barrier presently on the market. The heavy-duty material can be used for crawlspace vapor barriers too.

With a permeability of 0.0052 to 0.038 Perms, the yellow, white, blue, or black 20-mil can also be used as a pond liner, making it an excellent choice for an under concrete moisture barrier.

Conclusion

Concrete pads that will be inside heated structures or those outside that will be coated with an impermeable material should have a vapor barrier. Concrete that will be exposed to the elements to weather naturally can be poured without a vapor barrier.

However, if you’re thinking about maybe enclosing it later in a heated structure or painting it with an impermeable seal, put a vapor retarder under it. Hopefully, you have a better understanding of what a vapor barrier is and how to select and install one.