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

Section R506 of the IRC identifies how concrete floors should be built, as does the ACI. 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.Ĭompaction of the base materials is necessary to ensure it is stable. Site prep involves the removal of vegetation, roots, and foreign material. Proper site preparation is important when planning a concrete pour. Heavy loads placed on or moving across the concrete can also cause it to fail or crack.

Excess ground moisture can also degrade the concrete causing it to fail or crumble, especially if there is any freeze-thaw action.

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Pouring concrete directly onto dirt can cause the pad to crack or shift out of level as the soil underneath settles. While it is acceptable to pour sidewalks, patios, driveways, and shed floors directly on undisturbed or compacted soil, it isn’t recommended. Site preparation for slab-on-grade or concrete pads on the ground is important to the structural integrity and life span of the concrete. ASTM E-1745 also advocates for thicker poly or other membrane material to be used to form more of a barrier against vapor. 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. The American Concrete Institute (ACI) recommends a minimum of 10-mil polyethylene or thicker for vapor retarders under concrete. Patios, walkways, carports, and driveways likewise don’t require a barrier unless they will be enclosed within a heated structure in the future. 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. It also requires seams or joints in the barrier to have a minimum 6” overlap sealed with compatible material. 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. Is a Vapor Barrier Required by Code?Ĭoncrete slabs that will be within heated structures must have a vapor barrier.

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Always check with the local building department or a professional before pouring. So, anything you want in or under the concrete needs to be there before the pour. Do You Need a Vapor Barrier Under a Concrete Slab?Īlthough 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.

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

A vapor retarder or barrier minimizes the movement of moisture through 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. The permeable nature of concrete allows ground moisture to penetrate upward to flooring or anything in contact with the concrete. A common practice is to use a thin layer of polyethylene (plastic) sheeting as a vapor barrier. True moisture or vapor barriers have a permeance of 0.00 Perms.

To be considered impermeable, a vapor barrier or retarder requires the permeance of 0.1 Perms IMP or less.

Best Moisture Barrier for Protecting Concrete Slabs and FloorsĪ vapor barrier is a material that retards or restricts moisture vapor penetration and is also known as a moisture or vapor retarder.

Vapor Barrier for Existing Concrete Slab.

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How to Install Vapor Barrier Under a Slab.What Can I Use for Vapor Barrier Under Concrete?.What Is the Difference Between a Moisture Barrier and a Vapor Barrier?.What Happens if You Don’t Use a Vapor Barrier?.Do You Need a Vapor Barrier Under a Concrete Slab?.