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Do You Need Plastic Under Concrete Driveway?

Concrete is a versatile material that you may use for various purposes, including flooring. Extensive planning is required when preparing to cast a concrete driveway or other construction. A critical choice is whether to install a vapor barrier underneath the concrete driveway or not.

Source: canva.com

Because concrete is porous, rainwater from the ground may permeate the surface. Concrete floors exposed to the outdoors should not be left unprotected unless you seal them. You should install a vapor barrier if the concrete flooring is covered with an impenetrable finish or is shielded and heated by a fixed structure.

This post will discuss what a vapor barrier is and its use. We’ll cover the distinctions between moisture and vapor barriers, what kinds to use and when you need plastic under the concrete driveway.

What Are the Characteristics of a Plastic Barrier Used in Construction?

A vapor or plastic barrier is a layer that prevents or slows the entry of moisture vapor. A moisture or vapor retarder is another name for it. A vapor barrier or retarder must have a permeance of 0.1 Perms IMP or less to be considered “impermeable.” Real moisture or vapor barriers have a permeance of 0.00 Perms, which implies nothing can get through.

Polyethylene (plastic) covering is often used as a vapor barrier. However, it isn’t always the best option.

Groundwater may seep up through the porous nature of concrete and into flooring or anything else in contact with it. Mold or mildew might form in a moist bottom, and ground vapor could escape and destroy flooring and concrete platforms. A vapor retarder, also known as a barrier, prevents water from passing through concrete.

Groundwater may flow up through the concrete in this situation. The permeance of standard concrete is 3.2 Perms IMP. Permeability of six-mil poly is 0.059 Perms IMP, whereas permeability of ten-mil poly is 0.3 Perms IMP. The thicker the poly, the more effective and long-lasting it keeps out things like water. Because one mil aluminum foil has a 0.00 Perms IMP, it’s worth noting that it doesn’t allow any air to pass through.

A thousandth (1/1000) of an inch is a mil. Mil is a unit of measurement used to quantify the thickness of various objects.

Is Installing a Vapor or Plastic Barrier Under a Concrete Driveway Necessary?

Although concrete may take up to a month to cure entirely, it begins to harden within an hour of pouring and cures entirely within 24 hours, resulting in a firm slab that you can utilize immediately. As a result, whatever you intend to put in or beneath the concrete must be present before pouring. Always check with the local building department or an expert before pouring concrete.

Is Having a Vapor Barrier Necessary by Code?

All concrete slabs used in heated structures must have a vapor barrier installed. A minimum of at least 6-mil polyethylene vapor retarder must be installed between the concrete and prepped ground, according to the 2018 IRC. It also requires a 6-inch overlay of seams or joints in the barrier, which you must seal with a suitable material.

According to the IRC, unheated structures, such as garages and utility buildings, are also excluded from the necessity for a moisture retarder unless they will be heated later. On the other hand, patios, walks, carports, and driveways do not need a barrier unless you ultimately enclose them inside a heated structure.

Vapor retarders under concrete should be at least 10-mil polyethylene or thicker, according to the American Concrete Institute (ACI). Thicker poly creates a better barrier and is more puncture resistant, according to the ACI 302.1R-15 Guide to Concrete Floor and Slab Construction. In addition, ASTM E-1745 suggests adopting a thicker poly or other covering material to offer a better vapor barrier.

What Will Happen if a Vapor Barrier Is Not Used Under Concrete Driveway?

When making a concrete plate or floor inside a heated building, most building codes require you to use a vapor barrier. A vapor barrier is under the concrete to keep the concrete from getting wet. People who live in this house might get a lot of dampness from concrete because of it.

To keep mold and mildew from growing in your home, you need to put something under your concrete floor. Without this, your home could become very wet, making it easy for mold and mildew to grow. Overlaid flooring and wall structures in contact with the base may get damp and expand or break down. It could also cause humidity and health issues on other floors of the building if too much moisture evaporates out of the building.

If you coat the concrete surface with an impermeable layer or substance, you should place a vapor barrier underneath the pad. The impenetrable surface will retain ground moisture traveling through the slab, leading the concrete to expand, cup, or otherwise deteriorate, eventually failing. You are not required to have a vapor or moisture retarding barrier for outside concrete pads or unheated buildings.

 Although a vapor or moisture retarder or barrier is not necessary beneath concrete floors used for unheated carports, driveways, and outbuildings, some DIY enthusiasts and builders recommend it. Moisture migrating through a concrete flooring into the building may produce dampness and even rust and discolor the concrete if the metal is lying on the concrete pad. Condensation may also cause impenetrable floor coatings or coverings to bubble and peel.

What Is the Difference in Terms of What Vapor and Moisture Barriers Do?

Vapor or moisture diffusers or retarders are part of a building’s moisture control. A vapor diffuser or retarder helps keep moisture vapor from entering a structure or material. Many commonly call a vapor barrier and a moisture barrier a vapor diffuser or retarder.

Builders use different barriers to prevent more moisture from coming through structures. Various products in each class are superior to others at stopping vapor motion and dispersion.

When you put concrete down, you need a Class 1 Vapor Retarder, which has a 0.01 Perm IMP or less score to keep moisture and vapor from getting in. The IRC’s lowest required barrier material score is six mil polyethylene, about 0.059 (or about 0.06) Perms IMP thick. If you use 10-mil polyethylene, you should get 0.03 Perms of permeance, which the AIC recommends.

Vapor barriers aren’t really what they seem to be. When it comes to vapor barriers, a true one has an impermeability value of 0.00 Perms IMP. It doesn’t let water or vapor through. Most of the barrier materials used in the construction industry are vapor or moisture diffusers or retarders. They stop most, though not all, of the moisture from moving.

What Kind of Vapor Barrier Can I Use Under Concrete?

To be safe, a vapor retarder placed beneath the concrete to prevent moisture movement must have a Class 1 overall score and permeability of 0.06 Perms IMP or lower, according to the code. A Perms IMP score of 0.059, which is roughly 0.06 Perms IMP. Polyethylene was not permitted to be thinner than six mils in 2018.

Pro Tip: You should choose the least breathable material and the most durable to put under the concrete flooring. American Concrete Institute or AIC says that 10-mil poly or plastic with a 0.03 Perms IMP score is the lowest barrier you should put down under residential concrete. Like 15-mil and 20-mil, thicker poly barriers are often less absorbent and durable.

We hope this article has been helpful. If you have any further questions or comments, we’d love to hear them!

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