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Is a 6-mil Vapor Barrier Needed Under a Shed?

6-mil Vapor Barrier

When you build your own shed, unless you live in a rather arid area, your shed will benefit from having a vapor barrier under most foundations. The exception would be something like a shed built with a very noticeable slope, meaning that most of the shed is very much above ground.

A 6-mil vapor barrier will keep ground moisture from seeping through a concrete slab foundation and into your shed. If you have a gravel foundation the barrier will help rainwater to run off more easily and keep your gravel from gradually sinking into the ground over time. It will also keep weeds from growing up through the gravel.

What Does 6 mil Vapor Barrier Mean?

A mil is a term of measurement that denotes the thickness of an item. One mil is equal to a thousandth of an inch. A 6-mill vapor barrier will be six-thousandths of an inch thick. The clear, reinforced polyethylene sheeting used for vapor barriers and other things is measured this way. The thinner the plastic, the less expensive your vapor barrier will be.

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Plastic rated as 6 mils has been popular for vapor barriers for some time. However, in recent years, thicker plastic has been recommended, as the barriers 6 mils thick have developed problems. Barriers 6 mils thick are susceptible to tearing when something is laid over them, such as gravel for a gravel foundation.

Even though the gravel needs to be tamped down firmly every two inches while it is being laid, the crushed gravel does have some sharp edges, and the 6-mil thick vapor barriers have been shown to be subject to tearing. This defeats the purpose of the barrier. It can tear when concrete for a slab foundation is being poured over it, even when the concrete is being poured over a rebar grid laid over the plastic.

In addition, 6-mil plastic has been shown to be permeable to moisture. If you have a concrete slab foundation, especially if you use that foundation for your shed floor, you really need to keep ground moisture from seeping up through the slab and causing moisture problems inside your shed, such as mildew.

In recent years, plastic 10-20 mils thick is recommended for a shed vapor barrier. The 20-mil thickness is especially recommended for concrete slabs. Besides withstanding the concrete pour, the thickness of the plastic will keep the concrete from curing too quickly. Concrete takes an average of 28 days to fully cure. If it dries too quickly, it can get weak and even crack.

Can I Install a Vapor Barrier Myself?

The plastic will come in a roll and is fairly easy to install, so you certainly can install it yourself. A typical roll will be about 2-1/2 feet wide by 40 feet long, adding up to 100 sq. ft. It does come in different widths, so you may choose to buy wider rolls to cut down on the amount of overlapping plastic you’ll need.

Do be careful when purchasing plastic for a vapor barrier. Stay away from any that is labeled as recycled or regrind, as this type usually contains impurities and tends to become brittle and tear over time. Some people are using black plastic as a vapor barrier, check out my article Can You Use Black Plastic for Vapor Barrier? for more information.

What Sealant Do You Use for Vapor Barrier?

Actually, you don’t need a sealant for the entire vapor barrier itself. What you will need to do is seal the overlapping strips of the barrier. There are special sealing tapes to join the sheets of plastic that are made for just this purpose. Often these tapes are double-sided. Ask your supplier what’s best for sealing the plastic you have chosen if you are in doubt.

There are caulking materials on the market as well that work for vapor barriers, but these are mostly used when installing a vapor barrier on interior walls to seal around electrical boxes and light fixtures.

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Vapor barriers are common when building homes, especially those with a concrete-lined basement. In that case, the concrete is often sealed once it fully cures, to keep any moisture that the concrete may absorb from transferring to other materials, such as wall studs or drywall. The barrier itself is not sealed as a whole.

Can I Use Plastic Sheeting as a Vapor Barrier?

Most vapor barriers are made of plastic, or polyethylene, especially those for smaller foundations, such as your shed’s. To provide even more moisture protection, some people like to use closed-cell foam insulation between the floor joists under the subfloor. While this is mostly to provide insulation to the shed, it also acts as another vapor barrier.

How Do You Install a 6 mil Vapor Barrier on Concrete?

First, you need to excavate your foundation space about six inches or so deep. This is to allow you to put a six-inch layer of crushed gravel into the space. Be sure it’s tamped down firmly. The best way is to tamp it down every two inches of depth, then pour in another layer.

In the case of a gravel foundation, the vapor barrier goes underneath the gravel. In the case of a concrete slab, the barrier goes on top of the gravel. If you plan to use a rebar grid to strengthen your slab, it goes right on top of the barrier.

One tip is that you need to make sure that your plastic covers not only the base of your foundation excavation but also goes up the sides. Your concrete can still absorb ground moisture from the sides of the slab.

In addition, if this happens while the concrete is curing, it may cause a situation in which the edges of the slab cure at a different rate than the center, which can cause real problems. While you may be tempted to cut your plastic to make it fit into the corners, it’s much better to fold the corners and seal them with tape to keep them firmly against the soil.

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A good rule of thumb is that your vapor barrier should cover any place your foundation material will touch, whether gravel or concrete. If you put gravel around your shed to channel the rain that runs off the roof, which is a good idea, you can also extend your plastic at least partway under this gravel. This will help hold it in place along the sides of your excavation while you install your foundation.

What’s the Difference Between Vapor Barrier and Moisture Barrier?

The two terms are mostly used interchangeably by many people. However, some sources differentiate the two when used on walls, saying that a moisture barrier keeps water from entering the interior of the wall cavity, while a vapor barrier prevents moisture from the air inside the building from seeping through the inner wall and condensing on the warm side of the insulation. If you have condensation in your shed right now you can read my article How to Deal with Condensation in Your Shed for ways to deal with the condensation before it damages the contents and mold begins to form.

Do Vapor Barriers Cause Mold?

The purpose of a vapor barrier is to keep moisture seeping from the ground into your concrete slab or into the shed itself. This will prevent moisture problems such as mold and mildew inside your shed.

When mold shows up inside a shed that has a vapor barrier under the foundation, it’s usually due to other reasons, such as a barrier that wasn’t installed properly or humidity inside the shed. For instance, some people advocate poking drain holes in the plastic after it’s installed, but that defeats the purpose of the barrier.

If you have mold or mildew problems developing in your shed, the likely culprit is not the vapor barrier. Usually, it’s caused by humidity in the air inside your shed condensing into moisture on various surfaces. If you have a concrete slab that doubles as the floor, the cool concrete is a great place for moisture in the air to condense, causing mildew and mold problems to anything in contact with the floor, such as items you place on the floor.

Another cause is an uninsulated shed. While most people don’t insulate the walls of a shed unless they are planning to heat the shed or install air conditioning, this can cause moisture problems, especially in cooler weather.

The interior air warmed from the sun on the roof inside the closed shed holds more moisture than the cooler air outside. This moisture can condense on any cool surface inside the shed, even the walls. This can eventually cause mold and mildew problems.

Opening your shed whenever possible will help this problem. Having a window in the shed that opens will help a great deal with airflow when the door is open as long as it’s not on the same wall as the door. Installing vents in the gable ends of your roof will help a great deal. Moisture forms on surfaces best when the air is still, so any airflow you can enable inside your shed will help.

There are products on the market that help absorb moisture from the air, usually in the form of absorbent granules that are replaced every so often. Some of these have granules that dissolve with moisture, collecting water in the bottom. You’ll be surprised at the amount of moisture that was in your shed. You can also use a cloth bag filled with charcoal briquettes or non-clumping kitty litter. These fillings must be replaced periodically as well.


Installing a vapor barrier underneath your shed is a really good idea. If you live in a humid climate, you’ll need all the help you can get to keep moisture and dampness out of your shed. A vapor barrier doesn’t cost a lot, especially if you install it yourself. It’s another thing you can do to protect your shed and its contents and provide some peace of mind. After all, you want your shed to last, so give it all the protection you can.