Can You Use Black Plastic for Vapor Barrier?


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When building a shed, installing the proper vapor barrier is vital to keep moisture from seeping up from the ground and into the wooden lower parts of the shed, such as the joists and flooring. This is especially important when using a concrete slab as a foundation, as concrete absorbs moisture. If it doesn’t dry quickly enough, the moisture eventually seeps upward, affecting any wood that contacts it.

Black plastic makes a good vapor barrier. It comes in rolls or rolls of sheets and is available in the same thicknesses as clear plastic. Black plastic is more effective at preventing plant seeds from sprouting. It is also more effective at stopping moisture evaporation up into your shed base. Clear plastic, on the other hand, has a tendency to sweat, adding an additional moisture source.

Some sources state that a vapor barrier is not necessary under certain conditions, such as having the shed raised to allow airflow. However, if you have your shed on a concrete slab it still is not a bad idea to install a barrier even if the shed is raised. Moisture seeps through concrete and then can condense on the underparts of the shed during humid weather.

It is also advisable to use it on gravel foundations. If you plan to install solid or mostly solid skirting around a raised shed, which will limit airflow, a vapor barrier is a good idea. If you have a shed on skids, you might consider attaching a vapor barrier to the bottom of the subfloor between it and the skids and joists.

What Kind of Plastic Do You Use for a Vapor Barrier?

Polyethylene is the usual choice for a vapor barrier. There are other alternatives, however, such as kraft paper either coated or impregnated with asphalt, and foil skim Kraft, which is paper-backed aluminum. These last two choices are less effective in keeping moisture out of your shed than polyethylene. For more information on vapor barriers read my article Do I Need a Vapor Barrier Under My Shed?

Do Vapor Barriers Cause Mold?

Vapor barriers actually prevent mold and mildew. Even if your shed is raised from the ground a few inches, allowing airflow, if you live in a humid climate, any moisture takes longer to evaporate, keeping the ground from drying. When it evaporates, moisture travels upward, going right up into the joists and subfloor of your shed.

A vapor barrier prevents this moisture from traveling to and through the wood, preventing mold and mildew. Once you have moisture problems in the base of your shed, not only does it help deteriorate the wooden structure but also affects anything stored in your shed.

What Thickness of Vapor Barrier is Best?

While a 6 mil thickness was traditionally used for decades, it has been discovered that a 10 mil thickness is preferred. For a foundation vapor barrier, a 20 mil barrier is recommended. Thinner plastic may tend to tear, especially when pouring concrete for a foundation pad. Rebar especially can punch through a thinner plastic.

Is Black or Clear Plastic Barrier Good for Under a Shed?

Either one is great for putting under a shed. As mentioned before, the black barrier does have advantages over the clear barrier, but either one will work.

How to Install Vapor Barrier?

On the ground – If you plan on a concrete foundation, you’ll want to install your vapor barrier over a tamped base of gravel. You can pour some sand onto the gravel to level it out. The plastic allows for better concrete curing, as without it, the edges tend to cure more quickly than the middle, which can lead to curling.

If you plan for a gravel foundation, dig out the space for the gravel, then install the vapor barrier before pouring the gravel. Spreading the plastic out around the perimeter of the space about a foot will allow you to spread more gravel there to catch and drain rain runoff from the roof. For more information on preventing water under your shed read my article How High Should a Shed Be Off the Ground?

On walls – Most sources state that a vapor barrier is generally not needed unless you plan to heat and/or air condition your shed, using it as a workshop or art studio. Where you install the barrier depends on your climate. Basically, the vapor barrier goes on the hotter side of the wall.

In a cold climate, the barrier goes on the inside of the framing. A barrier on both sides is definitely not needed and will actually cause moisture problems. If you are using insulation, install that first.

Check the joists, removing any large splinters, and making sure all nail and screw heads are flat. In other words, check for anything that may poke holes in the plastic. Measure your wall, then cut strips of the vapor barrier to the lengths needed. Use a staple gun and 1/2-inch long wood staples to attach it to the studs, overlapping the edges.

Make sure all the gaps are covered. If you haven’t installed your flooring yet, you can allow a longer length to be tucked under the flooring edge. The wall covering is installed over the vapor barrier.

In warm, humid climates, the barrier goes behind the outer wall covering depending on the type of insulation and siding you choose. Ask your siding supplier for recommendations.

On ceiling – Vapor barriers are generally not needed in sheds with adequate attic ventilation, even in colder climates. That being said, some people in cold climates choose to install them. As with the walls, the barriers are only used in heated and/or air-conditioned sheds.

Similarly, the vapor barrier is installed after the insulation, with the ceiling material over that Installing all the barrier you plan for before putting on any wall or ceiling material will allow you to join the barrier edges where the walls meet the ceiling.

Conclusion

Vapor barriers are an important part of protecting your shed from the weather. Your choice of foundation, climate conditions in your area, and whether your shed is intended to be climate controlled all play a part in deciding where a barrier should be installed. Your supplier should be able to advise you on whether you need a barrier in certain places in your shed.

Recommended Resources

In this area I will go over the best resources that I have found that you will find very helpful:

Here are my favorite eBooks for beginners as well as those of you who have a lot more experience with home projects.

I know how disappointing it can be to finally find some plans online only to find out after that theirs a lot of essential information missing making these resources useless and a waste of your time!

First is “Ryan’s Shed Plans”… Provides 1,000’s of shed plans, so there’s something for everyone with detailed cross sections and very easy to follow instructions. What I really like is the material and cutting lists which means you know how much material to get.

And if you act soon, you can also get some free books: Advanced Woodworking Tips, List of suppliers to get your materials even cheaper and for you woodworking types you also get 400 free wooding plans. Definitely worth every penny… Check it out here and get your free 8×12 plan just for looking.

Second is “Ted’s Woodworking” … You get thousands of woodworking plans and they come with step-by-step instructions, material and cutting lists, very detailed plans, something for beginners as well as the professional woodworker.

You’ll also get woodworking guides and a detailed book on how to start a woodworking business and how to sell your woodworking projects for profit. See for yourself all the projects you can do and start making impressive pieces right away. Check it out here.

Third is the “Ultimate Small Shop” … This guide walks you thru everything you need to get a small workshop set-up on a budget. Goes into detail what you need to set-up, organizing your space and laying out your work areas, tools list, safety and so much more. Covers everything you need to have a complete shop.

You also get some Free bonus: The workshop cheat list, shows you where to get cheap supplies and tools. You also get a lifetime subscription to the deal alert service and so much more, see it for yourself here.

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