If you are planning to build your own shed, planning to have a vapor barrier would greatly add to the longevity of your shed. If you plan to have a concrete slab base, the vapor barrier will keep ground moisture from seeping through the concrete into your shed.
This is especially important if you plan to use the slab as your shed floor. All kinds of moisture problems, including mold, mildew, and mustiness, can be caused by moisture seeping up through the concrete.
You don’t need a 6 mil vapor barrier in your shed if its not going to be insulated, and you’re not going to be heating or cooling it, however if you going to be using a concrete slab a 6 mil vapor barrier under the slab will prevent condensation from coming in from the ground.
If your shed will be based on deck piers, skids or sunken concrete piers, the airflow underneath the shed will do a great deal towards keeping moisture from your shed floor, but a vapor barrier will add to that protection.
A 6 mil vapor barrier means that the plastic is six-thousandths of an inch thick. While this thickness has been popular in sheds for some time, some authorities urge the use of 10 mil or even 20 mil thick vapor barriers.
This is because the 6 mil plastic can tear while installing or pouring concrete for a slab over it. It has also been shown to be a bit permeable to moisture, so if you live in a humid area or an area that gets a lot of rain, you may want to consider thicker plastic for your floor barrier.
How Do You Know if You Need a Vapor Barrier?
If you live in a dry area that doesn’t get much rain, you probably don’t need a vapor barrier, especially if you build your shed raised off the ground by a few inches. Most sheds, however, can benefit from the vapor barrier. It’s much better to install one even if you think you may not need it than to find out later that you should have installed one. It really doesn’t add much to the cost and is easy to install.
Just about any shed could use a vapor barrier for the floor. Whether you install one elsewhere in the shed depends on whether you insulate the shed and heat or cool it. If you need more information on preventing mold in your shed check out my article How to Prevent Mold in Your Shed.
Do I Need a Vapor Barrier When Insulating a Shed?
If you are going to insulate your shed, you should install a vapor barrier onto the walls. This is especially true if you plan to heat and/or air condition your shed. The difference between outside and inside temperatures will give moisture a place to form somewhere in the walls. This is where you don’t need to consider a barrier thicker than 6 mils. Where you install the barrier depends on your climate.
If you don’t plan to use heating or cooling inside, you actually shouldn’t use a vapor barrier on the walls. Even if your shed will be insulated, as long as there is no climate control, the barrier may actually help trap moisture that would flow out at night.
During summer, especially in hot climates, moisture will build up in the shed during the day when the air gets superheated. At night, however, the airflow through the shed will actually get rid of the day’s built-up moisture as drier air flows through. Of course, you will need sufficient ventilation built into your shed, such as gable vents and perhaps a roof vent of some type.
If your shed never gets any airflow unless your door and/or window are open, you could have problems. While it may seem like providing ventilation to your shed in the form of air vents and/or roof vents works against your climate controls, ventilation is key to keeping the air flowing inside and keeping moisture from forming. If your on a budget you may be wanting to use the cheaper black plastic as a vapor barrier, check out my article Can You Use Black Plastic for Vapor Barrier? for more information.
Where Should a Vapor Barrier be Placed in a Shed?
In a cold climate, moisture will condense where the warmer inside air meets the cold outside air, forming beads of water. In this case, the vapor barrier should be placed on the inside of the framing. It will prevent moisture from inside the shed from traveling into the insulation. Moisture in insulation lowers its insulating ability and can result in mold or rot.
If you live in a hot, humid area and you put air conditioning in your shed, the moisture will condense where the hot outside air meets the cooler inside air. This means that the vapor barrier should be installed on the outside of the framing to prevent the hot air from flowing into the cooler zone. If this seems a bit hard to remember, just keep in mind that the vapor barrier goes on the cool side of the studs.
Do I Need a 6 mil Vapor Barrier Under My Shed?
You do need at least a 6 mil vapor barrier underneath your shed unless you live in a very dry climate. You might consider using a thicker barrier on the underside of your shed and leave the thinner, 6 mil plastic for vapor barriers on the walls.
Even if you have a raised shed with at least six inches of airflow underneath, after heavy rain moisture will evaporate from the ground underneath your shed, and without a breeze, moisture will be able to condense on your joists and subfloor.
If you plan to enclose the space under the shed to keep out critters, you will need a moisture barrier. In addition, install vents in the enclosing material to keep air circulating underneath. In addition to keeping moisture from damaging your shed wood, remember that insects need moisture as well.
Termites are attracted to damp or moist wood, as it makes the wood easier to get into. And mosquitoes can lay eggs in as little as 1/8th inch of water, so if you have any small puddles that develop under your shed, you may have a mosquito breeding farm that is hard to get to. Get more information in my article Is a 6-mil Vapor Barrier Needed Under a Shed?
What Type of Vapor Barrier Do I Need?
- Ceiling – If your shed will be heated and/or cooled, you will also need a vapor barrier for the attic as well as on the walls. This can be the 6 mil thickness, but there is another option. Installing a drywall ceiling covered with a good coat or two of latex paint can actually act as a vapor barrier for your ceiling.
- Walls – Again, this is only necessary if you plan to have some sort of climate control inside your shed, whether heating or cooling. You can use the 6 mil thickness here or go with a thicker type if your climate dictates. Try to get wide rolls to eliminate the number of seams in your barrier.
Fasten the plastic to your top and bottom plates and studs with a staple gun or hammer tacker. The seams should overlap a few inches and be sealed with non-permeable plastic tape. Another method is to run a bead of caulk down the center of each stud and along the top and bottom plates and place your staples right through the caulk.
The caulk will then seal the staple holes as it dries. Staples should be placed every 12 to 18 inches. Be careful not to tear or break the plastic. If you do, you’ll need to tape the rips with the plastic tape or replace the whole sheet in case of a larger tear.
Some vapor barrier manufacturers provide seaming tape, but if you’re buying the usual poly rolls, you’ll have to ask your supplier to suggest sealants, caulk or joining tape as well as the best type of staples for your project.
- Floor – Even if you have a vapor barrier underneath your concrete slab or gravel base, if you are going to heat and/or air condition the shed and have a wood floor or subfloor, you will probably need an additional barrier to protect the floor.
The barrier will go between the joists or skids and the subfloor. Overlap the seams a few inches and use a sealant or double-sided tape to connect them. While the barrier underneath the base is advised to be thicker, this additional barrier will do fine with a 6 mil thickness.
Types of Vapor Barrier Materials
Most material sold as vapor barriers is reinforced polyethylene plastic sheeting. Other things that have been used include asphalt-coated paper or tar paper, and bitumen-coated Kraft paper. These materials will not give you the results that reinforced polyethylene will.
You will need to be careful in your selection, however. Most of the poly sheeting sold in hardware stores, for instance, can be made out of recycled or regrind poly. This type of plastic may contain impurities such as dirt or even moisture.
Black plastic sheeting is often thought of for vapor barriers, especially since it is better at keeping weed seeds from sprouting underneath than clear plastic and is a bit less moisture permeable, but it is made of recycled polyethylene. Be sure to ask for virgin polyethylene sheeting to ensure that your vapor barrier will give you the results you want.
Conclusion
Again, it’s much better to have a vapor barrier underneath your foundation, whether gravel or a concrete slab than to wish you had put one in once mildew or mustiness rears its ugly head. The more protection you can provide against moisture and the damage it can cause, the better. After all, you want your new shed to last a long time, just as you do your home.