Do you need a vapor barrier under a plywood subfloor?

To use a vapor barrier or not is indeed a profound question. There are many arguments for or against the use of them. It will ultimately depend on the moisture levels present in your home, whether you should use one or not.

Should you use a vapor barrier? The answer is yes, especially if a manufacturer requires it to keep your warranty intact. It would help if you kept in mind that a moisture barrier is not a substitute. Even when installing a moisture barrier, you should still do all that you can to ensure that your plywood is dried within an acceptable range of five to seven percent.

Vapor barrier under plywood subfloor

To check for moisture within your plywood, you will need a tool called a moisture meter. Whether you’re using a pinned meter or a meter without a pin, if your readings reveal that the plywood is not within an acceptable range, you should do everything you can to reduce the plywood’s moisture before installing your flooring. This could include installing a vapor barrier.


What Is a Vapor Barrier?

Although it may not be obvious, moisture is present throughout your house. As the seasons and the weather changes, you can guarantee that moisture levels in your home will rise. It isn’t visible to the naked eye, however. The only time you will know that you have high levels of moisture is when you smell musty odors or see mold and mildew growing.

A vapor barrier, or vapor retarder as it is also commonly referred to, is a sheet of plastic used to keep moisture from penetrating your subfloor and ruining your flooring. It is placed between the plywood and the new flooring to help reduce the moisture levels, mainly if the floor is above a crawl space.

It can also be used on the second floor, especially in a room above a kitchen or bathroom where constant moisture content changes. It is also helpful between concrete and flooring because, believe it or not, concrete does create moisture.

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Placing a sheet of plastic between the subfloor and the flooring will keep your flooring from getting damaged. If there is no barrier, once the plywood subfloor starts to develop moisture, the moisture can create mold, which can destroy your flooring.

If you have carpet, it can cause mold and mildew to grow through it. If you have hardwood, not only will it cause mold and mildew, but it will also start to warp or cup the hardwood. Having the right choice of vapor barrier will minimize this problem by keeping the moisture out.

Types of Vapor Barriers For Subfloors

There are many types of vapor barriers that you can use. The most common will be a polyethylene film. This is a plastic sheeting made from petroleum that keeps moisture out when it is used for construction. Vapor barriers can also help keep sound out. Here are some common types of vapor barriers on the market.


VisQueen is a low-density polyethylene film that was introduced in the 1950s by Visking Corporation. Today it is produced by British Polythene Industries Limited. It is used for various reasons, including as a drop cloth when painting, in crawl spaces, and between walls. VisQueen is also used as a vapor barrier between the ground and the home’s subfloor.

Painter’s Plastic

Painter’s plastic is a high-density polyethylene film used primarily by painters when painting home interiors or other interiors of buildings. Although this is its primary purpose, it is very versatile. You can use it for a variety of different projects and purposes. Because it is waterproof, it is often used as a vapor barrier to prevent moisture from damaging the subfloor.

Foam with Plastic

Another type of vapor barrier is a foam that has plastic attached to it. These are particularly good when you are installing carpet. You typically put the plastic side down and allow the foam to face up. This will keep the moisture from the concrete or the ground from seeping through into the carpet, creating vapor that can build mold and mildew underneath your carpet.

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Vapor Barrier When You Have Plywood Subfloors

Sometimes a vapor barrier is not recommended when installing hardwood floors over the plywood floors, particularly if you are gluing the hardwood to the subfloor. The best way to know if you should use it or not is to read the manufacturer’s warranty. Not using one when a manufacturer requires it may void your warranty.

If you want to be extra careful with moisture issues, you can install a vapor barrier before you install your subfloors, mainly if the subfloors are right above the ground. Placing subfloor directly onto the soil can cause not only moisture issues but can also invite termites and other vermin to start destroying your plywood subfloor.

Vapor Barrier Within a Crawl Space

Besides having a vapor barrier on top of the subfloor, it may be necessary to have it under the subfloor when the subfloor is over a crawl space. When you are prepping your subfloor to get it ready for your flooring, you not only want to check the moisture level of the plywood subfloor but also the humidity in the crawl space below.

If you have decided that you need to replace the subfloor before starting your floor project, but you have a crawl space beneath the subfloor, it’s a good idea to make sure that the crawl space is dry. So, your DIY project starts actually after you have fixed any problems in the crawl space.

You probably thought that your DIY project would be simple. Maybe you heard lots of squeaking and saw lots of damage with your subfloor and decided that not only did you want to install new flooring, but you need new subfloors, too. It is a good idea that you make sure to check for moisture issues within the crawl space. This could include standing water, musty smells, a damp, warm feeling in the space, or mold growth, or even the presence of pests.

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If you cut corners and skip the crawl space moisture issues, you will end up with a very big problem on your hand that will cost you even more money in the long run. Not only will you have to replace your hardwood flooring and subfloor again but you will also have to fix the crawl space moisture issue.

To ensure that your crawl space is adequately insulated, you may want to call in the experts. Simply laying down a vapor barrier may not be all that’s needed to fix the moisture problem in your crawl space. You may have to have the mold in the space removed, which may need to be remediated by specialists.

Also, the area may have to have the moisture removed with a dehumidifier before properly insulating it. However, once the crawl space is insulated, you can resume your flooring project, and rest assured that your subfloor won’t be destroyed by moisture coming from the crawl space.

Another Important Consideration – Install a Vapor Barrier If You Have An Indoor Pool

If you have an indoor pool located in your basement, it is essential to install a moisture barrier over the floor above the pool. Indoor pools create a high level of moisture within the house. Therefore, installing a vapor barrier in the floors and the walls in rooms surrounding the pool area is critical. Otherwise, over time, the moisture from the pool room will seep into the walls and floors surrounding it and start to cause mold and mildew issues.