Moisture Barrier: Protect Your Floor Before It’s Too Late

A moisture barrier offers more protection than you might think. If you don’t prevent moisture build up in your home, major problems will arise. You want to keep moisture under control and out of your home.  

moisture barrier

Protecting your home from water vapor can be the difference in it lasting ten years or a hundred. If you don’t know how to install a moisture barrier, you might want to learn. 

What Is A Moisture Barrier?

A moisture barrier blocks moisture. It’s also known as a water barrier or vapor barrier. However, the barriers aren’t the same. 

A vapor barrier blocks water. A water barrier blocks liquid. However, a moisture barrier guards against moisture, which is a combination of water and vapor.

Vapor barriers also safeguard against gas. However, a moisture barrier blocks all liquids.  

What Is An Air Barrier?

An air barrier prevents air from passing through walls, floors, and ceilings. It is a type of insulator for your home. The catch is that sometimes the two functions can overlap.

Air is harder to block than moisture, yet hybrid insulators block both air and moisture. 

Why Do I Need A Moisture Barrier?

What Is An Air Barrier?

Here are all of the reasons you should install a moisture barrier:

Termite And Pest Prevention

Moisture attracts pests like termites. Termites are the biggest threat to your home. Termites break down wood and can cause a home to collapse.

Climate Issues

A moisture barrier is a must if you live in a cold climate where it often snows, the snow will melt, and water will get inside your home. Also, if you live in a humid environment, a barrier will be necessary.

Dry or mild climates won’t need a moisture barrier. Most experts recommend moisture barriers in climates where the possibility of moisture entering a home due to weather is high. 

Moisture Areas

A moisture barrier or vapor barrier is recommended in crawl spaces and basements. The spaces have a lot of natural moisture buildup. A moisture barrier is necessary on walls, ceilings, and floors. 

Concrete Floors

One of the reasons that basements need a barrier is because they have concrete floors. If you are laying the floor over them a barrier between the concrete floor and top floor is necessary to prevent leakage and build up that could harm the floors above. 

Rot And Mold

If moisture gets in wood and other materials, it may rot and mold. Moldy wood can affect your health while rotting wood can collapse when it loses structural integrity.

Protect wood or mold-susceptible materials with a moisture barrier. With bathrooms and kitchens, you should be concerned with exterior moisture entering from above. 


Moisture causes humidity. The best way to reduce indoor humidity is to prevent moisture. You won’t need as much air if you have less humidity. Once this is achieved, your home will be energy-efficient and you’ll save money. 

Three Classes Of Moisture Barriers

Types Of Moisture Barriers

Moisture barriers come in three classes and are graded based on their materials. 

The three classes include:

  • Class I -This is the highest barrier class because it is impermeable. It’s also the strongest vapor barrier class. It includes glass, sheet metal, polyethylene sheets, and rubber. 
  • Class II – The barriers are semi-permeable. They block moisture, but not all of it. Class II materials include unfaced expanded or extruded polystyrene, 30-pound asphalt coated paper, plywood, and bitumen coated kraft paper.
  • Class III – The vapor barriers are permeable, making them the lowest forms of moisture barriers. It doesn’t work as a barrier but instead allows moisture to pass through. They’re made with materials like gypsum board, fiberglass insulation, board lumber, concrete block, brick, and 15-pound asphalt coated paper.

How To Install A Moisture Barrier

How To Install A Moisture Barrier

There are two main ways to install a vapor barrier and it all depends on where you’re installing it. Whether it is under a floor or in a crawl space. 

Installing A Moisture Barrier Under Flooring

Installing a moisture barrier under your new floor may be necessary, especially if it’s concrete. You can start by rolling the moisture barrier onto the floor. Make sure you lay it out with a few extra inches on both sides. 

You should also overlap each row with the next one so there isn’t a chance of anything leaking between them. Then, seal each seam with moisture resistant tape to create a secure hold. 

Finally, cut around the edges of the barrier either before or after laying the flooring. Each edge of the floor must be covered so you may have to wait until after you lay the floor to get a close cut. 

Installing A Moisture Barrier In A Crawl Space

One of the most common places to install a barrier is a crawl space. The small areas are hot spots for moisture in humid climates. 


The first thing you need to do to prevent the moisture is to create a drainage system around your house so water from rain flows out instead of in. Something like a French drain is an attractive and effective method.

Then, cover the bottom of the crawl space with a plastic moisture polyethylene vapor barrier. Just like when laying the barrier over a floor, you need to seal the seams with tape to prevent the moisture from crawling through the cracks.

You can bend the barrier to cover the crawl space walls. You can staple or tape them to secure the walls. Since the space isn’t meant to be seen, they don’t need to be cut. You can use fabric stakes in the dirt to secure the barrier.


You’ll need a drainage system for your pipes. If the pipes leak, you want them to drain outside of the house.  If water puddles inside your home it can cause severe damage. 

When you cover your walls with moisture resistant insulation, this will prevent your pipes from freezing. Finally, use caulk on areas or holes that can’t be protected otherwise. If you don’t know how to use caulk, learn how with this guide on caulk

Frequently Asked Questions (FAQ)FAQ

Is Underlayment And A Moisture Barrier The Same?

Some moisture barriers are simple, and others are part of the underlayment you would install anyway. Moisture barrier with underlayment makes installing over moisture prone subfloors easier.

Should A Moisture Barrier Go Under Subflooring?

Placing a sheet of plastic between the subfloor and the flooring will prevent flooring damage. If you don’t have a barrier, when your plywood subfloor develops moisture, then mold will emerge and ruin your flooring.

Do I Need A Moisture Barrier Under Vinyl Plank Flooring?

Vinyl plank flooring is durable. The flooring can resist dents, dings, and scratches. If you’re laying vinyl planks over an existing cushioned vinyl floor or a low-grade tile floor, you won’t need an underlayment. 

However, you’ll need one in other applications including covering existing hardwood floors, vinyl flooring that isn’t cushioned, and concrete floors. However, when vinyl plank flooring has underlayment, you may need to install a vapor barrier.

Do I Need A Damp Proof Membrane On Floorboards?

When installing wood flooring, solid or engineered, on a concrete base the use of a damp proof membrane is advised. No matter how dry your floor feels, it will still be damp. If left untreated, cupping, lifting, and other floor damage will occur.

What Are Some Examples Of Water Vapor?

Water vapor is water in a gaseous state. This happens when water is diffused as a vapor in the atmosphere and at a temperature below boiling point.

Steam is a form of water vapor. The floating mist that hovers over a boiling pot of water is vapor. A vapor barrier is installed to prevent moisture from accumulating inside walls.

Moisture Barrier Conclusion

If you aren’t sure if you need a moisture barrier, ask a professional. They will come to your home and survey your floor. You may discover that what you need is a vapor barrier.

Vapor barriers are designed to retard the migration of water vapor. They’re not intended to prevent air migration. The barriers were intended to prevent surfaces from getting wet, but they prevent them from drying.

Vapor barriers are necessary for crawl spaces. A moisture barrier is the minimum amount of protection you should have in your crawl space. Most people will forget about protecting their crawl spaces, and when they do, serious problems arise.