What is a Moisture Barrier and When is it Needed for Flooring?

Most people are unsure of what to do with a moisture barrier when dealing with flooring. Is it even needed? If so, when? More importantly, what does a moisture barrier do? 

Overall, vapor barriers are misunderstood as a material that is only used for building walls. In reality, it’s used for both walls and floors, but for floors, they’re primarily referred to as moisture barriers. 

What is a Vapor Barrier?

Also known as a moisture barrier, a floor vapor barrier is typically a sheet of plastic that slows moisture from moving through a wall or subfloor. A moisture barrier is used when installing floors or walls in areas prone to dampness or excess moisture

Flooring moisture barriers are placed beneath the floor’s underlayment. You can buy a barrier separately or get underlayment with a moisture barrier built-in. If any water vapor penetrates through a concrete subfloor, the vapor diffusion retarder slows the movement of the moisture, preventing damage to the floor. 

Vapor Barrier FAQ

Can you use plastic sheeting as a vapor barrier?

A moisture barrier is a material that reduces moisture from moving through other materials. Therefore, a plastic sheet can be a vapor barrier. In fact, plastic (or polyethylene) sheets are a Class I vapor barrier. Meaning, in terms of excellent moisture control, a plastic sheet is one of the best options. 

How long does a flooring vapor barrier last?

A vapor barrier underneath flooring will last as long as the floor. After the floor has been installed, the vapor barrier underneath will not get damaged over time since it’s not stepped on or exposed to other elements