What is the difference between underlayment and vapor barrier?

Author: Dr. Vito Dickens  |  Last update: Saturday, March 15, 2025

Key Distinctions and Considerations Materials and Installation: Underlayment materials vary based on flooring type and performance needs, whereas vapor barriers are typically constructed from moisture-resistant materials like polyethylene or specialized membranes.

Do you need both underlayment and vapor barrier?

Very often, particularly in a below-grade installation on a concrete basement floor, a vapor barrier is recommended in addition to the underlayment. That vapor barrier would go between the subfloor and the underlayment.

Is underlayment a vapor barrier?

Vapor barrier underlayment, also known as moisture barrier underlayment, is designed to prevent moisture from seeping into your floors. In regions where humidity levels fluctuate, or in installations over concrete subfloors, the risk of moisture damage is significant.

When should you not use a vapor barrier?

Because a vapor barrier is a low permeable barrier, it keeps moisture from passing through. This is beneficial to keep moisture away from a dry surface but will trap moisture behind it if installed on a water-damaged concrete wall, causing the wall to fail further.

What are the disadvantages of a vapor barrier?

The Cons of Vapor Barrier Installation

Although vapor barriers aim to prevent moisture from entering the structure, they can also trap moisture inside. If not properly installed or upheld, this could result in the proliferation of mold and other issues related to moisture.

Why You Need Underlayment For Your Floors

What is an alternative to vapor barrier?

Carton board air barrier paper is an excellent alternative to vapor barrier plastic.

What is the life expectancy of a vapor barrier?

A high-quality crawl space vapor barrier can be an effective solution for many years, as long as it is not damaged or punctured along the way. Generally, a crawl space vapor barrier that has been correctly installed by a professional and made of high-quality materials can have a lifespan of 20 years or more.

Will mold grow under a vapor barrier?

However, if not installed correctly or if damaged, vapor barriers can actually contribute to mold growth.

Where is the best place to put a vapor barrier?

Vapor barriers are installed along, in, or around walls, ceilings, and floors. Of course this is done to prevent moisture from spreading and potentially causing water damage.

Is Tyvek a vapor barrier?

Is DuPont Tyvek® a vapor barrier? No, DuPont Tyvek® is not a vapor barrier. It is made with unique material science to keep air and bulk water out while allowing moisture vapor inside walls to escape.

What goes under underlayment?

You have the top layer that is your floor covering (Hardwood, Laminate, Vinyl, Carpet, etc.), next you have the underlayment (usually a thin layer of fiber, foam, rubber), below that you have your subfloor (plywood, OSB, concrete), and lastly you have the joists which are the foundation to your floors.

Is foam underlayment a vapor barrier?

An underlayment that offers a vapor barrier is essential to protect any flooring installed over a subfloor that emits moisture, like concrete. A thin foam underlayment made of polyethylene or polypropylene usually works best.

Should you put a vapor barrier under vinyl plank flooring?

What You Need For Your Flooring Application. We recommend for you to read the flooring installation instructions for proper installation. 95% of the time, they will recommend a 6-mil vapor barrier to be installed underneath the flooring.

What happens if you don't have a vapor barrier?

“For minor projects, the absence of a vapor barrier leads to water intrusion, leading to an unstable foundation or a cracked slab.” The American Society for Testing and Materials (ASTM) outlines national standards for underslab vapor barriers.

Do you really need underlayment?

In most cases, underlayment offers multiple benefits such as added comfort, sound reduction, and moisture protection. While some flooring types like tile or engineered hardwood might not always require it, underlayment is often a good investment to ensure the best performance and longevity of your floors.

What happens if you put a vapor barrier on both sides of insulation?

The common taboo against a double vapor barrier arises from a very real concern: If you have a Class I vapor retarder (less than 0.1 perm) on both sides of a wall, that wall has virtually no drying potential in either direction.

Where to place a vapour barrier?

A vapour barrier is placed on the inner side of the insulation and is designed to stop moist air from passing through it. A breathable membrane, by contrast, is installed on the outer side of the insulation. Breathable membranes must repel water from the outside while still allowing moisture from the inside to escape.

How do I know if I need a vapor barrier?

If you live in a mixed climate – hot and humid with several heating months in the winter, you probably need a vapor retarder. Specifically, if you live in climate zones 4C (marine), 5, 6, 7 and 8. Not sure your climate zone? You can check here.

What is the best material to use for a vapor barrier?

Materials such as rigid foam insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion. These types of vapor retarders are usually mechanically fastened and sealed at the joints. Thinner membrane types come in rolls or as integral parts of building materials.

What's the difference between a moisture barrier and a vapor barrier?

Both are designed to keep moisture out of your home and can be made from various materials. But there are a few key differences, the most apparent being that moisture barriers are better at keeping out liquids, and vapor barriers are better at keeping out water in its gaseous state.

How much does it cost to have a vapor barrier installed?

An average vapor barrier installation costs $2 to $4 per square foot or between $1,200 and $4,000 for materials and labor. *Labor costs to install add $1.50 to $3.00 per square foot on average.

Does a vapor barrier need to be sealed?

It's also important to ensure that the barrier is completely sealed so that no moisture can get through it. Using staples or tape along all edges and seams will help create a strong seal.

What are the disadvantages of vapor barrier?

Cons: This material is fairly flimsy and can be easily damaged during installation. It incorporates limited resistance to punctures and tears. Any penetrations, such as for an electrical junction box, must be taped and sealed in order to form an effective barrier.

What is the best thickness for a crawl space vapor barrier?

By sticking with a thicker mil plastic, you'll have a dryer crawl space and better air quality. We recommend using a vapor barrier of 16 mil or higher. This translates into a permeance rate of only . 0015 This is considered impermeable in American building codes.

Can you put new vapor barrier over old?

However, putting new plastic over an old vapor barrier is not a recommended approach. Whenever you embark on a vapor barrier installation, take the time to assess the condition of the existing barrier, clean up any debris, and ensure a clean slate.

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