Moisture Barrier for Flooring: Types and Installation

Moisture Barrier for Flooring

In the world of flooring, moisture barriers – also called vapor barriers or moisture retarders – are a key component for protecting wooden floors. These barriers are moisture-resistant layers placed beneath flooring materials to prevent water from seeping through.

Their main job is to stop or slow down the movement of water – whether it’s vapor, condensation, or liquid – so that the flooring doesn’t get damaged. This is especially important for wood floors, including solid hardwood, engineered wood, and laminate planks. Without a moisture barrier, these types of flooring can suffer from problems like expansion, contraction, warping, and even rot. Using a moisture barrier helps keep your wooden floors looking great and lasting longer.

Learn more about choosing and installing moisture barriers for hardwood flooring.

The classification of vapor diffusers

Water vapor is measured in units called ‘perms’, which stands for permeability. To understand how well a material can block moisture, vapor barriers are divided into three classes based on how much water can pass through them.

  1. Class I vapor retarders: These are the most impermeable barriers, meaning they don’t let any water through. Examples include glass, sheet metal, plastic sheets, and rubber membranes. While highly effective at preventing moisture from passing through, class I barriers can sometimes be too impermeable for wood floors. They might trap moisture, leading to issues like mold or mildew between the barrier and the wood.
  2. Class II vapor retarders: These materials are semi-permeable, allowing some moisture to pass through. Examples are unfaced expanded or extruded polystyrene, 30-pound asphalt-coated paper, plywood, and bitumen-coated kraft paper. These are often a good choice for wood floors as they strike a balance between allowing some moisture to pass through and providing sufficient protection. 
  3. Class III vapor retarders: These are the most permeable barriers, meaning they allow water vapor to pass through easily. Examples include gypsum board, unfaced fiberglass insulation, board lumber, concrete block, brick, and 15-pound asphalt-coated paper. These can also be appropriate for wood floors in certain situations. They allow more moisture to pass through, which can be beneficial in environments where some breathability is needed to prevent moisture buildup. 

Types of moisture barriers for flooring

Choosing the right moisture barrier for your flooring installation depends on a few key factors. You’ll need to consider the type of flooring, the subfloor material, and the environmental conditions

It’s a good idea to consult with flooring professionals or manufacturers to get expert advice on the best moisture barrier for your project. Do not hesitate to reach out to First Atlanta Flooring if you need guidance. 

Here are some common types of moisture barriers for flooring:

  • Polyethylene sheeting: This is a popular and cost-effective vapor barrier. It comes in rolls and is laid over the subfloor before installing the flooring. This plastic sheeting effectively prevents moisture from rising through the floor, making it a common choice for concrete subfloor installations. It’s available in various thicknesses, with thicker sheets providing better moisture resistance.
  • Bitumen-coated kraft paper: Often used under hardwood flooring, bitumen-coated kraft paper is another reliable vapor barrier. It comes in rolls and is designed to block moisture transfer from the subfloor to the wood flooring. This barrier also provides a smooth surface for installation and helps reduce squeaks by minimizing friction between the subfloor and the hardwood.
  • Rubberized asphalt membranes: These are versatile vapor barriers suitable for various flooring types, including wood, laminate, and vinyl planks. These membranes come in rolls or sheets and present a rubberized asphalt layer that sticks to the subfloor, creating an effective moisture barrier. The flexibility of rubberized asphalt makes it an excellent choice for uneven subfloors.
  • Aluminum foil vapor barriers: These are great for concrete subfloors. The foil layer reflects heat and moisture, keeping them away from the flooring material. These barriers are often used with other underlayment materials to provide a great moisture defense system.
  • Epoxy moisture barriers: They are coatings applied directly to concrete subfloors, creating a seamless and durable moisture-resistant layer. They are particularly effective in areas with heavy moisture, like basements and high-rise buildings. Besides preventing moisture intrusion, epoxy barriers offer extra protection against chemical and environmental damage.
  • Cork underlayment: It offers both moisture resistance and sound insulation, making it a popular choice under laminate and engineered wood flooring. Cork’s natural properties resist microbial growth, adding an eco-friendly aspect to moisture control.
  • Moisture-resistant underlayment: Some flooring materials come with built-in moisture-resistant underlayment. Designed to address specific moisture concerns, these underlayments often feature high-density foam or rubber, which resist moisture and provide additional support and insulation.

Installing a moisture barrier for flooring

Before installing your moisture barrier, it’s important to understand the different types available:

  • Underlayment with an attached vapor barrier: This type simplifies the installation process. If your barrier is attached to the underlayment, simply install the underlayment, then lay your new flooring on top.
  • Glue-down moisture barriers: These barriers are adhered directly to the subfloor.
  • Vapor barrier rolls: These can be taped together to form a complete floating sheet and may come with a self-taping option. In this case, you’ll need adhesive, such as tape or glue, you’ll need the vapor barrier adhesive, a knife or scissors, and the vapor barrier itself. Here’s how to install it:

1. Lay out your moisture barrier in any area where moisture is a concern.

2. Start along one edge of the room and tape the sheet to the subfloor. When connecting two sheets, overlap them by six inches and use vapor barrier tape to ensure a strong bond and excellent moisture control.

3. Continue across the room, starting from the same edge, until the entire area is covered.

Your moisture barrier for flooring should be a continuous sheet without any damage or holes. If there are any punctures or tears, repair them before installation to prevent moisture from seeping through.

The pros of moisture barriers for flooring

There are generally no downsides to installing a moisture barrier with hardwood flooring, making the extra effort worthwhile. The benefits are substantial, including:

  • Mold-resistance: Mold thrives in damp environments. A moisture barrier prevents water or vapor from accumulating underneath the floor, stopping mold growth before it starts.
  • Prevents wood rot: Natural hardwood and many flooring composites can rot when exposed to water. A moisture barrier completely prevents this issue.
  • Prevents cupping and other issues: Wood is hygroscopic, meaning it absorbs and releases moisture from its environment. Excessive moisture can cause cupping and other problems in hardwood floors. Installing a moisture barrier helps prevent these issues, keeping the floors in better condition.

A moisture barrier installed beneath the flooring will endure for the same lifespan as the floor itself. Once the flooring is in place, the vapor barrier underneath remains undamaged over time, since it’s not subjected to foot traffic or other environmental damaging factors.

For advice on hardwood flooring or details about any of our product offerings, contact First Atlanta Flooring and we’ll respond promptly to assist you!