Benefits of Using 6 Mil Poly or Felt as a Moisture Barrier Under Flooring

Moisture is one of the biggest threats to the longevity and performance of flooring. Whether you are installing hardwood, laminate, or engineered flooring, moisture from the subfloor can cause warping, mold growth, and structural damage. Using a moisture barrier is a simple yet effective way to protect your flooring investment. Two popular options are 6 mil polyethylene (poly) sheeting and felt underlayment. This post explores why these materials are essential moisture barriers and how they help maintain the quality and durability of your floors.

What Is a Moisture Barrier and Why Is It Needed?

A moisture barrier is a layer of material installed between the subfloor and the flooring surface. Its main purpose is to prevent moisture from rising up from concrete slabs, crawl spaces, or damp subfloors and damaging the flooring above. Moisture can cause:

• Warping and cupping of wood floors

• Delamination of engineered flooring

• Mold and mildew growth under the floor

• Adhesive failure in glued-down floors

  
  

Concrete slabs, especially those below grade or in humid environments, often contain moisture that can seep upward. Even wood subfloors can hold moisture after rain or plumbing leaks. Without a proper moisture barrier, this moisture can compromise the flooring’s integrity and appearance.

Why Choose 6 Mil Polyethylene as a Moisture Barrier?

6 mil polyethylene sheeting is a plastic film that is 0.006 inches thick. It is widely used in construction as a vapor barrier because it is:

• Highly resistant to moisture penetration

• Durable and tear-resistant

• Affordable and easy to install

• Compatible with many flooring types

  
  

Key Benefits of 6 Mil Poly

• Effective Moisture Protection

The thickness of 6 mil poly makes it a strong shield against water vapor. It blocks moisture from concrete slabs and prevents it from reaching the flooring above.

• Prevents Mold and Mildew

By stopping moisture, it reduces the risk of mold growth under the floor, which can cause health issues and unpleasant odors.

• Easy Installation

Poly sheets come in large rolls that can be cut to size and overlapped to cover the entire subfloor. Tape seals the seams, creating a continuous barrier.

• Cost-Effective Solution

Compared to specialized vapor barriers, 6 mil poly is inexpensive and widely available at home improvement stores.

When to Use 6 Mil Poly

6 mil polyethylene is ideal for:

• Concrete slab subfloors

• Areas with high humidity or moisture concerns

• Under laminate, engineered wood, and vinyl flooring

  
  

It is not recommended for use under hardwood floors nailed to wood subfloors, as it can trap moisture and cause wood to rot.

Why Felt Underlayment Works as a Moisture Barrier

Felt underlayment is a dense fabric material made from recycled fibers or synthetic blends. It is often used under hardwood and laminate flooring to provide cushioning, sound absorption, and moisture protection.

Benefits of Felt as a Moisture Barrier

• Moderate Moisture Resistance

Felt absorbs some moisture but also acts as a barrier to slow moisture transfer from the subfloor.

• Improves Floor Comfort

Felt adds a soft layer underfoot, reducing noise and making floors feel warmer.

• Enhances Flooring Longevity

By controlling moisture and providing cushioning, felt helps prevent floor damage and squeaks.

• Easy to Handle

Felt rolls are flexible and easy to cut, making installation straightforward.

When to Use Felt

Felt is best suited for:

• Wood subfloors where some moisture control is needed

• Hardwood flooring installations that require cushioning

• Areas with moderate moisture levels

  
  

Felt alone may not be sufficient for concrete slabs with high moisture. In those cases, combining felt with a polyethylene vapor barrier is recommended.

Comparing 6 Mil Poly and Felt for Moisture Control

| Feature | 6 Mil Polyethylene | Felt Underlayment |

|--------------------------|----------------------------------|---------------------------------|

| Moisture Barrier Strength| High | Moderate |

| Cushioning | None | Yes |

| Sound Absorption | None | Yes |

| Installation Ease | Simple, requires taping seams | Simple, cut to fit |

| Cost | Low | Moderate |

| Best Use | Concrete slabs, high moisture | Wood subfloors, moderate moisture|

Choosing between the two depends on your subfloor type, moisture levels, and flooring material. For concrete slabs, 6 mil poly is usually necessary. For wood subfloors, felt may provide enough moisture control with added comfort.

Practical Tips for Installing Moisture Barriers

• Prepare the Subfloor

Clean and dry the subfloor before installation. Remove debris and repair cracks.

• Overlap Seams

When using 6 mil poly, overlap sheets by at least 6 inches and seal with waterproof tape.

• Extend Up Walls

Run the moisture barrier a few inches up the walls to create a complete seal.

• Avoid Punctures

Be careful not to tear the poly during installation. Use staples or tape carefully.

• Combine Materials if Needed

In some cases, use 6 mil poly under felt for extra protection on wood subfloors.

Real-World Examples of Moisture Barrier Benefits

• A homeowner in a humid climate installed 6 mil poly under laminate flooring on a concrete slab. After two years, the floor showed no signs of warping or mold, despite seasonal moisture fluctuations.

  
  

• A contractor used felt underlayment beneath hardwood floors in a dry wood-frame house. The floors felt warmer and quieter, and no moisture-related issues occurred.

  
  

• In a basement renovation, combining 6 mil poly with felt under engineered wood flooring prevented moisture damage that was common in previous installations without barriers.

  
  

These examples show how moisture barriers extend flooring life and improve comfort.

Final Thoughts on Using 6 Mil Poly or Felt Under Flooring

Installing a moisture barrier is a crucial step in any flooring project. Both 6 mil polyethylene and felt underlayment offer valuable protection against moisture, but they serve different needs. Use 6 mil poly for strong moisture resistance on concrete slabs and high-risk areas. Choose felt for cushioning and moderate moisture control on wood subfloors.