What Goes Under Hardwood Flooring: Essential Layers Explained

When installing hardwood flooring, it’s important to understand the components that lie beneath the surface. The materials used underneath significantly impact the longevity, stability, and performance of the floor.

Typically, a subfloor is the foundation upon which the hardwood flooring is laid. In addition, a moisture barrier is essential in preventing warping and maintaining the integrity of the wood due to humidity and moisture.

Lastly, underlayment provides a cushion between the subfloor and hardwood flooring, reducing noise and enhancing thermal insulation. These elements play a crucial role in achieving a successful hardwood flooring installation.

Types of Underlayment


Foam underlayment is a popular choice due to its affordability and versatility. It is available in different thicknesses and offers good sound absorption and thermal insulation.

  • Thickness: 2mm to 5mm
  • R-Value: Approximately 0.5 to 1 per inch
  • Sound Absorption: 55 STC to 70 STC (decibels)


Felt underlayment is made from recycled materials and offers excellent sound reduction and moisture resistance. It is suitable for floating, glue-down, and nail-down installations.

ThicknessR-ValueSound Absorption
3mm to 12mmApproximately 0.4 to 1 per inch58 STC to 74 STC


Cork underlayment is a natural and sustainable choice with effective sound absorption and thermal insulation properties. Due to its flexibility, it also helps to minimize floor imperfections.

  • Thickness: 3mm to 6mm
  • R-Value: Approximately 3 to 3.5 per inch
  • Sound Absorption: 53 STC to 75 STC


Rubber underlayment is a durable and versatile option that offers excellent sound resistance and cushioning, making it suitable for high-traffic areas.

ThicknessR-ValueSound Absorption
2mm to 6mmApproximately 1.2 to 1.8 per inch56 STC to 80 STC

Functions of Underlayment

Moisture Barrier

One of the primary functions of underlayment is to serve as a moisture barrier. This is especially important for hardwood flooring installations over concrete subfloors or in areas with high humidity. A quality moisture barrier underlayment helps protect the wooden floors from moisture-related damages by preventing water vapor from reaching the hardwood.


Underlayment provides cushioning for the hardwood floor, which can enhance the overall comfort and feel of the floor when walking on it. The cushioning effect can also help to reduce stress and wear on the hardwood flooring, prolonging its lifespan. Various types of underlayment materials offer differing levels of cushioning, so it is essential to choose one that best suits your flooring needs.

Thermal Insulation

Thermal insulation is another critical function of underlayment. Installing an insulating underlayment can help your hardwood floors retain heat during colder months, reducing energy costs and creating a more comfortable living environment. Some underlayment materials have a higher R-value (thermal resistance) than others, so it’s essential to compare products to find the best fit for your specific project.

Sound Reduction

The final essential function of underlayment is sound reduction. Sound-absorbing underlayments help to minimize the noise of footsteps and other impacts on hardwood floors, providing a more peaceful and quiet atmosphere. This characteristic is particularly important in multi-story homes or condominiums, where sound transmission between floors can be a concern. Various underlayment materials have different sound reduction ratings, so choose one that meets your specific needs.

Materials to Avoid

When installing hardwood flooring, it is important to be aware of the materials that should be avoided as a subfloor or underlayment. Some materials can cause more harm than good, leading to poor flooring performance, moisture issues, or even voiding the warranty of your hardwood.

Here are a few materials to avoid using under hardwood flooring:

  • Particleboard: This material may lack the necessary structural integrity and can be susceptible to moisture damage. Instead, choose plywood or oriented strand board (OSB) as more stable alternatives.
  • Low-quality foam underlayment: Cheap foam underlayments may not offer sufficient moisture protection or sound reduction. Look for high-density foam or specialized underlayment designed for hardwood flooring, such as cork or felt.
  • Smooth plastic sheeting: Although plastic is often used as a vapor barrier, smooth plastic sheeting can create a slippery surface, posing a risk for the hardwood flooring to shift. Instead, opt for a proper vapor barrier like 6-mil polyethylene or a specialized underlayment with an integrated moisture barrier.

When selecting materials for your hardwood flooring project, always consult with the manufacturer’s guidelines to ensure compatibility and to maintain the warranty. Additionally, adhere to local building codes and industry standards to achieve long-lasting and high-quality flooring results.

Considerations for Specific Subfloors

When installing hardwood flooring, it is important to consider the type of subfloor. In this section, we will discuss the specific considerations for concrete, plywood, and radiant heat subfloors.


Concrete subfloors are commonly found in basements and on ground floors. When installing hardwood flooring over concrete, moisture prevention is crucial. It is recommended to use a moisture barrier, such as a polyethylene film, to prevent potential damage.

Another consideration is to level the concrete surface before installation. Self-leveling compounds can be used to achieve a smooth and even surface. It is ideal for the concrete to reach a moisture content of no more than 75% relative humidity.


Plywood subfloors are typically found in residential construction. Before installing hardwood flooring over plywood, ensure that the subfloor is structurally sound and free of moisture. The plywood should have a minimum thickness of 5/8 inches for adequate support.

Additionally, it is important to properly prepare the surface by cleaning and leveling it. Use a combination of sanding and filling with a leveling compound to create a smooth surface for hardwood installation.

Radiant Heat

If installing hardwood flooring over a radiant heat system, there are some unique considerations to keep in mind. First, be sure to choose a hardwood species that is compatible with radiant heat, such as engineered hardwood. Solid hardwood species may expand and contract, leading to damages. “

  • Check the manufacturer’s guidelines for compatibility with radiant heat systems.
  • Allow proper acclimation of the hardwood to the installation site to minimize expansion and contracting.
  • Monitor and control the temperature and humidity levels to maintain proper conditions for the hardwood flooring.

By addressing these considerations for specific subfloors, you can ensure the successful installation of your hardwood flooring.

Wrap Up

At this point, you should have a better understanding of the essential components that go beneath hardwood flooring. These materials, such as:

  • Subflooring
  • Moisture barriers
  • Underlayment

are critical to the longevity and overall performance of your hardwood floors. Proper installation of these elements ensures that the flooring is stable, reduces noise transmission, and minimizes moisture-related issues. Remember, always follow manufacturers’ guidelines, industry standards, and best practices when installing hardwood flooring and its supportive layers.

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