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Quick Answer
Yes, underlayment is usually recommended for hardwood floors installed over concrete. Underlayment provides several important benefits:
- Moisture barrier – Underlayment creates a protective barrier against moisture coming up from the concrete slab.
- Sound insulation – Underlayment helps reduce noise transfer between floors.
- Comfort – Underlayment provides a “cushion” feel when walking on the floors.
- Smooth subsurface – It protects against imperfections in the concrete and helps create a smooth, even surface for installing the wood flooring.
While not always required, underlayment is considered best practice for hardwood flooring over concrete. It’s an important part of proper installation and maintenance.
The Purpose of Underlayment
Underlayment serves several key functions when installing hardwood floors on concrete:
Moisture Barrier
One of the main reasons to use underlayment is that it acts as a moisture barrier between the wood flooring and the concrete slab. Concrete naturally absorbs and emits moisture from the ground. Without a protective layer, excessive moisture can damage the wood over time, causing warping, cupping, gapping, and other problems.
The underlayment creates a moisture barrier to protect the wood flooring. Look for underlayments made with a waterproof polyethylene film or similar water-resistant material. This moisture protection is critical for hardwood flooring success over concrete subfloors.
Sound Insulation
Underlayment also helps absorb sound by providing a “cushion” between the flooring and rigid concrete. This reduces noise transfer between levels of the home. The sound dampening can significantly improve the ambience of the living space.
Thicker, high-quality underlayments provide better sound insulation. Cork and rubber underlayments, in particular, are known for their acoustic insulating properties.
Comfort Underfoot
Walking on hardwood floors over concrete can feel quite hard and unforgiving without underlayment. The underlayment acts as a buffer and absorbs some of the “shock” of foot traffic. This creates a more comfortable walking experience.
Underlayment also helps smooth out any small imperfections in the concrete subsurface. This provides a more uniform and comfortable floor surface.
Smoothing Imperfections
While concrete offers an excellent subsurface for hardwood flooring, it is still subject to some natural imperfections. Tiny deviations in flatness, grooves, unexpected cracks, and other flaws can potentially telegraph through the finished wood floor over time.
A quality underlayment helps compensate for these imperfections. Flexible foam or cork underlayments can conform around flaws and create a more uniform subsurface. This helps prevent irregularities in the concrete from impacting the finished wood floor above.
Types of Underlayment
There are several main categories of underlayment used beneath hardwood flooring on concrete:
Foam
Foam underlayments provide cushioning for comfort and noise insulation. Basic foam options are inexpensive but offer minimal moisture protection. Higher-density polyurethane foams resist moisture better while still smoothing out the floor.
Cork
Cork underlayment is naturally moisture-resistant while offering cushioning for comfort. It absorbs sound well for noise reduction. Cork also naturally conforms around imperfections in the concrete subsurface.
Felt
Felt or fiber underlayments are affordable options that provide basic smoothing and sound absorption under hardwood floors. However, felt does not offer moisture protection. An additional vapor barrier is required.
Rubber
Rubber underlayments are durable options that resist moisture and provide excellent sound insulation properties. They offer lasting protection under hardwood flooring over concrete substrates.
Hybrids
Some underlayment products combine layers of different materials to optimize moisture protection, sound insulation, and comfort. For example, a common hybrid pairs a moisture barrier film with a foam cushion layer.
How Thick Should the Underlayment Be?
Typical underlayment thickness ranges from 2mm up to about 5mm. Thinner underlayments, around 2-3mm, provide basic smoothing and noise reduction. Thicker options, 4-5mm, offer enhanced comfort and sound insulation.
Follow the flooring manufacturer’s recommendations for underlayment thickness. Using an overly thick underlayment under hardwood can potentially cause installation problems or telegraphing unevenness through the floor over time.
As a general guideline for underlayment thickness:
- 2-3mm: Basic sound insulation and smoothing
- 4-5mm: Enhanced comfort and acoustic performance
Also consider the type of hardwood flooring:
- Thinner for solid wood flooring
- Thicker for engineered wood flooring
Engineered floors can accommodate thicker underlayments better than more sensitive solid wood flooring.
Do You Need a Vapor Barrier?
In addition to underlayment, a separate moisture or vapor barrier is highly recommended for hardwood installations over concrete. This provides optimal protection against moisture-related damage.
The vapor barrier goes down first directly over the concrete, followed by the underlayment over top. Options include:
- Plastic sheeting – At least 6 mil polyethylene film
- Specialty vapor barrier materials
- Some foam underlayments have integrated moisture barrier films
Always check concrete subfloors for proper moisture levels before installation. Follow the wood flooring manufacturer’s recommendations for acceptable moisture limits and vapor barrier use.
Proper moisture testing and vapor barrier installation helps prevent costly moisture-related problems down the road.
Installation Tips
When installing underlayment for hardwood flooring over concrete, follow these best practices:
- Concrete should be properly cured and prepped – Clean, smooth, and flat within specifications
- Check moisture levels and humidity in the space
- Install any required separate vapor barrier first
- Roll out underlayment perpendicular to the direction of the flooring
- Butt edges together tightly, do not overlap seams
- Use the recommended underlayment thickness for the flooring type
- Tape seams as needed per manufacturer instructions
- Trim underlayment flush with walls after installing finished floor
Careful underlayment installation creates the ideal subsurface for top-quality hardwood floor results over concrete.
Underlayment vs. Plywood
Underlayments and plywood provide different functions beneath hardwood floors:
Underlayment
- Designed for usage directly under hardwood flooring
- Cushions for comfort and sound absorption
- Thin material (2-5mm thick)
- Conforms around minor concrete irregularities
- Options with moisture barrier films available
Plywood
- Not intended as underlayment, serves as a subfloor
- Builds up height to match other flooring surfaces
- Stiffer with no cushioning effect
- Thicker material (1/4″ or more)
- Requires additional underlayment on top
Plywood builds up the floor assembly thickness as needed over concrete, but an additional underlayment should still be installed directly beneath the hardwood flooring.
Common Mistakes to Avoid
Some common mistakes to avoid when installing underlayment for hardwood flooring over concrete include:
- Not moisture testing the concrete and addressing high moisture properly
- Installing underlayment that is too thick or soft for the wood flooring type
- Not taping seams according to manufacturer specifications
- Leaving gaps between underlayment rolls or at walls
- Failing to trim underlayment flush with walls after flooring install
- Using underlayment with inadequate moisture barrier properties
Taking care to avoid these issues will lead to better underlayment installation and hardwood flooring performance.
Conclusion
While not universally required, underlayment provides meaningful benefits beneath hardwood flooring over concrete. It acts as a moisture barrier, insulates sounds, and creates a more comfortable walking surface. Underlayment also smooths over minor imperfections in the concrete subsurface. Follow best practices for proper underlayment selection, thickness, installation, and seam sealing to get the best results under hardwood floors. When paired with appropriate moisture testing and vapor barrier use, quality underlayment helps ensure successful and durable hardwood flooring over concrete substrates.