What's That Paper Under Your Hardwood Floors? And Does It Really Matter?

During hardwood floor renovations, we've found just about everything beneath the boards: rosin paper, kraft paper, cardboard, plastic sheeting, and occasionally materials that leave us wondering what the installer was thinking.

Some were probably installed to reduce friction or noise. Others were intended to protect against moisture.

The trouble is that not everything sold as an underlayment—or placed beneath a floor—actually works as a moisture vapor retarder.

And using the wrong material can create problems that remain hidden for years.

What Does a Moisture Vapor Retarder Actually Do?

Wood absorbs and releases moisture as surrounding conditions change. A vapor retarder slows the movement of moisture vapor through part of the flooring assembly.

It doesn't waterproof the floor, stop a plumbing leak, or correct excessive moisture already present in the building.

The material selected depends on what's beneath the hardwood, how the flooring will be installed, and the conditions inside the property.

For nail-down hardwood over suitable plywood or OSB, we commonly use Aquabar B, an asphalt-laminated paper manufactured for applications that include wood flooring.

Unlike ordinary paper, Aquabar B has documented moisture vapor resistance. It slows vapor transmission without acting as a completely impermeable sheet.

That distinction matters, particularly over wood subfloors.

Not All Paper Belongs Under Hardwood

Rosin paper and ordinary kraft paper have appeared beneath countless hardwood floors. Cardboard occasionally makes an appearance, too.

But their presence doesn't establish that the flooring has adequate moisture protection.

There are legitimate asphalt-saturated kraft products used as vapor retarders. Ordinary kraft paper isn't automatically one of them.

Plastic sheeting presents another concern. An impermeable layer installed directly above a wood subfloor can interfere with drying and contribute to trapped moisture under certain conditions.

That doesn't mean plastic is always wrong. It means its location and intended application matter.

Even approved products can be installed incorrectly.

Aquabar B requires proper overlaps and subfloor preparation. We've encountered installers who misunderstand the overlap requirements or assume the orientation of every paper product is the same.

Instructions should come from the manufacturer of the material being installed, not from what someone remembers doing on another job.

Concrete Is Another Matter

Concrete requires a different approach, particularly with glue-down hardwood installations.

We regularly use liquid-applied moisture-control products such as Bona R540 and WAKOL PU 280 in approved flooring systems.

These products aren't interchangeable with paper retarders. They're applied directly to properly prepared substrates under specific conditions.

And applying them correctly involves more than opening a bucket and reaching for a roller.

Surface preparation, moisture testing, coverage rates, drying times, and the number of coats all matter.

Depending on the product and installation system, one coat may be permitted while another application requires two.

Too little material may compromise the intended moisture protection. Additional coats don't automatically improve the system either.

The adhesive and flooring must also be compatible with the selected moisture-control product.

What About Floating Floors and Crawl Spaces?

Floating floors may require different underlayments, sometimes combining vapor protection with cushioning or sound reduction.

Those products have their own limitations and installation requirements. The word underlayment alone tells us very little.

Crawl spaces introduce another issue.

A vapor retarder beneath the hardwood won't correct moisture rising from exposed soil or other moisture problems below the building.

Ground coverings, drainage, crawl-space conditions, and the building's moisture-control design need to be considered separately.

Installing another layer beneath the flooring won't necessarily solve what's happening below it.

Testing Comes Before Installation

We place considerable importance on moisture testing before selecting and installing a flooring system.

Wood subfloors and concrete require different testing methods. Results must be considered alongside the flooring manufacturer's requirements and the limits of the proposed moisture-control product.

An acceptable moisture reading today also doesn't guarantee that a building has no ongoing moisture problem.

We've occasionally encountered flooring failures involving incorrect materials, poor installation, or moisture conditions that were never properly addressed.

Sometimes the material was unsuitable. Sometimes the installation was wrong. Sometimes both.

A moisture vapor retarder is a relatively small part of a hardwood flooring project, but choosing and installing it correctly deserves considerably more attention than simply covering the subfloor with whatever happens to be available.