hardwoods below ground level

Recent advances in technology have made hardwood floors in underground (below grade) basements a safe and reliable option. All concrete floors emit water vapor, and basements in particular are notorious for damp conditions and water seepage through the slab. However, as long as proper precautions are taken and the appropriate products used, you can still have a hardwood floor in your basement that is fully covered by the manufacturer's warranty.

In the case of my latest installation, this involved a number of steps which I've outlined below. Hopefully this information will help other homeowners in evaluating the potential of their basements for hardwood flooring.

Preparation

A chat with the homeowners, and a quick inspection after a record rainfall, revealed no evident problems with water seepage through the floor. The basement was clean and mold-free. A hygrometer measurement indicated moderately high levels of humidity compared to the rest of the house--about 70%. The homeowners purchased a dehumidifier and a hygrometer to address this issue. Within hours the humidity had dropped to 50%, which is a nice median percentage. Maintained year-round, the floor should experience minimal swelling or shrinking.

The concrete floor underneath the carpet had been polished and treated with an epoxy or oil finish. Using a diamond-blade grinder attachment on a buffer, I removed the coating and achieved a light surface texture. These steps ensured maximum adhesion when it came time to glue down the wood floor.

original surface

prepared for adhesive

With a clean substrate, it was time to measure the vapor emissions from the concrete, using two calcium chloride tests from Taylor Tools. I placed a dish containing a known weight of calcium chloride on the concrete floor, enclosed it under a plastic dome that sealed tightly to the concrete, and waited 65 hours. 

Then, with the help of a friendly pharmacist, I had the dishes re-weighed, calculated the increased weight, and ran the figures through the formula provided by the manufacturer. The test indicated that emissions were just a hair over 5 pounds of water per 1000 square feet per 24 hours--fairly dry conditions that, especially in light of the excessive rainfall this summer, indicated no major concerns with the concrete slab.

Products

For extra security, and to allow room for possible increase in moisture emissions down the road, we used Bona's R851 adhesive, which is warranted to protect wood floors up to 12 pounds of vapor emission. Since our slab was right around 5 pounds, the hardwood floors should be amply protected. The calcium chloride test results were independently verified by a lab, giving both Old Town Wood Floors and the homeowner proof that the adhesive warranty conditions were complied with.

As a second precaution, this time to guard against possible elevated humidity in the future, we used Owens Plankfloor engineered wood instead of the typical solid oak. With multiple layers of hardwood plies glued perpendicularly to each other, the danger of humidity-related swelling or shrinking is significantly reduced. Engineered wood can remain stable under more extreme moisture changes than solid wood. The best feature of the Owens Plankfloor products, however, is that on top of the engineered foundation, each board has a 4.5mm thickness of solid, unfinished oak. Thus we were able to match the floors in this house both above and below grade, using the same grade of red oak, and employed identical sanding and finishing techniques for a flawless match across all three stories of this house. And just like the rest of the house, the basement floors can be refinished multiple times down the road.