The Importance of Moisture Testing Concrete Subfloors

Concrete is a permeable substance capable of absorbing and releasing moisture, which can continue to affect the RH of a slab long after it's been placed.

Groundwater, high humidity and leaking plumbing are the top causes of moisture problems in a concrete slab.
Groundwater, high humidity and leaking plumbing are the top causes of moisture problems in a concrete slab.
Wagner Meters

Many people don’t realize how important moisture testing is with concrete subfloors, especially with old concrete. After all, if the slab was poured 10 years ago, it must be dry, right? And if it had a floor installed all that time, there can’t be a problem, right? 

Maybe. Maybe not.

Here’s a true story from our files. A customer called from the Northwest. They were installing a very expensive wood floor over a concrete slab that was poured in 1952. The area had just experienced historically heavy rainfall in the past few months. The customer performed an in situ relative humidity (RH) test as part of the process, and the readings pegged at 99% RH. That’s more than enough to cause a flooring system to fail horrendously. 

So, how is that possible with a slab that’s more than 60 years old? For the answer to that, we need to first look at how moisture gets into concrete. 

Sources of Moisture in Concrete

There are many ways that moisture gets into a concrete slab. Ground moisture can enter either through capillary action or as water vapor. Groundwater might be present due to a high-water table or poor drainage. Other ways include high air humidity, high RH in the environment or leaking plumbing that goes through the slab. 

The high moisture in our story isn’t surprising given the age of the slab and the recent weather. Older slabs were often constructed without moisture barriers, or the moisture barrier has degraded over time. Or maybe a plumbing leak has developed. 

High moisture levels in the concrete could also be hidden by the old flooring system. Older flooring adhesives and sealants were typically more moisture-resistant than today’s lower VOC products. You might remove an older floor that has performed perfectly well for years and find that the slab is too wet for today’s less moisture-resistant flooring systems. 

Moisture Damage to Floors

Installing any type of floor over concrete that isn’t sufficiently dry can be disastrous. For wood floors, slabs with excessive moisture can cause adhesive failure, wood warping or cupping, gaps and creaking. Floor coatings can suffer from blistering and delamination. Sheet vinyl and vinyl tile floors can suffer adhesive failure and blistering. Mold and mildew are also common problems with excessive moisture. 

Experienced flooring installers know that you can’t look at the surface of a concrete slab and know if it’s dry enough to install a floor. There may be large amounts of moisture hidden deep in the slab. After you install a floor, that hidden moisture can rise to the surface of the slab and cause the floor to fail. Over the years, various methods of moisture testing have been developed, but most of them are not scientifically based, accurate and reliable. 

Only the RH test has been scientifically proven to give reliable and accurate results. Research going back to the 1960s found that a moisture gradient forms within a concrete slab as it dries, so the slab is drier at the surface and wetter deep inside. That’s why surface tests are not a reliable indicator of the slab’s moisture condition. Further research in the 1990s established procedures for testing at specific depths within the slab using an RH probe. In 2002, ASTM International developed the F2170 standard for in situ RH testing based on this research.

Today, there are various RH probes on the market to help contractors test moisture levels. Wagner Meters’ Rapid RH L6 system is one of the most cost-effective systems for RH testing concrete slabs in compliance with ASTM F2170. The system uses single-use sensors for speed, economy and ease of use. Once the L6 sensors are installed in the slab and equilibrated, there’s no need to move them from location to location and wait for them to equilibrate again. Repeat readings can be taken without additional equilibration time. And unlike reusable probes, the L6 sensors never need calibration because they come calibrated from the factory and include a NIST-traceable certificate of calibration to keep with your records regarding the concrete RH testing.

The Rapid RH L6 system’s Total Reader reads, displays and transmits temperature and RH data via Bluetooth to a mobile app, DataMaster L6, with no manual intervention and no need to record readings on paper. The app allows users to store, display and report data on their mobile device. Users can even email PDF format reports to clients and all interested parties. Backup copies of readings are stored in the cloud and in the sensors that are permanently installed in the slab. This unbroken digital path from the sensor to the final report, plus automatic data backup, ensures the highest data integrity, accuracy and peace of mind. 

Jason Spangler, Wagner Meters’ flooring division manager, has more than 25 years’ experience in sales and sales management across a broad spectrum of industries. He has successfully launched a variety of products to the market, including the original Rapid RH concrete moisture test. Spangler is involved with the National Wood Flooring Association (NWFA), the International Certified Flooring Installers Association (CFI), and is vice chairman of associations for The Flooring Contractors Association (FCICA).