Hi Ben. OK, we can see why you may have moisture in that room. Also, make sure that water easily and quickly drains away from the area between the house and retaining wall or the slab will soak it up like a sponge. It sounds like the contractor used a pinless moisture meter. The 17% does not help us unless we know in what measurement reference it is related to. However, if it was determined that the readings were sufficient for a moisture blocking epoxy, then the humidity in the room will decrease quite a bit as a result. This is assuming the main source is from the floor and no where else. Humidity in the room cannot build up if moisture is being blocked from entering.

Hello,

The slab is not below grade, we are at the bottom of a hill, there is a retaining wall about 1.5 metres away from the house.
They tested the moisture with a surface probe, no drilling. I am not sure of what the 17% measurement is myself.
The room with the higher than average floor moisture reading seems to be very humid, it has bad ventilation and we live in a tropical climate area. I am just worried with bad ventilation, them installing vapour blocking epoxy on the floor, this might cause humidity build up between the epoxy and engineered timber flooring that will go over the top of the expoxy. My big concern is mold forming.

Hello Ben. Is your slab below grade or up against a hill? The 17% RH reading does not provide enough information for us since we don’t know exactly what type of RH test was performed. Most require drilling a hole in the concrete in various places to place meters. What we can tell you, however, is that moisture vapor blocking epoxy primer coatings do just what they say – block moisture vapor transmission. It will not cause damage to the concrete. This is a common practice in the coating and flooring industry providing that the moisture vapor readings are not too high. We are assuming that the company who recommended this process has already verified that the moisture vapor content does not exceed the parameter of the coating. I’m not sure why you believe that humidity will be trapped inside the room when moisture vapor is being blocked. Can you elaborate?

Hi Tim. Yep, the oily substance is the byproduct of moisture in the concrete mixing with minerals and other ingredients in the concrete. Is your slab below grade or up against a hill or bank? If so, that could explain your moisture issue. Water that does not drain away from the slab or pools against the foundation can cause moisture issues as well. Generally, humidity will not affect the test results. However, if the dome of the test kit was not sealed properly to the concrete it can skew the results. In addition, if water had been introduced to the surface within the previous 48-72 hours, it may not have had time to completely dry throughout the slab and the heat would cause the moisture vapor from within to rise and produce high readings. If any of those two were the case, you may want to test again to see if you get numbers low enough that will qualify for a moisture blocking epoxy primer.

Garage floor mats are made of polyvinyl and are not breathable. They will block moisture transmission and get wet underneath. If you want a breathable option, we would highly recommend polypropylene (hard plastic) interlocking garage floor tiles. They are breathable and allow for air circulation under the tiles. You can learn more about them here.