How to Remove Trapped Moisture From a Floating Subfloor

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  • 4-15 hours
  • Intermediate
  • 0-300
What You'll Need
Plywood or OSB Sheets (floating subfloor)
Moisture Barrier
Stapler or Nailer
Pencil and Marker

A floating subfloor is the layer of flooring installed above concrete and under the finished flooring. The floating subfloor may get moisture trapped and cause cracks in the cement flooring. In order to prevent damage to the concrete and the finished flooring, follow the steps below to remove any trapped moisture.

Step 1 - Inspect for Trapped Moisture

Moisture usually gets trapped inside the floating subfloor when the concrete below the structure has a high moisture content and has not been coated with a moisture barrier. Usually, the trapped moisture in the subfloor can be spotted easily because of the darker coloring it has compared to the normal color of the concrete. If some areas of the subfloor do have trapped moisture in it, then make sure to take action before installing the finished flooring above it.

Step 2 - Remove the Affected Subfloor

It is best to remove all the subfloor if this does not cause an issue. The reason is that if the concrete has a high moisture content, it needs to be coated with a moisture barrier first to prevent any moisture issues in the future. When the subfloor has been removed, mix the moisture barrier product in a container and apply it to the concrete floor using a notched trowel. Install a dehumidifier in the room to allow the moisture to evaporate quickly. This will also be useful in creating a moist-free space.

Step 3 - Allow the Floating Subfloor to Dry

While waiting for the moisture barrier to dry, place the subfloor material in a well-ventilated area in order to allow the trapped moisture to evaporate. Sometimes, the affected parts of the subfloor may warp or distort. In this case, make sure to have replacements ready. It is not a good idea to have the subfloor dry under the sun when the moisture is still fresh.

The reason is that the quick drying process will distort the size and shape of the material, especially if the moisture is quite high. Therefore, just let the material dry out naturally in a well-ventilated room.

Step 4 - Reinstall the Subfloor

When the subfloor material has dried out, check for parts that need to be replaced with new ones. Use a pencil and a marker to mark areas that need to be cut. Cut these areas and prepare replacements. Afterwards, reinstall the floating subfloor on top of the coated concrete. Lay the pieces carefully and staple them together to form the subfloor.

As long as the concrete is coated properly with the moisture barrier, the subfloor will never have issues again with trapped moisture. Just to make sure the moisture does not become an issue in the future, make sure to install exhaust fans, dehumidifiers or ventilation materials in order to keep the area free from high humidity.