New Construction Gypsum Concrete Floor Underlayment Installation And Moisture Issues

Gypsum concrete floor underlayment (i.e., Gyp-Crete®, FIRM-FILL® Gypsum Concrete, etc.) is used extensively in the construction of multi-family apartments and condominium buildings – with a reported installation exceeding four billion square feet. It is lighter than traditional concrete, which reduces the structural requirements, yet meets the noise control and fire building codes of many municipalities. Gypsum concrete is mixed on-site and pumped onto a structurally sound, broom-clean subfloor. It self-levels and fills the space where the drywall meets the floor, completely sealing room perimeters and protecting the base plates from the spread of fire. Gypsum concrete also reduces horizontal and vertical sound transmission. It sets up quickly and can be walked on after 90 minutes – allowing other light sub-trades to begin work the next day. Gypsum concrete is considered a “green” building material, and also meets the stringent volatile organic compound (VOC) requirements of GREENGUARD Gold Certification.

Overall, gypsum concrete provides a cost-effective means of providing fire and sound barriers, without impacting the project schedule. While it often appears to be the right product for the job, there have been numerous construction defect and product liability claims and losses related to its use. Water damage and mold have been reported on several construction projects using gypsum concrete – resulting in multi-million dollar losses and schedule delays.


The source of these reported water damage and mold losses may be related to the moisture within the    product and the building materials located adjacent to and around it; the ambient moisture levels when the product is poured; and the installation, monitoring, and final cleaning of the material.

Typically, drywall is installed prior to gypsum concrete installation. At this stage of construction, the building may be in a dry-in state (roof covering, exterior doors, windows and walls installed), but the ventilation system is not being operated. Exterior doors and windows may be left open at this phase to allow ambient air to flow through the facility. If the ambient air has elevated humidity levels, then the drywall may absorb ambient  moisture until an equilibrium is reached. The increased moisture content within the drywall may then result in mold growth.

As stated earlier, gypsum concrete is a wet product that is mixed on-site and pumped onto the subfloor material. The rate at which the product releases moisture is related to the environmental conditions where it is used. Installation instructions typically request that the general contractor supply mechanical ventilation and heat if necessary to remove moisture from the area until the gypsum concrete floor underlayment is dry.  Manufacturer product sheets indicate that a 3/4″ thickness of gypsum concrete pour sets up quickly, but the drying time is usually five to seven days. This “typical” drying time may occur when the  ambient humidity   levels are low and the building materials located adjacent and near the gypsum concrete are relatively dry. However, when these conditions are not present, drying times may extend several days if mechanical ventilation and dehumidification are not used.

If moisture barriers are not installed or installed improperly at the base of the drywall, prior to gypsum concrete pouring, then moisture from the gypsum concrete may wick up the drywall. This may result in water damage and subsequent mold growth on the drywall. Additionally, since the sub-trades can walk and work on the gypsum concrete the next day, the moisture emissions are further delayed if cabinets and wall coverings are installed before the gypsum concrete and drywall release the excess moisture. The resulting moisture emissions can affect the cabinets, resulting in water and mold damage to the underside and base of the cabinets. Some gypsum concrete manufacturers recommend that users follow ASTM F2170, Standard Test Method for Determining Relative Humidity in Concrete Floor Slab Using in situ Probes. They do not recommend, however, the use of ASTM F1869, Standard Test Method for Measuring Moisture Vapor Emission Rate of Concrete Subfloor Using Anhydrous Calcium Chloride, because it may provide erroneous measurements. They also recommend that contractors follow the respective floor goods manufacturer’s recommendations for relative humidity requirements, prior to installation of floor coverings over the gypsum concrete floor underlayment.

HETI…Help with Prevention and Response to Moisture/Mold Issues

So, in the end, the architect, general contractor, subcontractors, and sub-trades are all responsible for recognizing the potential issues with using gypsum concrete and implementing measures to reduce the potential for mold growth and project delay.

HETI can assist with the prevention and response to gypsum concrete-related moisture and mold issues. Our staff of EHS professionals and industrial hygienists has the experience and technical expertise to assist with the specification development and review, and appropriate response to moisture and mold issues occurring from the improper installation and management of gypsum concrete floor underlayment installation − as well as other water intrusion issues.