What is the industry Standard for Moisture in Gypsum Board?

Donald E. Smith

March 2005

I work for a contractor, and we have been asked to help investigate a hospital that is about two years old and has mold forming in a few locations on the interior side of the exterior walls. We are trying to determine the acceptable level of moisture in gypsum board, and who the governing authority on this subject is. If there are no governing authorities, what is the industry standard, and is it published?

First of all, gypsum board products have a certain level of water in the paper, and water is chemically bound into the gypsum itself. Under normal circumstances, this water is not detrimental to the performance of the gypsum board. It is well known that gypsum board products are subject to damage by water and high humidity. This damage surfaces as delamination of the face paper, and deterioration of the core of the panels. Remember that gypsum board can act as a sponge in sucking up moisture and humidity. The key factor to consider is the relative humidity in the space. The relative humidity should be no higher than 50 percent.

If mold is growing on the interior surface of an exterior wall, the best bet is that moisture is penetrating the wall assembly and settling at the bottom of the wall. This buildup of moisture will take the route of least resistance, usually to the warm side of the wall.

To my knowledge, there are no specific requirements in industry standards pertaining to moisture content or methods described to test the moisture content. Neither are there specific requirements about what to do with wet gypsum board. All the standards I could find to search are very specific about keeping gypsum board dry, and storing it in spaces with controlled humidity. I suppose it is just common sense that water-damaged or wet gypsum board should be disposed of and not used. If the board is in fact wet or has been exposed to high humidity, it will not hold screws or nails.

Robert Scharff in his book, Drywall Construction Handbook, states: "Water vapor is absorbed by gypsum board. This causes the core to soften and the paper to expand, ultimately causing the panel to sag between ceiling supports. Consistent high humidity, whether from atmospheric conditions or the installation of moisture-laden construction products concrete, stucco, plaster or spray fireproofing, gives the galvanized steel components of the panels more opportunity to rust, especially if the humidity comes from salt water. Job delays might result from high moisture content in the air due to extended drying time between coats of joint compound.”

I am a property manager for four buildings constructed with EIF systems. The buildings are 10 years old and require cleaning—algae is growing in many areas. However, the developer used the EIFS material to paint the buildings with bold color and artwork. We have tried the specifications from Web sites as well as commercial cleaning products. Do you have any recommendations on other products or cleaning methods?

The most straightforward information available is from STO, one of the manufacturers of EIFS products. I suggest you try a small area in an out-of-the-way location to test the cleaning process before you undertake the entire job.

Depending on the size of the area to be cleaned and the tenacity of the dirt, algae or mildew accumulation, the tools used for cleaning will vary. For example, hand clean with a soft to medium bristle brush for isolated surface spots, use a garden hose for a full elevation, and pressure washing for entire buildings and/or difficult to remove accumulations

A generic solution consists of one to two cups trisodium phosphate detergent and a gallon of warm water.

Add one-half to one quart of bleach to remove algae (usually green stains on the surface of the finish) or mildew (generally black stains that look like dirt). Bleach is necessary to kill micro-organisms that create the algae or mildew on the finish. If no bleach is used or if it is not allowed sufficient time (about 15 minutes) to sit on the surface and kill the micro-organisms, algae or mildew will re-occur. Here are some other commercially available cleaning solutions that can be used:
  • General Cleaner by Wind-lock Corp., www.wind-lock.com.
  • Miracle Mildew Remover by Wind-lock Corp., www.wind-lock.com.
  • Wash Down by Demand Products, www.demandproducts.com

Apply the cleaning solution to the wall surface by brush or spray, and allow it to soak for 15 to 20 minutes. For heavy deposits, lightly scrub the affected area with a soft, medium bristle scrub brush. If a pressure washer is used, keep the nozzle several feet from the surface. Do not exceed 500 psi pressure, as the EIFS surface integrity can be destroyed with higher pressure washing. Some variation of these instructions may exist for proprietary brand cleaners. Always read the label and follow directions.

After the cleaning solution has soaked the surface, rinse the surface thoroughly with clean water and allow the surface to dry.

  • Do not use solvent based cleaners (acetone, gasoline, ketones, mineral oils or turpentine) because a solvent can penetrate the EIFS finish and base coat materials and damage the insulation board.
  • Do not use steam cleaning or other high temperature cleaning methods. The EPS board in EIFS has a service temperature limit of about 165 degrees F.
  • Do not use excessive scrubbing, stiff bristle or wire brushes. This type of abrasion can damage the EIFS finish.
  • Do not use high pressure washing in excess of 500 psi, or sandblasting. The EIFS lamina can be damaged with excess pressure or abrasion.

Always check local regulatory requirements for disposal of cleaning solution and waste water.

About the Author
Donald E. Smith, CCS, is AWCI’s director of technical services. Send your questions to smith@awci.org.