When drywall gets wet, mold can grow, even through paint. Note the line of
swelling and intense mold growth near the bottom of the photo, where common
(non-moisture-resistant) joint tape and joint compound absorbed water, swelled and deformed.

Effective prevention requires that the entire drywall system, not just the wallboard, be resistant. Little attention is paid to the tape and joint compound used to finish gypsum board, and the most commonly-used materials actually provide a perfect growth medium for hazardous mold species. Since joint compound typically covers 40% of wall surface, it can form a big “weak link” in the chain of prevention.

Fortunately, mold- and moisture-resistant joint materials are available to complete the system. You just need to know what to look for.

Mold Basics
Mold is a micro-fungus commonly found in the exterior environment. It grows from spores, microscopic “seeds” that are virtually everywhere outdoors. They can easily become airborne, and can also “piggy-back” rides on moving objects such as people and their clothing. This means that mold spores are, potentially, everywhere indoors, as well.

For the spores to grow into an infestation, they require two conditions: moisture and a source of nutrition.

The U.S. Environmental Protection Agency (EPA) stresses moisture-control to stop mold growth, which solves part of the problem. Sealing out water leakage from the exterior can help prevent mold in places that would normally be expected to be dry.

However, there are some places that are wet by design or by the nature of their intended use. Bathrooms, kitchens, maintenance areas, locker rooms, and any place that must be washed down frequently must be expected to be wet on a regular basis. Other areas where moisture might be present or water might spill, such as areas adjacent drinking fountains, must also be considered.

In climates that are naturally moist, almost any part of a building can be a mold site. 50% relative humidity is the minimum threshold for fungal growth. In a moist climate, therefore, prevention efforts should not be limited to areas where liquid water is present.

Mold Resistant Materials
Places where moisture is present or could be present are the normal sites for installation of mold- and moisture-resistant drywall. These products prevent mold by denying nutrition to the spores.

Mold’s food is organic matter, including the cellulose that makes up the bulk of the paper. Common paperfaced drywall is, therefore, a feeding ground for certain widely-distributed mold species that are known to cause health issues, such as the black mold Aspergillus. If ordinary drywall is used in a moist location, mold can grow.

Mold-resistant wallboards are either paperless or made with specially treated mold-resistant paper. Construction, remodeling or repair in potentially moist locations should include mold-resistant wallboard. In places vulnerable to liquid wetting, board that is both mold- and moisture-resistant is necessary to prevent mold and moisture-damage.

Equally important are substances used to finish the wallboard, including:

• Joint tape used to seal the gaps between panels;
• Joint compound used to embed joint tape and cover fastener heads;
• Paint.

Beyond Wallboard: Joint Tape
Paper joint tape can feed mold. Fiberglass joint tape contains nothing organic, and is designated for mold and moisture resistance. It is also stronger than paper tape, making it a good choice in repair applications. Wherever mold-resistant wallboard is in use, fiberglass tape should also be employed.

Joint Compound
Joint compound is an equal concern for mold resistance and, unfortunately, is generally equally overlooked. Finishing practices standardized by the Gypsum Association dictate that up to 40% or more of wallboard surface is covered with joint compound for most painted walls.

While mold- and moisture-resistant joint compound is readily available, selecting the right compound is not as obvious as the choice between paper and fiberglass tape. Joint compounds fall into two general categories:

- Drying-type compounds
- Setting-type compounds

Drying-type compounds harden by water in the compound evaporating; they are not moisture-resistant. They are prone to soften and deteriorate when wet. Most drying-type compounds contain organic glue or – mold-food – so they are not moldresistant either. All “ready-mixed” joint compounds are drying-type. Some drying-type compounds are shipped as powder that must be mixed with water. Setting-type compounds are based on cement or cementlike products. They harden because of a chemical reaction that begins when water is added. Setting-type compounds harden even if they remain thoroughly wet. They are moisture-resistant.

However, not all setting-type compounds are mold-resistant. Some also contain organic matter such as starch.

Fortunately, there is a simple way to tell if a joint compound is mold-resistant. There is a standard test for moldresistance in wall finishes, ASTM International D 3273, Standard Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber. Any joint compound that has passed this test with the highest score – 10, no mold growth – can be trusted.

One example of a mold resistant compound is a widely available cement based, rapid-setting compound that can be applied in a single pass. It is designed for joint taping as well as patching and repairs. As a setting-type compound it is moisture-resistant. It has been tested for mold-resistance according to the ASTM standard, and that is stated clearly on the packaging. It also offers the properties you would normally seek in a joint compound: non-shrinking, extremely hard and durable, sands easily and paints within 90 minutes of application.

Paint
In mold-vulnerable environments, paint that is mold-retardant or has been treated with a mold-retardant additive is a must. However, it cannot do the job alone.

Many people make the mistake of thinking that paint can prevent mold. Mold spores may be present even on brand new wallboard materials, and can grow underneath paint if moisture and nutrition are present. Complete mold-resistant substrate systems, as discussed above, must be considered part of any effective mold prevention program.

If ordinary drywall, paper tape or ordinary joint compound has become mold-infested, it should probably be removed and replaced with a complete mold-resistant drywall system.

To protect paint from chemical burns, be sure to select a joint compound with low pH. This is especially important in repair situations where painting may be done soon after the joint compound has set.

Mike Van Kleeck, CSI, is a 28-year veteran of the industry who provides technical support for Rapid Set® OnePass®, a mold- and moisture-resistant joint compound made by CTS Cement Manufacturing Corporation. He can be reached at mvankleeck@ctscement.com.

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