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Testing for Fungal Growth in Building Products: A Collaborative Effort

by Pamela Hargrove

Public concern about fungal growth in buildings and the possibility of health effects associated with exposure continues to grow. In response to this concern, building materials manufacturers continue to stress proven moisture management techniques as the best and most appropriate response to limiting fungal growth in buildings. Building design, product choice, good construction practices and proper maintenance are all important factors in reducing the risk of mold growth in buildings.

Building product manufacturers are working to inform architects and contractors about the proper storage, installation, and use of their products to ensure that they will perform as needed. They also recognize the need to develop more robust building products. There is also an associated need to classify what constitutes “mold-resistant” products so that they can be specified and used appropriately.

ASTM Standards

To understand fungal resistant properties and develop products offering better fungal resistance, manufacturers of panel products, including wallboard, ceiling tile and underlayment materials, searched for published ASTM test methods that might be suitable. In the absence of a test method specifically written for panel products the industry generally relied on two methods: ASTM G 21, Practice for Determining Resistance of Synthetic Polymeric Materials to Fungi (under the jurisdiction of ASTM Committee G03 on Weathering and Durability), and ASTM D 3273, Test Method for Resistance to Growth of Mold on the Surface of Interior Coatings in an Environmental Chamber (under the jurisdiction of ASTM Committee D01 on Paint and Related Coatings, Materials and Applications).

Practice G 21 is an agar plate method used to test synthetic polymers including poly(vinyl chloride) and plastics. Samples are placed on a carbon-free nutrient salts agar and directly inoculated with a known concentration of fungal spores. A concern about the use of this method to test bulk building products is that the direct inoculation severely wets the sample and can give a false negative result in regard to fungal resistance.

The other ASTM method used by the building products industry to test the performance of their products, D 3273, though designed as a method to evaluate coatings performance, represents a more realistic exposure environment than G 21. In D 3273, samples are suspended in a chamber maintained at 32.5º C and 95 to 98 percent relative humidity. Temperature and humidity are maintained by heating water in the bottom of the chamber with a temperature-controlled element. A soil bed above the water is inoculated with various strains of fungi. Samples are exposed to this environment for one month and rated weekly for fungal growth. Ratings are based on a set of photographic standards published in ASTM D 3274, Test Method for Evaluating Degree of Surface Disfigurement of Paint Films by Microbial (Fungal or Algal) Growth or Soil and Dirt Accumulation.

Test methods D 3273 and D 3274 are well established and accepted by the paint and coatings industry. But they have not always provided reproducible results when used to evaluate bulk building materials, including gypsum board, ceiling tile, gypsum wood fiber, or cement board. Aspects of the method that may contribute to the variability of results seen when evaluating building products include:

• The severe humidity in the test environment and the population of fungal organisms that occur naturally in the potting soil;
• The fact that fungal organisms specified in the test method may not be the most appropriate for the types of building materials being tested and haven’t been validated for these product types; and
• The fact that photographic standards are difficult to apply to textured surfaces such as ceiling tile or cement board and contribute further to variability in reading the test results.

Improvements to Existing Standards

Because of this variability, a task group was formed in Subcommittee D01.28 on Biodeterioration in June 2003 to develop a test method for coated building products. The test method will be used to evaluate any coated panel product such as ceiling tile or wood fiber. The decision to develop the test method in this subcommittee was based largely on the need to incorporate the expertise of its members in the effort. Many of the members of D01.28 have strong backgrounds in microbiology and mycology and would therefore be able to supply strong technical insight on mold behaviors, detection and measurement.

Since forming the task group in June 2003, participation has grown to include many of the major building material manufacturers as well as end users and independent labs. This broad representation assures that a fair test method will be drafted that will serve to measure the relative fungal resistance of existing building products and help building material manufacturers develop new products.

To address the same need for uncoated building products including gypsum board and cement board, a second task group was formed in February of this year within Subcommittee G03.04 on Biological Deterioration. This task group will work cooperatively with D01.28 to draft an initial standard in Committee D01. From this initial standard a second standard will be drafted specific to uncoated building products, which will be balloted in G03.04.

Currently, work is under way to evaluate various chamber designs and inoculation methods by work groups within Subcommittees D01.28 and G03.04. This information is shared during virtual meetings coordinated through ASTM, which allow members to view and edit documents during live teleconferencing over the Internet. This tool has been useful in viewing and discussing test results as well as continuing work on the draft between scheduled ASTM meetings. By taking advantage of this technology, it is hoped that the new method will be ready for balloting by January 2006.

Through these efforts, new ASTM test methods written specifically for coated and uncoated building materials will be developed. These test methods will provide information about the relative fungal resistance of building products to aid manufacturers in the development of improved products. Sound building practices will continue to be emphasized, but the development of more fungal resistant building products will further minimize fungal growth in buildings. //

Individuals who are interested in participating in the development of these new test methods are invited to contact the task group chairs of D01.28, Pamela Hargrove (phone: 847/970-5195) or Donald Knutson (phone: 952/888-7795), or the task group chair of G03.04, Scott Brown (phone: 302/472-135) for information. Additional information is available from Tim Brooke, ASTM International (phone: 610/832-9729).

Copyright 2004, ASTM International

Pamela L. Hargrove is a member of technical staff at the USG Research and Technology Center in Libertyville, Ill. She is a formulator in the Interior Finishing Systems Laboratory, having responsibility for ready-mixed joint compounds manufactured in the eastern and southeastern United States. Pamela also is responsible for USG’s Microbiology Lab, which conducts both in-house and ASTM testing of building products.