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Enter Sandman

September 1, 2006
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Q: I have a question about sanding as it pertains to drywall. I know that the S520 standard does not recommend this practice, but I am preparing to assist on a job where another remediator has suggested it.



Specifically, he suggested "vacuum sanding the drywall" because there is no visible mold on the backside. Have you ever heard of this being done? Would vacuum sanding be good for small spots?

A: You are correct that the S520 does not recommend sanding drywall. I can only assume that the drywall is a painted surface and not covered with wallpaper. The removal of the wallpaper and sanding of the drywall would probably be more time consuming and costly than removing the drywall. With that in mind, let's first of all exam how drywall gets wet. There are a number of ways that drywall can get wet enough to support mold growth. Some examples include:
  1. water that flows along the floor.
  2. water that comes from above.
  3. water that moves through a concrete slab that results in wet drywall.
  4. moisture or vapor that moves through the building envelope from the outside to the inside.
  5. moisture or vapor that moves through the building envelope from the inside to the outside.

Water that flows across a floor or water that comes from above can result in walls coming into direct contact with the water. When water flows across a floor, it can also be drawn into the drywall by capillary action. In these cases both sides of the drywall will probably be wet. The interior of the wall cavity will most likely remain wet longer that the side of the drywall that faces the living area. This is probably not the scenario that caused the problem that you are describing. The reason is that it is more likely to have mold growth on the interior of the wall cavity.

The third example will most probably result in both sides of the drywall becoming wet. The same results exist here as with the previous example. In this case, there is a greater probability of mold problems since the walls are likely to be wet longer than in the first example. Again, this is not the scenario that caused the problem you are describing.

The fourth and fifth examples are more likely to have caused a condition where there can be mold growth on the side of the drywall that faces the living area only. When moisture moves through the building envelope from the outside to the inside, it is possible for mold to grow wherever the moisture vapor comes into contact with a surface that is cold enough to result in condensation. That surface could be inside the wall cavity or in some circumstances the water vapor will move through the interior drywall and condense on vinyl wallpaper.

In other instances, the moisture can move through the drywall and possibly condense on other surfaces adjacent to the wall. We were asked to investigate a mold problem in a school after the janitors had moved file cabinets away from an exterior wall for summer cleaning and maintenance. Once the cabinets had been moved, mold was discovered on the painted drywall. The school was located in California, where the outdoor climate was hot and dry, while the inside of the school was air-conditioned. In order to maintain the grounds, the lawn was regularly watered. The sprinklers were directed at the exterior of the wall where mold had been discovered. It was believed that moisture had moved from the hot exterior to the cooler interior resulting in mold growth on the drywall. It was determined that there was no mold growth on the interior of the wall cavity. The water, in the form of a vapor, had moved through the exterior cladding, through the wall cavity, then through the drywall and condensed on the back of the metal file cabinets. The file cabinets were close enough to the wall to result in the drywall becoming wet. There was mold growth on the side of the drywall that faced the living area only. Another way that this same pattern of growth could develop is if the indoor relative humidity condensed on the drywall and resulted in mold growth only on the side of the drywall that faced the living area.

When there is mold growth on the surface of painted drywall, S520 makes the following recommendation:

"Small isolated areas of mold growth on a surface layer of condensation on enamel-painted walls or other non-porous surfaces, and mold growth has not resulted in concealed areas, usually can be removed by HEPA vacuuming and damp wiping as part of a regular maintenance program."

This is a restatement of what was recommended by the American Conference of Governmental Industrial Hygienists (ACGIH) in their publication "Bioaerosols: Assessment and Control." ACGIH makes the following suggestion and comment:

15.2 Removing Existing Contamination

Growth that has occurred in a surface layer of condensation on painted walls or non porous surfaces (including wood) can usually be removed by (a) vacuuming using equipment with high efficiency filters or direct air exhaust to the outdoors, (b) washing with a dilute solution of biocide and detergent, or (c) cleaning, thorough drying, and repainting. Porous materials that have sustained extensive microbial growth must often be removed. Examples of porous materials are ceiling tiles, installed carpeting, upholstered furnishings, and wallboard. Extensive microbial growth refers not only to the extent of the area affected but also the degree to which microorganisms have degraded a material for use as a food source.

The latter document recommends either cleaning the surface or replacing the material. S520 follows the same line of thought.

What purpose would be served by sanding drywall? If the growth is so extensive that it requires sanding, then I would suggest it fits ACGIH's description of a material that has been degraded sufficiently to warrant replacement. It seems to me that sanding of drywall would be more time consuming and costly than cleaning. Additionally, there might be more cost to prep the surface for repainting.

Finally, I would think that HEPA vacuuming the surface, then cleaning would be less likely to cause an aerosolization of mold spores. That means that there would less risk of contaminating the surrounding area, and less risk of litigation.

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