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When is Dry Really Dry?

September 15, 2008
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Q: I recently restored a water-damaged carpet that later had a musty odor. The building owner had me clean it again and the odor came back after a couple of days. I thought that molds couldn’t grow if the carpet was dry. What else could be causing the odor?

A:Odors can be caused by molds, bacteria or a combination of both. You are correct that neither microorganism will grow unless there is moisture present. However, the carpet does not necessarily have to be noticeably wet in order for there to be sufficient moisture for an odor to develop. Small pockets of dampness can be sufficient for bacteria amplification and a consequential odor problem.

In a recent study conducted by our firm, we looked at drying of carpet over concrete. Our intention was to look at bacterial amplification in carpet that was dried in-place. In conducting the study we attempted to follow similar studies that also involved flooded carpets. Specifically, we looked at the Society of Cleaning and Restoration Technicians published study entitled “Water Extraction-efficiency Testing (WET) Study.” This paper was a well-designed project that looked at the extraction efficiency of various systems.

In the WET study, it was discovered that an average of 3.22 gallons of water per yard of carpet was needed to fully saturate the test samples of carpet and pad. In our case, we likewise flooded carpet and pad that was installed over concrete with approximately 3 gallons of water and then extracted that water after 4 hours. The carpet and pad was then dried using the IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration recommendations for air movers and initial dehumidification requirements.

Prior to the installation of the carpet and pad over the concrete, we looked at the moisture conditions of the concrete and found that both a non-penetrating and pin-type moisture meter indicated the concrete was dry. In addition, we used a Tramex Concrete Encounter, which indicated a reading of 2.5. After the extraction was performed, we found that we did not extract as much water from our carpet and pad as was extracted in the WET study. In fact, we were able to remove only 66 percent of what was extracted in the WET study.

I recently read an article that discussed water adsorption into concrete. In that article, it was noted that a test slab of concrete was poured, then cured. The slab was considered cured at the point that the slab was emitting around 3 pounds of moisture as determined by a calcium chloride dome test. They then poured 12.5 pounds of water on the slab and allowed it set for a period of time.

It was documented that after two hours the slab had adsorbed 4.5 pounds of the water. It should also be noted that the study showed that further curing of the slab and subsequent flooding resulted in less water being adsorbed. If we were to use the sample adsorption characteristics of this study and combine that with what we were able to extract from our test, the resulting volume of extracted water would come close to the amount recovered in the WET study. It is our opinion that the difference between our study and the WET study can be accounted for in that our study was conducted over unsealed concrete and, based upon a discussion with one of the WET study’s authors, the WET study was conducted over sealed concrete. This in no way minimizes the value of the WET study.

Going back to our study, we dried the carpet and pad and used a moisture sensor to determine that the carpet and pad was dry. We followed that with a non-penetrating moisture meter, which indicated it was dry. Finally, we used a penetrating moisture meter that also indicated that it was dry. After we removed the carpet and pad, we used the Tramex Concrete Encounter to take one last look at the slab. The remaining moisture in the slab pegged the moisture meter.

This raises several concerns about drying carpet and pad in-place. We need to be careful about how we determine that the carpet, pad and substrate were dry. We need to be concerned about any residual moisture that might be emitted from a concrete slab and results in rewetting of the pad sufficiently that bacteria might be able to continue to amplify, and we need to determine a method of evaluating the efficiency of our drying projects in broader terms than we have considered until now. When we repeat our study, we intend to perform calcium chloride dome testing before the wetting of the test carpet and pad, and after the drying is complete to further investigate and quantify the moisture adsorption into the concrete.

The information provided here might or might not apply to your specific circumstance. However, I would recommend that you consider further investigation of other sources of moisture or areas where drying might have been impeded. The problem might not be mold growth, but bacteria growth instead.

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