Figures: http://www.aiha.org/aihce03/handouts/ps402spurgeon.htm

WHY SAMPLE WALL CAVITIES?

INTRODUCTION

There seems to be a significance divergence in views among mold investigators concerning the utility of sampling wall cavities for mold. Some think it is an indispensable tool when performing mold investigations, while other investigators will argue that this technique has little merit. So, is it even worthwhile to collect wall cavity samples? I'm one of those investigators that say “yes”, and allow me to explain why by addressing the following questions:

• Why sample wall cavities?
• Which wall cavities should be sampled?
• What about the sample volume?
• What are we looking for in wall cavities?
• How should wall cavities be sampled?

Why sample wall cavities?

There are five basic reasons that I collect wall cavity samples. The five reasons are:

• 80 percent of the houses I am asked to investigate do not have any visible mold;
• To find old leaks, using mold as a surrogate for moisture;
• To identify sites for destructive testing, and confirm the results of the destructive testing;
• To define a scope of remediation, or assess the effectiveness of a mold restoration; and
• To assess whether or not hidden fungal reservoirs are affecting the indoor environment.

The primary reason that I use wall cavity sampling is that I am more likely to encounter hidden fungal reservoirs than visible mold growth during an investigation. Probably more than 4 out of 5 houses that I am asked to investigate do not have any visible mold, whereas probably 2 out of 3 houses have hidden mold. Yet, even though the mold is hidden, the client was still concerned enough to ask for an investigation. Many water intrusions that result in mold growth initially involve wall cavities and other inaccessible areas of the structure. The mold remains hidden from view, and it only becomes visible if the mold happens to grow through the drywall or other material.

The second reason is to locate areas of both current, as well as past, water intrusion. Although I am sampling for mold, mold follows moisture. Therefore, mold can be used as a surrogate for moisture. For example, suppose a home owner states that during the last rain, a significant leak occurred in the living room window. However, if the last rain was six months ago, the wall will be dry, and a moisture meter is not going to be very useful in verifying that the leak occurred. However, if significant amounts of water did enter the wall cavity during the last rain, there is a good chance that detectable amounts of mold spores will still be present in the wall cavity under the window.

The third reason I collect wall cavity samples is to identify the most productive sites for destructive testing; and, to confirm the results of the destructive testing. Destructive testing is relatively expensive, it disrupts the occupants living environment, and it may be subject to false negatives. Therefore, in many mold investigations, it is used sparingly.

I recently performed a mold investigation on a house that had 118 windows, some of which leaked. Using wall cavity sampling, it was possible to identify a group of windows for destructive testing. This was accomplished at a relatively small cost, and minimized both the disruption and cost of the destructive testing by “pre-qualifying” the test sites.

The fourth reason for sampling wall cavities is to assist the occupants in fulfilling their objectives. I may be asked to define a scope of remediation, help them obtain legal assistance, or assess the effectiveness of a recently completed mold restoration (as opposed to a mold remediation).

It's my experience that wall cavity samples are generally required to adequately accomplish each of these tasks. For example, it is not unusual to sample directly in the drying holes cut in walls and toe kicks while drying the materials in place, and to then detect significant amounts of mold spores inside those cavities. Unfortunately, those drying holes provide an open pathway between the fungal reservoirs and the breathing zones of the occupants.

Question: During a restoration, why not use simple multi-port plastic manifolds to blow dry air into the bottom of a wet wall cavity and a similar manifold to exhaust the moist, contaminated air outdoors?

The fifth reason to collect wall cavity samples is to assess whether or not hidden fungal reservoirs are affecting the indoor environment. One study has concluded that fungi contained in intact wall cavities sometimes enter the indoor air [Morey, P, Andrew, M, Ligman, B, Jarvis, J. Hidden Mold Sometimes Enters the Indoor Air. In Indoor Air 2002: Proceedings of the 9th International Conference on Indoor Air Quality and Climate, Vol. 2, H Levin, ed., Indoor Air 2002, Santa Cruz, California, 2002, pp. 455-460].

Which wall cavities should be sampled?

Although wall cavity sampling is an essential part of my consultant's “toolbox”, the indiscriminate sampling of wall cavities may not be very productive. I generally confine wall cavity sampling to the following situations:

1. Plumbing walls: Behind showers, toilets, sinks, laundries, wet bars
2. Base cabinets: The toe kick area under base cabinets that have been wet
3. Penetrations in perimeter walls: Windows, sky lights, sliding glass doors, cantilevered beams
4. Planter boxes: Adjoining perimeter walls
5. Exterior wall: Large cracks, areas lacking drainage (covered weep screed, etc.), nonporous wall covering, sub-grade walls
6. Irrigation problems: Visible problems, high soil, poor drainage, etc.
7. Restoration: Areas of inadequate remediation

This is not an exhaustive list of potential sampling locations, but they do indicate the types of locations in which wall cavity sampling may prove useful.

How should wall cavities be sampled?

In my opinion, a wall cavity sampling device should have the following characteristics:

• Collects both total fungal spores and culturable fungi;
• Has a high collection efficiency for fungal spores, and collection efficiency does not vary with either fungal or debris concentration;
• A clean sample probe is used for each sample;
• Has a solid sample probe that will not “crimp” inside a wall cavity;
• Has a low airflow rate to avoid the collection of heavy debris loadings, combined with a low sample volume to minimize sampling bias.

The Bi-Air cassette, which is the sampling device that I use, has all of these characteristics, because I designed it to have these characteristics.

SUMMARY

Wall cavity sampling is often viewed as “looking for mold”. But the real objective is to identify areas that either are, or were, affected by water intrusion. The mold may simply be the “marker” for the moisture damage. I consider it to be an essential part of my consultant's “toolbox”.

Other than airborne samples, it is probably the most common type of sample that our staff collects. It can be used to identify areas that are currently wet, and those that were previously wet. It can be used to pre-select areas for destructive testing, prepare a scope of remediation, and assess the effectiveness of “restored” areas. These tasks can be accomplished at a reasonable cost and without disrupting the indoor environment.

Finally, what I consider to be essential characteristics of a wall cavity sampling device were discussed. Although some consultants may disagree with me, the characteristics I