Don't Get Sandbagged by Moisture

July 6, 2008
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Q: I recently inspected a sandbag-constructed home with a concrete/stucco-type coating on both the exterior and interior of the building. The building was constructed around the mid 1960s. The indoor relative humidity was almost 100 percent, and the walls pegged my moisture meter. The home had a strong musty odor and there was what appeared to be mold growth on the exposed ceiling near the eaves. With this type of construction, could the mold also be embedded in the sandbags? What other concerns are there for this type of construction?

A: Historically, sandbag-type construction has been used in many parts of the world. Sandbag buildings were constructed using burlap bags filled with sand, soil or other earthen materials. The filled bags were stacked in an overlapping pattern to form walls that can be up to a couple of feet wide for stability. Frequently, strands of barbed wire are run between the layers of the bags to help improve stability. In the United States, sandbag construction was performed primarily by the military to create strong protective barriers and structures. In the 1980s, the use of sandbags surfaced once again, made popular by architect Nader Khalili at the California Institute of Earth Art and Architecture. Today the sandbag construction technique is often referred to as “Earthbag” construction, and has gained popularity as a free-form “green” construction technique with curving walls and dome-type roofs.

The structure and content of the sandbags is important. Burlap and/or jute constructed bags will rot when exposed to moisture, leading to eventual failure of the structure. Modern “Earthbag” constructed buildings typically use a polypropylene bag which doesn’t rot from exposure to water. However, the polypropylene bag must be protected from sunlight. Exposure to sunlight will cause the polypropylene to breakdown. The contents of the sandbags can be mixed with varying amounts of organic materials such as rice hulls or nut shells. These materials can also be prone to mold and rot.

The home that is mentioned above predates many of the modern techniques that are now being utilized. The description indicates that the structure was coated inside and out with a concrete/stucco material. To determine an appropriate course of action, it would be important to know what was used as the sandbag filler. As wet as the building is, any organic material present in the bags are likely to swell and cause cracking or other damage to the surface coatings. This could account for the root-brown grassy substance growing out from a crack on the exterior material as noted from our telephone conversation. It is possible there is sufficient soil combined with moisture to allow seeds to germinate and grow. If the building continues to remain wet, this condition can be expected to worsen. The cracks in the cement/stucco may also provide additional entry paths for moisture to enter into the wall assemblies.

Additionally, concrete/stucco mixtures have not fared well as a finishing material when used to coat “natural” materials such as clay, soil or sand. According to Cedar Rose Guelberth and Dan Chiras, authors of The Natural Plaster Book, the use of cement stucco on any natural building material “represents a colossal mistake that will come back to haunt people in the years to come.” They continue, “cement stucco wicks moisture into the interior of walls. It cracks and lets moisture in as well. Cement stucco traps moisture” and “prevents the moisture trapped inside a wall from escaping, which could cause considerable damage to … earthen materials.” In The Last Straw, a quarterly journal that focuses on alternative construction methods such as straw bale, Michael Smith wrote: “Unable to evaporate through the cement stucco, this moisture accumulates over time, saturating and weakening the wall, especially at the point where earth meets cement.” He goes on to talk about the problems when historic adobe buildings in New Mexico were plastered with cement stucco. “The St. Francis Church in Rancho de Taos, New Mexico, built in 1815 with massive adobe walls and thick buttresses, was plastered with cement stucco in 1967.” Over the course of the next decade the water trapped by the cement stucco eroded the wall, resulting in the stucco having to be removed and the wall rebuilt. These same principles may very well be at work in the sandbag structure.  

It was also noted that there was no mold growing on the surface of the interior walls. Based on the level of moisture present and the age of the structure, it is possible these surfaces also contain lead-based paint. Lead was used in paint as an antimicrobial. It does an excellent job of preventing mold growth on painted surfaces, but may present exposure issues for occupants. If there is lead-based paint present, then the abatement should be performed by a trained lead abatement company. Those working around lead-based paint should take appropriate precautions during the restoration or remediation activities to ensure that the material is not disturbed.

Howard Wolf of HW3 Consulting in Richfield, Wis., has experience drying similar structures. He indicated that, “naturally occurring silica and lime in the sand can be a deterrent for mold growth.” Wolf also suggested the reason that mold is appearing on the eaves where they meet the walls is that, with this type of construction, there is frequently a very poor seal at the top plate between the sandbag walls and the roof joists. This provides an infiltration point for cold air, which would reach dew point and result in condensation when it comes into contact with the water vapor inside the building.

A pre-remediation assessment by an indoor environmental professional might be able to help determine if mold spores have already been released and spread; if the growth can be controlled at the surface using containment; or if it is also present in concealed areas such as the ceiling cavities or other interstitial spaces.

The first thing that you need to determine is whether the walls are wet due to water intrusion or wet on the interior surfaces due to condensation. Solving or controlling the moisture problems in the building is necessary before performing mold remediation. This does not mean the building has to be totally dry, however; it is necessary to ensure that the building is drying enough so that secondary damage does not continue to occur from the moisture in the air. That your moisture meter indicated 100-percent relative humidity and a non-penetrating meter indicated moisture was off scale does not answer the question of how wet the walls actually are. If the walls are saturated, there could be hundreds of gallons of water absorbed through the depth of the sandbag. On the other hand, the high moisture readings might be from surface condensation. One possible way to determine whether you have a wet building envelope or condensation is, rather than penetrate the interior plaster, to access the wall through or around any penetrations for plumbing or electrical with insulated probes attached to your moisture meter.

If the walls are saturated, drying this building will be a challenge. According to the S500 Standard and Reference Guide for Professional for Professional Water Damage Restoration, this would be considered a “Class 4 (specialty drying situations). These consist of wet materials with very low permeance/porosity (e.g., hardwood, plaster, brick, concrete, light weight concrete and stone). Typically, there are deep pockets of saturation, which require very low relative and specific humidity for drying.” Drying the building using air movers would likely aerosolize mold spores from the surfaces. Care should be taken to not contaminate otherwise uncontaminated areas or surfaces. The remediation, following the IICRC S520 Standard and Reference Guide for Professional Mold Remediation, should be performed prior to drying. If walls are saturated it is likely that dehumidification or a hot-air drying system will be necessary for an extended period of time. Only after the building has been returned to and able to maintain a normal moisture level will you be able to address the mold-growth issue. As long as the building remains wet the mold will continue to grow. It may be that sealing the air gaps at the joists would stop potential infiltration and condensation. Also, it is possible that the HVAC system was not been properly sized or installed to provide the appropriate ventilation and moisture control. Unless the current HVAC system was engineered by someone familiar with this form of construction, it is likely they installed a unit that was designed for more traditional construction. (Author’s Note: supplemental information was requested and provided in order to answer this question).

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