The ‘mold crisis’ has been around long enough now that hopefully, it is becoming obvious the real culprit is excess moisture in buildings. Mold needs water to grow. The intent of this article is to provide you with some insights as to when/where moisture may be a problem, and practical information that can help you prevent that water problem from turning into a mold problem.

Remember that Moisture Comes in Two Forms
Both water vapor and liquid water may lead to mold growth, the liquid form being much more important. Studies have shown that it takes weeks or months at sustained high humidity conditions before mold growth begins, while under wetted conditions mold growth may begin in days. This is not to say that high humidity is not a concern, but more specifically it is the potential for condensation (i.e. formation of liquid droplets on surfaces) that is the real problem. And the risk of condensation is related to the surface temperature of materials exposed to moist air.

Beware of Cold Surfaces
When moist air comes into contact with a “cold” surface that is below the dew point temperature of the air (i.e. the temperature at which water vapor will begin to condense), liquid droplets will form on the surface. Everyone is familiar with condensation on toilet tanks during humid summer months. Other surfaces that may also become condensation sites during warm, humid weather include plumbing and fixtures, un-insulated chilled water pipes and air-conditioning ducts, and below-grade surfaces. Also consider the exterior walls and windows of a building during a northern winter. Although air in occupied spaces during the heating season is generally very dry, surface temperatures can be low enough for condensation to occur, particularly on single-pane windows, walls with poor or missing insulation, or areas where there is contact with structural components. There are two strategies for eliminating the potential for condensation; reducing the moisture content of the air (e.g. dehumidification) or raising the temperature of exposed surfaces (e.g. insulating pipes and ducts).

Liquid Water Gets Into Walls
When liquid water is present (from condensation, pipe breaks, or roof or window leaks), cleaning up the visible puddles is often inadequate to prevent mold growth. This is because you didn’t get all the water – a fair amount of it may have wicked into the walls or under the flooring. Many building materials can absorb large amounts of water within hours or even minutes. Gypsum wallboard may become wet 12”-18” above the floor if the bottom edge is left in contact with standing water. So the clean up also requires drying of any materials that were in contact with the water.

Some building materials absorb water faster and retain more than other materials. The good news is that the absorbent materials also tend to dry faster. The drying rate will depend on amount of exposed wet surface area and the amount and dryness of air moving across the wet surface(s). Drying cannot occur through surfaces that are covered by impermeable layers such as vinyl cove base, epoxy paints, or vinyl wallpaper. If possible, these should be removed to help dry the underlying wet material. Fans can enhance drying, but you won’t dry inside wall cavities unless there is a direct path for the air (a hole). Insulation in wall cavities further hinders drying by retaining water and reducing airflow. Wet insulation should be removed and replaced with dry insulation after the wall cavity is dried. Wall cavities without insulation or any impermeable covering can generally be dried successfully when one surface is exposed to lots of dry air, but exposing both surfaces is always best.

A good practice to follow when installing new drywall is to install the panels leaving a small gap between the floor and the bottom edge of the panel, thus preventing standing water from wicking into the material.

Consider the Carpeting
Carpeting, padding and the flooring under carpeting will also retain moisture. The best combination for mold prevention is thin, un-backed carpet on concrete. The carpet will generally dry quickly and the concrete is unlikely to support mold growth. Carpet padding increases the absorbing capacity and thus increases drying time. If the flooring under the carpet is potentially susceptible to mold (e.g. wood planks), it is important that it also be thoroughly dried. In this case, raising the carpet and blowing air underneath it to dry the underlying padding and/or flooring is generally necessary. A common mistake to avoid when drying flooring and floor coverings is failing to move furniture (especially file cabinets) that may trap water and prevent drying of the underlying floor covering.

Speedy Cleanup and Correction is Critical
The single most critical mold prevention technique is the speedy recognition and correction of water problems. Sometimes the “recognition” step is easy – a main water line break is hard to miss! But other times the problem is not in readily observable locations (such as condensation on ducts or pipes inside wall or ceiling cavities), or it may seem too minor to receive immediate attention. This is when a simple program to teach occupants and maintenance personnel about moisture awareness can mean the difference between a small inexpensive repair and a significant mold remediation effort.

Dr. Doll is an Associate at EH&E where her current work is focused on indoor environment investigation, in particular the interrelationship of mold and moisture.