Kill Mold by Controlling Moisture

Various Natural MoldsWhat to do about mold? Homeowners and facility managers have witnessed an explosion of mold in recent decades. While its threat is often exaggerated, mold really can make us sick. And while illness is certainly the first concern, let’s not forget that mold in significant quantities doesn’t smell very good. So even if mold is not always harmful, we can probably all agree that it’s not desirable. But what to do about it?

Searching for Mold

Start with the basics. Mold is all around us–it’s natural to the same environment we are. Mold is a living organism and has certain needs in order to survive. By controlling these needs we can mitigate the conditions in which mold can thrive. Mold must have three things: moisture, temperature, and food. The temperatures of our homes and offices are quite comfortable to mold, and the sugars in our wood and paper building products are excellent food for mold. These are tough factors to control. That leaves moisture as the one variable that we can use to combat mold. But with tighter homes (a good thing) trapping more moisture, and newer building materials that don’t hold as much water, excess moisture is more a problem than ever.

Moldy Basement Crawlspace

Excessive moisture in this crawl space has resulted in moldy wood and failing insulation.

Finding mold is not at all hard–it’s everywhere. Understanding when there is too much mold, and how it may be adversely affecting building occupants is a more complicated undertaking. Airborne sampling and lab analysis can verify that mold is present and make some extrapolations about quantity. But sampling is of limited help since mold is present everywhere, distribution levels can fluctuate widely, and different people react to it differently. One of the best ways to locate a mold problem is often through the very means that brought it to light in the first place: someone sees it or reports an unpleasant smell. People, it turns out, are good at detecting when a building feels stuffy, overly humid, and musty–all signs of a possible mold issue.

Delmhorst Moisture Meter

Moisture meters can indicate moisture content of a variety of building materials. Pictured: Delmhorst BD-2100, an industry standard.

Moisture Sources

So it comes down to finding the moisture. If you solve the moisture problem, you solve the mold problem. Locating the moisture often involves two things: having a good working knowledge of construction and building science, and having the diagnostic equipment to make the most of the knowledge. The water that feeds the mold could be coming from outside the building: through bad flashing on roof protrusions or in valleys; through the lack of a drainage plain behind siding; as a result of hydrostatic pressure on a foundation wall or slab; or through capillary wicking of building materials below grade. The water could also be coming from inside the building itself: from undetected plumbing leaks; from water vapor from showering or cooking; from numerous people or plants; or even from an improperly vented combustion appliance.

Testo Moisture Humidity Meter

Some instruments combine the functionality of a moisture meter and a humidity meter. Pictured here is the Testo 606-2.

Tools for Locating Moisture

Tools for finding the source of the moisture can be as simple as your eyes and hands, and as far reaching as a thermal imaging camera or ultrasound. Often it takes multiple tools to locate and verify the source of the problem. Thermo-hygrometers are a good first step in identifying excessive humidity in a structure. Better ones are able to measure ambient air temperature, relative humidity, calculate dew point, and more. This data can be helpful in determining whether the water is from inside or outside the building, and from vapor or liquid sources. A quality moisture meter is another indispensable tool in the search. A moisture meter will give a good indication of a material’s relative wetness, while some meters are calibrated to precisely measure moisture content of certain materials such as wood, drywall, and concrete.


A thermal camera can identify the extent of the moisture problem and help locate its source.

A thermal camera can scan a building for moisture in the walls or roof much more quickly and thoroughly than is possible with a moisture meter. Because water transmits heat more readily than most dry building materials, under the right conditions it is visible as a distinct pattern on a thermal camera. This pattern may also be helpful in determining the origin of the water, since it may enter a building cavity at one point then migrate to another. If you suspect that elevated humidity inside or outside the structure may be condensing on a cold exterior wall surface and feeding the mold, some thermal cameras offer an elegant solution. The isotherm function can show all surfaces below a certain temperature–in this case the dew point reading from the thermo-hygrometer. Now you can visualize all surface areas at risk of condensation and thus mold.


What seemed to be a minor leak from this water heater turned out to be contributing a great deal of moisture to the surrounding wall. The drywall in this utility room was quite wet yet showed no signs of water or stains.

Tougher investigations may involve a better understanding of a building’s pressure and airflow. A high-resolution differential manometer can indicate whether certain rooms or an entire structure is pressurized or depressurized relative to the outside, giving a partial indication of air and vapor flow. A blower door will quantify just how much air may be entering and exiting the building at any given time, carrying water into and through the walls. Duct pressure testers can be used to locate duct leaks that may be dumping cold air into an unconditioned, humid crawl space or attic–a recipe for condensation. To understand whether an HVAC system is properly dehumidifying a space, airflow instruments such as a capture hood or an anemometer can measure airflow at supply and return registers.


The FLIR E60 Thermal Camera and FLIR MR77 can be paired to capture moisture and thermal data simultaneously.

Final Thoughts

Moisture and mold are always tied together, and moisture is the one parameter of mold’s growth that we can easily control. If a house or office has mold, it has a moisture problem. Conversely, if a building has a moisture problem, it will likely soon have a mold problem if left unchecked. Controlling moisture not only reduces high levels of mold, but has the added benefits of greater occupant comfort, less building decay, fewer insect infestations, and higher energy efficiency. While finding mold is of some value, it’s only after finding and controlling building moisture that we can all breathe easy again.


Using the dew point measurement from the FLIR MR77, this isotherm image from the FLIR E60 is highlighting in grayscale those areas at risk of condensation. This bathroom window sill and hamper are below dew point and will be wet when the bathroom is humid.


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