This is the million dollar question for the mold detection industry. Much is invested in research to build tools that "see" the mold behind building materials. Radar, ultrasound and yes, even Infrared thermography are tools currently being investigated as detection tools to “see” mold behind building materials. So far development is still in progress and it is not known if a product is just around the corner or if any of these tools will become cost effective.
Current investigative technologies to determine mold growth include; wall sampling (AIHA Journal 64:40-47 2003, A Method for Detecting Fungal Contaminates in Wall Cavities), surface sampling of the exterior (visible detection) and air quality sampling of the inside. All techniques provide some clue as to what is “hidden”, but do not easily define how much or where the mold is.
As far as a correct way, most indoor environmental professionals (IEPs) utilize visual growth, air sampling (IAQ) and their understanding of building science to infer possible growth hidden within the building material. Wall air sampling is controversial; some use it many more do not. The paper referenced above provides information on its use and efficacy.
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