Moisture Meters and Thermography
Q: I recently purchased a thermographic camera. Do I still need to use moisture meters?
A: To answer this question, we need to discuss how thermography and moisture meters work and what they actually measure. Oddly enough, neither type of instrument really measures moisture content. Instead, they measure other factors that are often, but not always, associated with moisture.
Thermographic cameras record surface temperatures. Differences in surface temperature are often an indicator of the presence of moisture, but are not definitive, as a number of other factors or conditions affect surface temperatures.
One of the main ways moisture creates a temperature differential is through evaporative cooling. As liquid water evaporates from a surface, the surface becomes cooler. The faster the evaporation, the lower the temperature created. However, there are a number of factors that impact whether and to what extent evaporation, and therefore evaporative cooling, occurs.
The lower the relative humidity of the air in contact with the surface, the more rapidly evaporation takes place. Conversely, the higher the humidity, the slower the evaporation. Thus, in extremely humid rooms, such as are commonly found while doing initial inspections of water-damaged structures, evaporation from a surface, and therefore the resultant evaporative cooling, may be considerably reduced from what it would be in more "normal" conditions. Less evaporative cooling means a lowered temperature differential, and thus a lowered ability to detect wet surfaces using thermography.
Airflow across a surface accelerates evaporation. The more rapid the airflow, within limits, the greater the evaporation will be. Therefore, one potential way to make wet surfaces more visible using thermography is to create rapid airflow across the surface in question, accelerating evaporation and evaporative cooling.
Some surfaces are impermeable to moisture. For example, evaporation will not occur through materials, such as vinyl wallpaper and tile, at least not quickly enough to create significant evaporative cooling. In these cases, the moisture behind the surface material may not show up using thermography, but could be detected with proper use of an appropriate non-penetrating moisture meter.
Water also tends to form a heat sink. In general, dry materials tend to change their temperature more quickly than wet materials. Quickly lowering the air temperature in a room may make wet areas more readily visible, as they will not cool down as quickly as adjacent dry surfaces.
A similar approach can be used to detect wet areas under a roof membrane. When the sun is out, it heats up the entire roof, both wet and dry areas. When the sun goes down, the dry areas cool off more quickly than the wet areas, making them readily visible through a thermal camera.
False positives can occur when using thermography to look for wet materials or areas. While it is true an area on the wall may be at a lower temperature because it is wet, it may also be because an air conditioner vent is blowing directly on that spot. Gaps in insulation may create cool spots that appear to be wet but are not. For the same reason, framing, both wood and metal, may show up well with thermography when contrasted to the adjacent insulated cavities, even though both are dry.
For these reasons and others, while thermography is an excellent tool for rapidly and efficiently locating possible areas of moisture intrusion or accumulation, temperature differentials cannot be considered definitive proof that materials are wet. Assumptions made based on thermography should always be checked with moisture meters to confirm that they are accurate.
Like thermography, moisture meters do not directly measure moisture content. Instead, they measure electrical resistance/conductivity, radio wave reflectance or other factors that correlate closely to moisture content in many materials. However, as with thermography, they are not foolproof. It is important to understand the operational differences of the various types of meters.
Many factors may cause false positive readings with the moisture meters that measure electrical resistance. Metal structural components or finishing materials will read positive, including such common materials as metal framing, drywall corner metal, and foil wallpaper. Accumulations of certain salts due to efflorescence can also read positive, despite being dry.
Moisture meters can also give false negative readings. An obvious example would involve simply taking the measurement in the wrong location; most non-penetrating meters cannot read across even a small air gap, potentially leading you to believe a wall interior is dry when it is not.
Using meters in inappropriate ways can also cause problems. For example, if you know the surface of a hardwood floor plank is wet, but you want to know the moisture content of its interior, you must use a penetrating meter with insulated pins that measure only between the tips. If you use un-insulated pins with a penetrating meter, or using a non-penetrating meter, you will detect the surface moisture, which will prevent you from getting an accurate reading of the subsurface conditions. This can be critical when it comes time to decide whether a wet surface is the result of a wet building envelope or of condensation due to excessive humidity in the structure. The answer to this question can have great importance in establishing a scope of work for correcting moisture problems and resultant microbial growth.
Non-penetrating meters vary widely as to how deep into materials they can detect moisture. If the moisture accumulation is beyond this point, the meter will give a false negative.
Moisture meters may drift off accurate calibration over time. Recently, we tested six identical non-penetrating moisture meters on the same wet materials. No two of the meters read exactly the same! This variability is a major reason technicians should always use the same meter throughout a project to avoid problems.
Each type of meter has its own peculiarities of operation. It requires comprehensive training by experts who fully understand both effective restoration practices and proper use of the instruments, and even then a familiarity with one type of meter may only partially transfer to another type, even though it appears quite similar.
In summary, while thermography is a wonderful tool, it is by no means a replacement for moisture meters. It is rather a great supplement and complement to them. And when the two methods of detecting moisture are used together, each tends to compensate effectively for the weaknesses of the other, allowing the restorer to produce truly accurate moisture surveys.