If it is moisture you are looking for using a thermal imager, a Delta-T between the two units is not always necessary. I am assuming there is insulation between the framing over the block separating firewall. If water is entering the block fire wall and the insulation is wet, you should be able to see the anomaly. Because water has a relatively high thermal conductivity, wet insulation will transfer heat more rapidly than dry insulation and will affect the IR patterns accordingly. This happens as a cool spot when evaporation is taking place.
You can verify the anomalies with a moisture meter.
If you need to see more detail, there are several options. First you can cool of the unit that you are viewing from and or heat up the opposing unit. There are also other options that are more difficult like loading the wall with evenly distributed heat and than letting it cool off this can be a little tricky. This is similar to roof inspections using solar loading or doing EIFS inspections.
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