"I'm looking for moisture in wall cavities and insulation, all in the interior of homes. I can't even see the studs let alone the screws."
To find moisture in building materials, the infrared camera must see a temperature difference on the surface being viewed: no temperature difference, no detection. The question is; under what conditions can a temperature difference due to moisture occur?
If the relative humidity where you are trying to inspect is 90%, you are not going to get much evaporation, hence not much cooling, hence very low temperature differences that might not be spotted with your IR camera. In such a situation, use a dehumidifier in the room to lower the humidity (it also adds heat to the room which raises the temperature and also lowers the humidity as well).
Always confirm your readings with a moisture meter.
Wet insulation has a lower R value, or resistance to heat flow compared to dry insulation. So in situations where you have a temperature difference on either side of a wall, the wet insulation would allow heat to flow more easily, hence a thermal indication on the wall. Again, you need a temperature difference between both wall surfaces that has been there for several hours.
Will the wet areas appear hot or cold? That depends on the temperatures on the wall. For instance in the winter, where the inside is warmer than the exterior, you would expect to see a cool area when viewing from the interior. In the summer where the exterior wall temperature is higher than the interior, you would expect to see a warm area from the inside. Again, always confirm with a moisture meter, because there can be other reasons for hot or cold areas as well (like missing insulation, poorly fitted insulation, air leakage, heating ducts, etc.)