The method
Before using the result
- Compare representative local air and surface measurements. A positive margin is not a guarantee against hidden condensation, sensor error or future conditions.
Worked examples
These original training examples illustrate the method for InterNACHI Certified Professional Inspector (CPI). Use the matching “Load example” button above to explore a variation; example values are not equipment settings or prescribed criteria.
1. Worked example
Compare a measured surface temperature with the dew point of nearby air. Explain the limits of a single inspection-time measurement when documenting moisture concerns.
- Dry-bulb temperature
- 25 °C
- Relative humidity
- 60 %
- Absolute atmospheric pressure
- 101.325 kPa
- Measured surface temperature
- 15 °C
- Dew point = 16.701131854221103 °C.
- Margin = 15 − 16.701131854221103 = -1.701131854221103 K.
- Dew point: 16.70113 °C
- Surface margin: -1.701132 K
Measured surface is at or below the calculated dew point.
2. Comparison example
Compare a different supplied condition using the same method. For Home Inspect Cert, the important check is that the input meaning, units and method assumptions still match the problem.
- Dry-bulb temperature
- 25 °C
- Relative humidity
- 60 %
- Absolute atmospheric pressure
- 101.325 kPa
- Measured surface temperature
- 20 °C
- Dew point = 16.701131854221103 °C.
- Margin = 20 − 16.701131854221103 = 3.298868145778897 K.
- Dew point: 16.70113 °C
- Surface margin: 3.298868 K
Measured surface is above the calculated dew point for these inputs.
Check your understanding
1. Which relationship is used?
Surface margin = measured surface temperature − calculated air dew point.
2. Which assumption should you check first?
Compare representative local air and surface measurements. A positive margin is not a guarantee against hidden condensation, sensor error or future conditions.
3. How should you verify a different unit system?
Convert every input to the displayed calculation units first. Equivalent physical inputs must give equivalent results; a unit label alone does not convert a number.
Continue with the matching exam topic
This tool connects to InterNACHI Certified Professional Inspector (CPI) free practice. Work through a question independently, then use the calculation steps to identify where your method differs. The tool stays free whether or not you choose paid preparation.