CPR Psychrometric Drying Principles 3 — Questions and Answers
Question 1: A restorer raises the temperature inside a water-damaged structure from 65°F to 80°F without adding or removing moisture. What happens to the relative humidity?
- It increases because warmer air holds more moisture
- It decreases because the air can now hold more moisture (Correct answer)
- It stays the same because the moisture content did not change
- It increases to 100% causing condensation
Correct answer: It decreases because the air can now hold more moisture
Raising temperature increases the air's capacity to hold moisture, so the same amount of water vapor represents a lower percentage of saturation, reducing relative humidity.
Question 2: According to IICRC S500 guidelines, what is the target indoor relative humidity range for effective structural drying?
- 10–25%
- 30–50% (Correct answer)
- 55–65%
- 70–80%
Correct answer: 30–50%
IICRC S500 recommends maintaining indoor relative humidity between 30–50% to create conditions favorable for moisture migration out of wet building materials.
Question 3: What is the primary purpose of calculating the number of air changes per hour (ACH) during a drying project?
- To determine the dehumidifier's refrigerant charge
- To ensure adequate airflow for moisture removal from structural cavities (Correct answer)
- To calculate the enthalpy difference at the dehumidifier inlet and outlet
- To measure the wet bulb temperature differential
Correct answer: To ensure adequate airflow for moisture removal from structural cavities
ACH determines whether the air movers are circulating enough air volume to effectively move moisture from wet surfaces into the air stream for dehumidifier extraction.
Question 4: On a psychrometric chart, moving horizontally to the right along a constant dry bulb temperature line indicates:
- A decrease in relative humidity
- An increase in humidity ratio (more moisture in the air) (Correct answer)
- A decrease in dew point temperature
- No change in air properties
Correct answer: An increase in humidity ratio (more moisture in the air)
Moving right along a constant dry bulb temperature line on a psychrometric chart means the humidity ratio increases — more moisture is present in the air.
Question 5: Equilibrium Moisture Content (EMC) in a building material is best defined as:
- The maximum moisture the material can absorb before failure
- The moisture level at which the material neither gains nor loses moisture to the surrounding air (Correct answer)
- The moisture content required to begin microbial growth
- The percentage of free water versus bound water in a material
Correct answer: The moisture level at which the material neither gains nor loses moisture to the surrounding air
EMC is the stable moisture level a material reaches when it is in balance with the humidity of its surrounding environment.
Question 6: A restorer observes that a dehumidifier is running but the indoor relative humidity is not dropping. Which psychrometric scenario most likely explains this?
- The dew point is below the wet bulb temperature
- Moisture is evaporating from wet materials faster than the dehumidifier can extract it (Correct answer)
- The enthalpy of the indoor air is too high for condensation
- The humidity ratio has reached equilibrium with the specific volume
Correct answer: Moisture is evaporating from wet materials faster than the dehumidifier can extract it
If evaporation from wet materials exceeds the dehumidifier's extraction rate, RH will not drop — indicating the need for additional dehumidification capacity.
Question 7: Which factor most directly increases the rate of evaporation from a wet building material?
- Higher relative humidity in the surrounding air
- Increased airflow across the material's surface (Correct answer)
- Lower dry bulb temperature
- Higher dew point in the structure
Correct answer: Increased airflow across the material's surface
Increased airflow removes the saturated air layer at the material's surface and replaces it with drier air, accelerating the evaporation rate.
CPR Psychrometric Drying Principles 3
A restorer raises the temperature inside a water-damaged structure from 65°F to 80°F without adding or removing moisture.
What happens to the relative humidity?