22 Psychrometry & Drying Science Practice Questions & Answers

Every Psychrometry & Drying Science practice question from the IICRC WRT Water Damage Restoration Practice Test, with the correct answer and a short explanation.

Start practice test →

1. In structural drying, relative humidity expresses which of the following?
- A.The actual weight of water vapour carried in every pound of dry air in the space
- B.The force that water vapour molecules exert against the surface of a wet material
- C.The share of moisture the air holds compared with what it could hold at that temperature✓ Answer
- D.The temperature at which the air becomes saturated and its vapour starts condensing
Relative humidity is a ratio, not a quantity. It compares the vapour present with the maximum that air could hold at its current dry-bulb temperature, so the same reading taken at two different temperatures represents two different amounts of water.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, psychrometry and the properties of air and water vapourReport a problem with this question
2. The air in a sealed drying chamber is heated, and no water is added to or removed from it. What happens to the readings?
- A.Relative humidity rises while the humidity ratio drops by the same share
- B.Relative humidity stays the same while the humidity ratio climbs steadily
- C.Relative humidity and the humidity ratio both rise as the air gets warmer
- D.Relative humidity falls while the humidity ratio remains exactly the same✓ Answer
Heating enlarges the air's capacity to hold vapour, so the same water becomes a smaller share of that capacity and relative humidity falls. The humidity ratio counts the actual weight of vapour per pound of dry air, and nothing was added or removed, so it does not change.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, relationship between temperature, relative humidity and humidity ratioReport a problem with this question
3. Only the temperature of the air in a drying chamber is raised. What happens to the dew point?
- A.It rises, because the relative humidity reading has dropped
- B.It stays the same, since no moisture entered or left the air✓ Answer
- C.It rises in step with the dry-bulb temperature reading
- D.It falls, because warmer air is able to hold more moisture
Dew point is set by the quantity of vapour the air is carrying, not by how warm that air is. Heating alone changes neither the humidity ratio nor the dew point, which is why a falling relative humidity reading after heating is no proof that moisture has been removed.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, dew point and saturation of moist airReport a problem with this question
4. Heat is shut off in a drying chamber overnight and the air cools, with no moisture added or removed. What is the most likely result?
- A.The humidity ratio falls, making the air a stronger drying agent
- B.The dew point falls with the air, removing any condensation risk
- C.Evaporation speeds up because cool air has more room for vapour
- D.Relative humidity climbs toward saturation and surfaces may sweat✓ Answer
Cooling removes no water; it only shrinks the air's capacity, so relative humidity rises toward saturation. Any surface at or below the dew point will collect condensation, which puts water back into materials instead of taking it out.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, condensation and control of temperature during dryingReport a problem with this question
5. What does a humidity ratio reading, expressed in grains per pound, tell the restorer?
- A.The share of the air's total capacity for vapour that is currently in use
- B.The weight of water vapour carried by each pound of dry air in the space✓ Answer
- C.The temperature at which the vapour in that air would begin to condense
- D.The volume that one pound of that air-and-vapour mixture happens to fill
The humidity ratio is an absolute quantity of moisture and does not change when the air is merely heated or cooled. That is why restorers compare humidity ratio, rather than relative humidity, between the affected area, an unaffected area and outdoors.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, humidity ratio and psychrometric measurementReport a problem with this question
6. Two rooms both read 50% relative humidity; one is at 85°F and the other at 65°F. Which statement is correct?
- A.The warmer room carries more water in each pound of its dry air✓ Answer
- B.Neither room can dry materials until relative humidity drops lower
- C.The cooler room carries more water because cool air resists vapour
- D.Both rooms carry the same weight of water in each pound of air
Because warm air can hold far more vapour than cool air, half of a large capacity is a much greater weight of water than half of a small one. Matching relative humidity readings therefore say nothing about how much water each room actually holds.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, interpreting relative humidity against humidity ratioReport a problem with this question
7. Vapour pressure determines the direction in which moisture moves. Which direction does it move?
- A.From the drier material into the wetter material lying next to it
- B.From the area of lower relative humidity toward the higher reading
- C.From the area of higher vapour pressure toward the area of lower✓ Answer
- D.From the cooler side of an assembly toward the warmer side of it
Water vapour migrates from higher vapour pressure to lower — wet moves toward dry and warm moves toward cool. Lowering the vapour pressure of the air beside a wet material is exactly what makes that material give up its moisture.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, vapour pressure differential and moisture movementReport a problem with this question
8. Evaporation from a wet material requires which combination of conditions?
- A.Airflow across the surface and a relative humidity below the halfway mark
- B.Energy at the wet surface and a difference in vapour pressure with the air✓ Answer
- C.Heat at the surface and surrounding air that is already near saturation
- D.A temperature difference between the material and the surrounding air only
Evaporation is a change of state that needs an energy source to free the water molecules and a lower vapour pressure in the adjacent air to receive them. Take away either one and drying stops, no matter how much equipment is running.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, principles of evaporation in structural dryingReport a problem with this question
9. Net evaporation from a wet material stops at the moment when:
- A.the vapour pressure at its surface equals that of the surrounding air✓ Answer
- B.the surrounding air has been warmed above the material's temperature
- C.the chamber air has been exchanged several times with outside air
- D.the temperature of the material equals that of the surrounding air
Moisture leaves a material only while its surface vapour pressure is higher than that of the air beside it. Once the two pressures equalise the material is in equilibrium with the air, and further drying requires lowering the air's vapour pressure again.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, equilibrium between materials and ambient airReport a problem with this question
10. Why does directing air across a wet surface speed up evaporation?
- A.It cools the surface, which lowers the material's vapour pressure
- B.It lowers the humidity ratio of the air throughout the whole chamber
- C.It replaces the moisture-laden layer of air resting on the surface✓ Answer
- D.It supplies the thermal energy that turns the liquid water into vapour
A thin boundary layer of nearly saturated air forms against any evaporating surface and its vapour pressure quickly matches the material's. Sweeping that layer away with drier air restores the pressure difference that keeps evaporation going.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, airflow and the surface boundary layerReport a problem with this question
11. Air movers are running, the chamber air is close to saturation, and moisture content readings have stopped falling. Which conclusion is correct?
- A.Evaporation has stopped because the materials have reached the drying goal
- B.Evaporation is continuing, but the meter is reading the wrong material layer
- C.Evaporation will resume as soon as more air movers are placed in the room
- D.Evaporation has stalled because the air can accept hardly any more vapour✓ Answer
Airflow helps only while the surrounding air is still able to accept moisture. Once the air is near saturation its vapour pressure has risen to meet the material's, and nothing improves until humidity control removes the vapour already in the air.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, balancing airflow with humidity controlReport a problem with this question
12. Air movers run for a day in a closed structure with no dehumidification and no ventilation. What happens to the water?
- A.It stays inside the wet materials, because airflow alone cannot evaporate it
- B.It passes out through the building envelope as quickly as it evaporates
- C.It condenses back into the same materials it came from once night falls
- D.It evaporates and is absorbed by dry materials elsewhere in the building✓ Answer
Air movers cause evaporation but remove nothing from the structure. Without dehumidification or ventilation the vapour raises indoor humidity and hygroscopic materials in previously dry areas absorb it, spreading the loss as secondary damage.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, secondary damage from elevated humidityReport a problem with this question
13. A drying system is described as balanced. What does that mean?
- A.Temperature and humidity readings are identical in every affected room
- B.Dehumidification keeps pace with or slightly exceeds the evaporation rate✓ Answer
- C.Evaporation is deliberately held slightly above the dehumidification rate
- D.Air movers and dehumidifiers are installed in equal numbers per chamber
If materials release vapour faster than the equipment can remove it, humidity climbs and moisture simply relocates into other materials. Keeping removal at or slightly ahead of evaporation holds the air dry enough for drying to continue.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, balanced drying systemsReport a problem with this question
14. Chamber air has a dew point of 62°F. A window's glass surface measures 55°F. What happens at the glass?
- A.Water condenses, because the surface sits below the dew point of the air✓ Answer
- B.Nothing forms, because the room's relative humidity is below saturation
- C.Nothing forms until the whole body of air has cooled to the dew point
- D.Water evaporates from the glass, because the glass is cooler than the air
Air touching a surface colder than its dew point is chilled to saturation and gives up vapour as liquid water. Condensation therefore appears on glass, uninsulated walls and cold pipes long before the bulk air in the room reaches its dew point.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, condensation on cold surfaces during dryingReport a problem with this question
15. When air inside a drying chamber reaches saturation, which statement is true?
- A.Dry-bulb, wet-bulb and dew-point temperatures match, and drying stops✓ Answer
- B.Wet-bulb rises above dry-bulb, and evaporation reaches its fastest rate
- C.Dry-bulb is above wet-bulb, and dew point sits well below both readings
- D.Dry-bulb equals dew point while the wet-bulb reading keeps on falling
At saturation the air holds all the vapour it can at that temperature, so the three temperatures converge and wet-bulb depression disappears. With no capacity left to receive moisture, evaporation ceases and condensation begins instead.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, saturation, wet-bulb depression and the psychrometric chartReport a problem with this question
16. Why does raising chamber temperature eventually stop improving the drying rate?
- A.Warm air holds less vapour, so evaporation slows once the room is hot
- B.Heat raises the air's dew point even when no moisture has been released
- C.The vapour released raises the air's vapour pressure unless it is removed✓ Answer
- D.Heat lowers the humidity ratio, so the difference in vapour pressure vanishes
Temperature does two jobs: it energises the wet material and it enlarges the air's capacity. But the vapour driven out has to be taken away, or the air's own vapour pressure climbs until the difference driving evaporation narrows to nothing.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, thermal energy and its limits in the drying processReport a problem with this question
17. A wet wall assembly is exposed to dry air on one face only. How does moisture move within it?
- A.Out of both faces equally, because the assembly dries as a single unit
- B.Toward the face with the lower vapour pressure, the exposed dry side✓ Answer
- C.Toward whichever face is coldest, regardless of the vapour pressures involved
- D.Nowhere until the surface layers have dried completely, then outward
A drying gradient is simply a vapour-pressure gradient across the material: moisture migrates from the wetter core toward the face where pressure is lowest. If only one face is exposed to dry air, the assembly can dry in one direction only.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, drying gradients within materials and assembliesReport a problem with this question
18. A technician logs the humidity ratio of the air entering a dehumidifier and of the air leaving it. What does the difference between the two tell them?
- A.Whether the unit is actually pulling water vapour out of the chamber air✓ Answer
- B.Whether outside air is dry enough to be used for ventilation drying
- C.Whether the materials have reached the drying goal set in the drying plan
- D.Whether the chamber has enough air movers for its wet surface area
The drop in humidity ratio across the unit, known as grain depression, is direct evidence that vapour is being taken out of the air. Little or no drop means the equipment is not lowering the vapour pressure that drives evaporation.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, monitoring dehumidifier inlet and outlet conditionsReport a problem with this question
19. Daily readings show the affected room's humidity ratio well above that of an unaffected room in the same building. What does the comparison indicate?
- A.The moisture meters used in the affected room need to be recalibrated
- B.The affected room is at equilibrium and equipment can now come out
- C.Vapour is entering that room's air faster than it is being removed✓ Answer
- D.The unaffected room has been contaminated by air from the affected room
An unaffected area of the same building serves as the reference condition. A much higher humidity ratio in the affected room means evaporation is outrunning removal there, so humidity control has to be increased before the excess vapour damages more materials.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, daily psychrometric monitoring of affected and unaffected areasReport a problem with this question
20. On a Category 1 loss, outside air measures a lower humidity ratio than the air inside the structure. What does that comparison support?
- A.Ventilating with outside air could lower the indoor humidity level✓ Answer
- B.Outside relative humidity by itself settles whether ventilation will work
- C.Ventilating with outside air would raise the indoor humidity level
- D.Indoor air has to be cooled before any outside air can be brought in
Open or ventilation drying works only when outdoor air carries less water per pound than indoor air, which is a humidity ratio comparison, not a relative humidity one. It also demands constant monitoring and is unsuitable where contamination could be spread.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, open (ventilation) versus closed drying systemsReport a problem with this question
21. Chamber A reads 85°F and 55 GPP; Chamber B reads 85°F and 35 GPP. Both hold the same wet materials. Which chamber dries faster, and why?
- A.Neither, because both chambers share the same dry-bulb temperature
- B.Chamber A, because air nearer to saturation moves moisture more quickly
- C.Chamber A, because its air carries more energy to the wet materials
- D.Chamber B, because its lower humidity ratio widens the pressure gap✓ Answer
At the same dry-bulb temperature, the air carrying fewer grains per pound has the lower vapour pressure. That widens the difference between the wet material's surface and the surrounding air, which is what actually drives moisture out of the material.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, comparing psychrometric conditions and drying potentialReport a problem with this question
22. From day 1 to day 2 a chamber's dry-bulb rose from 78°F to 88°F and relative humidity fell from 60% to 45%, while the humidity ratio stayed at 84 GPP. What do these readings show?
- A.The materials released far more vapour into the air than on day 1
- B.The dehumidifiers pulled a large amount of water out of the air
- C.The air is warmer but carries just as much water as the day before✓ Answer
- D.The falling relative humidity proves the materials are nearly dry
Relative humidity fell only because heating enlarged the air's capacity. An unchanged humidity ratio means the weight of water in the air is exactly what it was, so the lower relative humidity is no evidence that any moisture left the chamber.
Source: ANSI/IICRC S500 Standard for Professional Water Damage Restoration, interpreting day-to-day psychrometric readingsReport a problem with this question

Practice questions based on the ANSI/IICRC S500 Standard for Professional Water Damage Restoration and standard restoration practice. IICRC and WRT are marks of the Institute of Inspection, Cleaning and Restoration Certification; this site is not affiliated with or endorsed by the IICRC. The S500 is revised periodically, and mould remediation and some restoration work are separately licensed in certain states — always follow the current standard, your licensing obligations and the authority having jurisdiction. About the WRT certification →

1. In structural drying, relative humidity expresses which of the following?

2. The air in a sealed drying chamber is heated, and no water is added to or removed from it. What happens to the readings?

3. Only the temperature of the air in a drying chamber is raised. What happens to the dew point?

4. Heat is shut off in a drying chamber overnight and the air cools, with no moisture added or removed. What is the most likely result?

5. What does a humidity ratio reading, expressed in grains per pound, tell the restorer?

6. Two rooms both read 50% relative humidity; one is at 85°F and the other at 65°F. Which statement is correct?

7. Vapour pressure determines the direction in which moisture moves. Which direction does it move?

8. Evaporation from a wet material requires which combination of conditions?

9. Net evaporation from a wet material stops at the moment when:

10. Why does directing air across a wet surface speed up evaporation?

11. Air movers are running, the chamber air is close to saturation, and moisture content readings have stopped falling. Which conclusion is correct?

12. Air movers run for a day in a closed structure with no dehumidification and no ventilation. What happens to the water?

13. A drying system is described as balanced. What does that mean?

14. Chamber air has a dew point of 62°F. A window's glass surface measures 55°F. What happens at the glass?

15. When air inside a drying chamber reaches saturation, which statement is true?

16. Why does raising chamber temperature eventually stop improving the drying rate?

17. A wet wall assembly is exposed to dry air on one face only. How does moisture move within it?

18. A technician logs the humidity ratio of the air entering a dehumidifier and of the air leaving it. What does the difference between the two tell them?

19. Daily readings show the affected room's humidity ratio well above that of an unaffected room in the same building. What does the comparison indicate?

20. On a Category 1 loss, outside air measures a lower humidity ratio than the air inside the structure. What does that comparison support?

21. Chamber A reads 85°F and 55 GPP; Chamber B reads 85°F and 35 GPP. Both hold the same wet materials. Which chamber dries faster, and why?

22. From day 1 to day 2 a chamber's dry-bulb rose from 78°F to 88°F and relative humidity fell from 60% to 45%, while the humidity ratio stayed at 84 GPP. What do these readings show?