Quick answer
Psychrometrics in water damage restoration is the measurement of moist air, using temperature, relative humidity, dew point and grains per pound to show how much water the air holds and whether it can take up more.
Relative humidity changes whenever the air is heated or cooled, even if no water is added. Grains per pound counts the water itself, so drying crews log both numbers, along with the dew point, every day.
Quick facts
The key facts about Psychrometrics in Water Damage Restoration, with sources:
- Relative humidity (RH)
- The ratio of the vapor pressure in the air to the vapor pressure at saturation at the same temperature 14
- Grains per pound (GPP)
- Grains of water vapor per pound of dry air; 7,000 grains make one pound 1
- Humidity ratio
- The mass of water vapor divided by the mass of dry air 1
- What a dehumidifier shows
- Some units display inlet and outlet readings and a grain depression in GPP 7
- EPA indoor target
- Keep relative humidity below 60%, ideally between 30% and 50% 5
Key takeaways
- Relative humidity is a percentage of what the air can hold at its current temperature, so it moves when the temperature moves 1.
- Grains per pound counts the water vapor in the air itself. Heating or cooling a room does not change it 1.
- The dew point is the surface temperature at which condensation starts. Any surface at or below it can get wet from the air 4.
- A drying log should hold temperature and relative humidity, indoors and outdoors, every day 6.
- Every example here is computed from a published formula with illustrative inputs. It is not a reading from a real building.
Psychrometrics in plain English
Air carries some water vapor. A drying job is a contest over that vapor. Wet materials give water to the air, and the equipment pulls it back out.
To follow the contest, a technician needs to know how much water is in the air and how close the air is to full. Psychrometrics is the name for measuring and relating those facts.
Four numbers drive the work. Temperature is the first. Relative humidity is the second, defined by the Pacific Northwest National Laboratory’s building moisture tool as the ratio of the current vapor pressure to the saturation vapor pressure 1.
EPA gives the same definition in its moisture control guidance and calls it technically more accurate than the everyday description 4. The dew point and grains per pound are the third and fourth.
The dew point is the temperature at which water vapor reaches its saturation point, or 100% relative humidity, and condensation begins below it 1.
Grains per pound is the humidity ratio written in drying-trade units. The humidity ratio is the mass of water vapor compared with the mass of dry air, and there are 7,000 grains in a pound 1.
The IICRC, the certification body that publishes the S500 water damage standard, lists psychrometry and drying technology among the topics the standard covers 10. The standard is a purchased document and this page does not reproduce it. The relationships below are general moist-air physics, taken from public government sources.
How the four numbers connect
Think of the air as a bucket that gets bigger when it is warm. The steps below give the links between the psychrometric numbers, in order.
- Temperature sets the size of the bucket. Warmer air can hold more vapor before it saturates. EPA notes that when the air is very dry the dew point is low, and surfaces must be much cooler than the air for condensation to occur 4.
- Grains per pound is how much water is in the bucket. It is the actual water, so it stays the same when you heat or cool the room 1.
- Relative humidity is how full the bucket is. The same water in a bigger, warmer bucket gives a lower percentage 1.
- The dew point is the temperature at which the bucket is exactly full. Cool the air to that temperature and it holds all it can 1.
- A cold surface acts like a dew point test. If the dew point is above the surface temperature, moisture condenses onto the surface. If it is below, it does not 4.
- TemperatureSets how much vapor the air can hold
- Relative humidityHow full the air is at that temperature
- Dew pointTemperature at which the air is full
- Grains per poundThe water itself, in grains
- Vapor pressureThe pressure the water vapor exerts
Temperature and any one of the other four fix the rest at a given air pressure. Source: [1] [4]
The readings compared: what each one tells you
| Reading | What it measures | Changes when you heat the room? | Best used for |
|---|---|---|---|
| Temperature | How warm the air or surface is | Yes | Setting how much vapor the air can hold 4 |
| Relative humidity | Vapor pressure as a share of saturation 1 | Yes, it falls when air warms with no water added | Comparing to EPA’s 30% to 50% target 5 |
| Dew point | Temperature where vapor saturates 1 | No | Predicting condensation on cold surfaces 4 |
| Grains per pound | Water vapor per pound of dry air 1 | No | Judging how much water a dehumidifier has to remove |
| Vapor pressure | Pressure exerted by the water vapor 1 | No, for the same air pressure | Understanding which way moisture moves |
Relative humidity alone can mislead
EPA notes that relative humidity is a relative measurement and not one that expresses the absolute amount of water in the air. Change the air temperature and the relative humidity changes, so knowing only the RH is not much help in predicting condensation 4.
Example: how relative humidity maps to grains per pound and dew point
The table below is computed, not measured. The inputs, 70°F air at six relative humidity values, are illustrative.
The vapor pressure comes from the National Weather Service El Paso formula e = 6.11 × 10^(7.5T ÷ (237.3 + T)) with T in °C, in hectopascals 2.
Grains per pound use the NWS mixing-ratio formula, 621.97 × e ÷ (P − e) in grams per kilogram, then multiply by 7 for grains per pound, since 7,000 grains make a pound 31.
Air pressure P is 1013.25 hPa, the standard sea-level value. These formulas are approximations, so a published chart may differ by a fraction of a grain.
| Illustrative air at 70°F | Vapor pressure (hPa) | Grains per pound | Dew point |
|---|---|---|---|
| 30% relative humidity | 7.5 | 32.5 | 37°F |
| 40% relative humidity | 10.0 | 43.5 | 45°F |
| 50% relative humidity | 12.5 | 54.5 | 51°F |
| 60% relative humidity | 15.0 | 65.5 | 56°F |
| 80% relative humidity | 20.0 | 87.8 | 64°F |
| 100% relative humidity | 25.0 | 110.4 | 70°F |
Two checks support the method. The PNNL page gives a humidity ratio of 0.008 as equal to 56 grains per pound, which is the same multiplication 1.
And EPA’s guidance reads the dew point of air at 85°F and 60% relative humidity as 70°F from a psychrometric chart 4. The same formula gives about 69.5°F, a close match.
Example: heating a room changes relative humidity but not the water
Take the 70°F, 50% relative humidity air from above. It holds about 54.5 grains per pound. Now suppose a heater warms the room and adds no water. The grains per pound and dew point stay put. Only the percentage moves. This second table uses the same NWS formulas and is illustrative 231.
| Same air, warmed with no water added | Relative humidity | Grains per pound | Dew point |
|---|---|---|---|
| At 70°F | 50% | 54.5 | 51°F |
| Warmed to 80°F | 36% | 54.5 | 51°F |
| Warmed to 90°F | 26% | 54.5 | 51°F |
| Cooled to 60°F | 71% | 54.5 | 51°F |
e = RH × 6.11 × 10^(7.5T ÷ (237.3 + T)) GPP = 7 × 621.97 × e ÷ (P − e)
T- Air temperature in °C (subtract 32 from °F and divide by 1.8)
RH- Relative humidity as a fraction, such as 0.50
e- Vapor pressure of the air in hPa
P- Station air pressure in hPa; 1013.25 at standard sea level
GPP- Grains of water vapor per pound of dry air
The vapor pressure formula and the 621.97 mixing-ratio constant are from the NWS El Paso weather calculator pages; the 7,000 grains per pound is from PNNL. Treat results as close estimates.
This is why a heated room can show a falling relative humidity while the dehumidifier has removed nothing. A technician watching only the percentage could think the air is drying when the water content has not changed. Grains per pound and the dew point do not have that problem 1.
Where you see these numbers on a job
On the drying log. NIH’s procedure describes a log that records daily temperature and relative humidity readings of both indoor and outdoor air, along with moisture readings and the drying goal and dry standard 6. Our moisture log page shows how it is kept.
On the dehumidifier. The owner’s manual for one manufacturer’s LGR dehumidifier lists an inlet and outlet display and a “GRAIN DPR” menu item, which shows grains per pound when set to Fahrenheit and grams per kilogram in Celsius 7. Our dehumidifier page explains the machine itself.
On a hygrometer. A hygrometer reads temperature and relative humidity. A technician can convert them to dew point and grains per pound with a chart or calculator, as EPA shows with its simplified psychrometric chart 4.
In a conversation about condensation. If a room feels clammy or a window fogs, the dew point and the surface temperature explain why. See the page on moisture intrusion vs condensation for how to tell a leak from vapor condensing out of the air.
What reading psychrometrics gives you, and what it does not
What it gives
- A way to tell whether the air is actually drier, not just warmer 1.
- A prediction of condensation. If the surface is below the dew point, expect it to get wet 4.
- A fair comparison between dehumidifier inlet and outlet air, shown as grain depression on some units 7.
- A common language with the technician, so the daily log makes sense to you.
What it does not give
- A measure of the water inside building materials. That needs a moisture meter and a dry standard.
- A finish line. Drying ends when materials reach their goal, not when the air looks dry 6.
- A safety check. Air readings say nothing about contamination or mold.
- A single answer for the whole building. Conditions differ from room to room and from inside a wall cavity to the room air.
Limits of psychrometric numbers
Air readings describe the air, not the building. A room can sit at a low relative humidity while a wall cavity stays wet, because water in a cavity does not reach the room air until it has a path out.
That is why NIH asks for moisture readings of the materials themselves, at recorded locations, as well as air readings on the log 6.
The formulas are approximations. The NWS pages give the vapor pressure and mixing ratio formulas as working calculations for a weather calculator, and the examples above inherit that precision 2. They are also pressure-sensitive.
The mixing ratio formula takes the station pressure as an input, so the same temperature and relative humidity give a different grain count at altitude 3.
Readings depend on where the instrument sits. A sensor near a hot machine outlet or a cold window does not describe the room. NIH says readings should come from the same instrument each time, which keeps day-to-day comparisons honest 6.
Psychrometrics also does not decide how much equipment a job needs. Equipment choice depends on the amount of wet material, the volume of the space and the conditions. The IICRC’s S500 standard is where the industry frames that decision, and it is a purchased document 10.
How psychrometrics drives equipment cost, without a price tag
We do not publish prices and no source we loaded gives a fair national figure. What psychrometrics does is decide how hard the air has to work, and that decides how many machines run and for how long.
A building with humid outdoor air and leaky walls carries a larger moisture load than a tight one. EPA notes that outdoor air is the largest source of humidity in many buildings during the cooling season 4.
Three things move the equipment line. Warm, humid weather raises the grains per pound of the air that has to be removed. Open doors and windows let humid air in again. Low temperatures reduce how well refrigerant dehumidifiers work.
ENERGY STAR notes that frost can form on the coils of a household unit when a space falls below 65°F, which hurts performance 8. One manufacturer’s manual gives an operating range of 33°F to 100°F for its LGR model 7.
To see how duration is affected without a promise, use the drying time tool. The cost guide shows how an invoice is built. Neither replaces a written drying plan that names the goal.
How a psychrometric reading is taken and logged
A restoration technician takes the psychrometric readings, and you can follow along. No permit is needed to read air conditions.
- Read temperature and relative humidity indoors, in the drying area, and outdoors, with the same instrument each time 6.
- Convert them to dew point and grains per pound with a chart or calculator. EPA shows how to find the dew point from temperature and relative humidity on a simplified psychrometric chart 4.
- Read the dehumidifier inlet and outlet where the unit shows them, and note the grain depression 7.
- Write the numbers, the time and the instrument on the log next to the material readings 6.
- Compare today’s grains per pound with yesterday’s. A falling number in the drying area suggests the air is taking up less water from the materials.
- Adjust. If the air is not drying, check for open doors, a full condensate drain or an undersized unit before adding days.
Keeping humidity in range after the job
After drying, the goal is to keep the air from drifting back into the damp range. These steps follow EPA and ENERGY STAR guidance 58.
- Aim for 30% to 50%. EPA says to keep indoor humidity below 60%, ideally between 30% and 50%. ENERGY STAR calls 30% to 50% the optimum range 58.
- Use a humidity meter. EPA notes a moisture or humidity meter can measure relative humidity 5.
- Run exhaust fans. EPA advises running the bathroom fan or opening a window when showering and using exhaust fans while cooking or dishwashing 5.
- Keep air conditioner drip pans clean. EPA says to keep the drain lines unobstructed and flowing properly 5.
- Add a dehumidifier when needed. EPA suggests air conditioners and dehumidifiers when humidity stays high 5.
Warning signs the air is too wet
- Visible condensation on windows, pipes or cold surfaces. EPA says condensation can be a sign of high humidity, and the cure is to dry the surface and reduce the moisture source 5.
- Relative humidity stuck above 60%. EPA’s upper limit for indoor air is below 60% 5.
- A dehumidifier running constantly with no change in the grain count. That points to a moisture source nobody found, or an undersized unit.
- Clammy or musty air. A musty smell can mean moisture is trapped. Ask for readings at the logged points and a thermal camera check.
- Cold supply ducts sweating in warm weather. EPA describes humid outdoor air condensing on a cold supply duct as an example of high-dew-point air meeting a cold surface 4.
Air readings do not show whether a space is safe to enter
Never enter a flooded or fire-damaged building before it has been judged safe. If water came from sewage, outside flooding or a source you cannot name, a humidity number tells you nothing about contamination. Let a qualified professional assess the space first.
Standards and guidance behind the numbers
The restoration standard. ANSI/IICRC S500, Standard for Professional Water Damage Restoration, lists psychrometry and drying technology among its topics. The IICRC lists the fifth edition as the 2021 edition and says a new consensus body has begun work to revise it 10. It is voluntary and purchased, and this page does not reproduce it.
Federal guidance. No federal law sets an indoor humidity limit for homes. EPA’s guidance is to keep indoor humidity below 60%, ideally between 30% and 50% 5.
EPA’s separate moisture control guidance explains dew point, relative humidity and condensation for building owners 4. ENERGY STAR gives the same 30% to 50% range as optimum 8.
Your contract. A restoration contract or an insurer may name the S500 standard or require a drying log. Policy and contract wording varies, so read both. Neither the formulas on this page nor the NWS pages are a standard for restoration work.
Psychrometric terms compared with related drying terms
| Term | What it means | How it differs |
|---|---|---|
| Relative humidity | Vapor pressure as a share of saturation at the same temperature 1 | Moves with temperature; see the separate relative humidity term |
| Dew point | Temperature at which vapor saturates and condensation starts 1 | Does not move when the room warms 4 |
| Grains per pound | Humidity ratio times 7,000 1 | The absolute amount of water in the air |
| Vapor pressure differential | The gap in vapor pressure between a wet surface and the air | The force behind evaporation; psychrometrics supplies the numbers |
| Equilibrium moisture content | Where wood settles at a given temperature and humidity 9 | About a material; psychrometrics is about the air |
Relative humidity
- A percentage of what the air can hold
- Changes when the room is heated or cooled
- Compared with EPA’s 30% to 50% range
- Read directly from a hygrometer
Grains per pound
- The actual water in the air
- Stays the same when the room is heated
- Shows what a dehumidifier removes
- Calculated from temperature and RH
Common myths about psychrometrics
- Myth A lower relative humidity means the air holds less water.
- Reality Warming the air lowers the percentage without removing any water. The grains per pound and dew point stay the same 1.
- Myth A room with no visible moisture has no problem.
- Reality Water vapor can move through solid-looking materials by diffusion even when nothing appears wet 4.
- Myth Dew point only matters outdoors.
- Reality EPA uses the dew point to explain condensation inside buildings, such as on a cold supply duct in a humid climate 4.
- Myth One humidity reading describes the whole building.
- Reality Conditions differ by room and by location, which is why NIH asks for readings at recorded locations 6.
- Myth A dry-looking air reading means materials are dry.
- Reality Materials are judged against a goal set from a dry standard, not by the air alone 6.
Climate and altitude change the numbers
Two regional variables change psychrometric readings. The first is climate. EPA’s example is humid outdoor air in Miami, which has a dew point above 70°F through most of a typical year, enough to condense on a 55°F supply duct 4. The second is air pressure.
The NWS mixing ratio formula takes station pressure as an input, so a higher site gives different grains for the same temperature and relative humidity 3.
As an illustration only, using 850 hPa as an example station pressure for a high-altitude site, 70°F air at 50% relative humidity computes to about 65.1 grains per pound, against 54.5 at standard sea-level pressure.
One manufacturer lists a maximum use altitude of 10,000 feet for its LGR dehumidifier 7.
When psychrometrics matters, and what to do next
Psychrometrics matters on any job that uses a dehumidifier, when a room feels damp after “drying,” and when condensation shows up on windows, ducts or cold walls. It matters less for a small, clean spill that dries on its own.
The next step is simple. Ask the company to show you the day’s temperature, relative humidity and grains per pound for the drying area and outdoors, and to explain any change.
If the numbers are not recorded, see structural drying for what a measured job looks like and the air mover page for the other half of the equipment pair.
- If Relative humidity falls but grains per pound do not
- Then The air was warmed, not dried
- If Grains per pound falls steadily
- Then The air is getting drier; check the materials against the goal
- If A surface is colder than the dew point
- Then Expect condensation on it
- If Humidity stays high with a dehumidifier running
- Then Look for a moisture source, open doors or an undersized unit
Questions about Psychrometrics in Water Damage Restoration
What is psychrometrics in water damage restoration?
It is the measurement of moist air using temperature, relative humidity, dew point and grains per pound. It shows how much water the air holds and how close it is to saturation. Drying crews use it to judge whether the air can take more moisture from wet materials and whether equipment is working.
What does grains per pound mean?
Grains per pound is the number of grains of water vapor in a pound of dry air.
There are 7,000 grains in one pound, so a humidity ratio of 0.008 equals 56 grains per pound.
It measures the actual water in the air, so it does not change when the room is heated or cooled with no water added.
Is relative humidity or grains per pound more useful?
They answer different questions. Relative humidity tells you how full the air is at its present temperature and is easy to read from a hygrometer.
Grains per pound tells you how much water is in the air, so it shows whether a dehumidifier is removing any. Drying logs record temperature and relative humidity, and technicians convert to grains.
What is the dew point, and why does it matter?
The dew point is the temperature at which water vapor reaches saturation. Below it, moisture condenses. EPA explains that if the dew point is above a surface’s temperature, moisture condenses onto that surface. It helps explain sweating pipes, fogged windows and damp cold walls, which relative humidity alone cannot predict.
What relative humidity should my home have after water damage?
EPA advises keeping indoor humidity below 60%, ideally between 30% and 50%. ENERGY STAR also calls 30% to 50% the optimum range for a building. In cold weather, ENERGY STAR suggests 30% to 40% to avoid window condensation. A hygrometer lets you check the number yourself.
Why does the relative humidity drop when the heater is on?
Warmer air can hold more water vapor, so the same amount of water becomes a smaller share of the total. As an illustration, air at 70°F and 50% relative humidity computes to about 36% if warmed to 80°F with no water added. The grains per pound and the dew point stay the same.
How do you calculate grains per pound?
Find the vapor pressure from temperature and relative humidity, then use the mixing ratio formula: 621.97 times vapor pressure divided by the difference between air pressure and vapor pressure, in grams per kilogram.
Multiply by 7 for grains per pound. The National Weather Service El Paso pages give the vapor pressure and mixing ratio formulas used here.
Does altitude change grains per pound?
Yes. The formula includes air pressure, so the same temperature and relative humidity produce a different grain count at a different pressure. As an illustration, 70°F air at 50% relative humidity is about 54.5 grains per pound at standard sea-level pressure and about 65.1 at 850 hectopascals. Instruments and charts need the right pressure.
Where does psychrometrics appear in IICRC S500?
The IICRC’s public S500 page lists psychrometry and drying technology among the topics the standard covers. The standard is a purchased document, so this page does not reproduce its text. Ask your restoration company which edition it works to and how it records air conditions on the drying log.
Can humidity readings show that mold is not growing?
No. Humidity readings describe the air, and mold depends on moisture in materials. EPA’s guideline is to dry water-damaged areas within 24 to 48 hours to prevent mold growth. A single air reading cannot show a building is mold-free, and a musty odor is a reason to ask for material readings.
Sources
- Pacific Northwest National Laboratory, Building America Solution Center: Moisture Metrics (definitions of relative humidity, dew point, humidity ratio), retrieved .
- National Weather Service El Paso: Vapor Pressure and Mixing Ratio calculator formulas (weather.gov, wxcalc), retrieved .
- National Weather Service El Paso: Mixing Ratio calculator formula (weather.gov, wxcalc), retrieved .
- U.S. EPA, Moisture Control Guidance for Building Design, Construction and Maintenance (EPA 402-F-13053, December 2013), retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- National Institutes of Health, Moisture and Mold Remediation Standard Operating Procedures (2023), retrieved .
- Legend Brands (Dri-Eaz), LGR 130 Dehumidifier Owner’s Manual #108997, retrieved .
- ENERGY STAR, Dehumidifiers (product guidance), retrieved .
- USDA Forest Products Laboratory, Wood Handbook, Chapter 4: Moisture Relations and Physical Properties of Wood (FPL-GTR-190, 2010), retrieved .
- IICRC, ANSI/IICRC S500 Standard for Professional Water Damage Restoration (standard page), retrieved .
Expert review
Written by the HyreRestore Research Desk. Not yet reviewed by an outside expert. We say so rather than imply a review that has not happened; see our editorial policy.
How the numbers were checked: Definitions were read from the PNNL moisture metrics tool and EPA’s moisture control guidance (EPA 402-F-13053).
Formulas for vapor pressure and mixing ratio were read from two National Weather Service El Paso calculator pages and applied by the Research Desk; every value in the tables and the bar chart is computed from them at 1013.25 hPa unless stated, using illustrative inputs, and is not a measurement.
The EPA cross-check (85°F and 60% RH gives a 70°F dew point) matched the formula within about half a degree. The IICRC S500 standard is purchased and was not read. ASHRAE psychrometric tables were not loaded, so chart values may differ slightly. No prices are given.
Suggest a correction. We fix errors and say what changed.