Quick answer
Relative humidity is the amount of water vapor in the air compared with the most vapor that air can hold at its current temperature, written as a percent, so 100 percent means the air is saturated.
Because warm air can hold more vapor than cold air, the same air has a lower relative humidity when it is heated and a higher one when it cools. That is why RH changes without any water being added or removed.
Quick facts
The key facts about Relative Humidity, with sources:
- Unit
- Percent; 100% means saturated air, where the dew point equals the air temperature 1
- EPA indoor target
- Below 60 percent, ideally 30 to 50 percent 2
- Depends on temperature
- Air can store much more water vapor as temperature rises 6
- NIH mold flag
- Some mold growth occurs when room humidity stays above 60 percent with no water source 4
Key takeaways
- Relative humidity is a comparison, not an amount. It says how full the air is at its current temperature 1.
- Heat air without adding water and RH falls. Cool it and RH rises, up to 100 percent, where water can condense 16.
- EPA says to keep indoor humidity below 60 percent, ideally 30 to 50 percent 2.
- A drying crew records RH and temperature daily because both drive how fast materials dry 4.
- RH alone is not enough. Dew point or specific humidity gives the actual moisture in the air.
What relative humidity is
Air holds some water as an invisible gas called water vapor. How much it can hold depends on temperature. Relative humidity compares what the air holds now to the most it could hold at that temperature.
The National Weather Service describes it as a measure of the actual amount of water vapor in the air compared with the total amount the air can hold at its current temperature, expressed as a percent 1.
The word “relative” is the catch. A reading of 50 percent at 70°F means half-full for 70°F air. The same vapor in 90°F air would read much lower, and in 50°F air it would read much higher. The reading moves with temperature even when the amount of water in the room has not changed.
The scientific definition is a ratio of the actual vapor pressure to the saturation vapor pressure at the air temperature 5. Saturation pressure rises quickly with temperature. Building Science Corporation’s fundamentals slides put it simply: air can store much more water vapor as temperature increases 6.
In a wet building, RH matters because wet materials give off vapor, and the air has to take it away. When room air is too humid, drying slows and mold risk rises. This page gives the basics; dew point covers the temperature at which the air saturates.
How relative humidity behaves, step by step
Follow one volume of air through a day to see why RH moves.
- Air holds some vapor. A room has a certain amount of water vapor in it, from breathing, cooking, showers and wet materials.
- Warm air can hold more. As temperature rises, the most vapor the air can hold rises quickly, so the same vapor is a smaller share of the maximum 6.
- RH falls on heating. With no water added, the percent drops as the room warms.
- RH rises on cooling. As the room cools, the same vapor becomes a larger share of a smaller maximum, until it reaches 100 percent.
- At 100 percent, the air is saturated. The dew point equals the air temperature, and water can condense on surfaces at or below that temperature 1.
- Adding or removing water changes the total. Wet materials add vapor. Dehumidifiers remove it 4.
- Same vaporWater in the room is unchanged
- Air warmsIt can hold more
- RH fallsSame vapor, bigger maximum
- Air coolsIt can hold less
- RH risesUp to 100% at the dew point
Ways to express humidity, and the RH ranges public sources name
| Measure or range | What it tells you | Source |
|---|---|---|
| Relative humidity (%) | How full the air is at its current temperature | 1 |
| Dew point (°F or °C) | The temperature at which the air would be saturated; shows the actual moisture, not just fullness | 1 |
| Specific humidity | Moisture in the air by mass, recorded alongside temperature and RH in NIH’s restoration records | 4 |
| EPA indoor range | Keep below 60 percent, ideally 30 to 50 percent | 2 |
| UNL checklist range | Below 50 to 60 percent; optimum about 30 to 50 percent; basements 30 to 50 percent | 3 |
| NIH mold flag | Mold growth can occur above 60 percent without a water source; NIH says “usually due to inadequate ventilation” | 4 |
Example: the same air at five temperatures
The values below are calculated, not measured. They use the Magnus formula for saturation vapor pressure with the coefficients Lawrence (2005) cites from Alduchov and Eskridge, for air that starts saturated at 50°F and is warmed with no water added 5.
Values are rounded and show the pattern, not a reading from any building. They agree roughly with the 100, 50 and 25 percent pattern at 50, 72 and 95°F in Building Science Corporation’s slides 6.
| Air temperature | Relative humidity (calculated) | What changed |
|---|---|---|
| 50°F | 100% | Starting point: saturated |
| 60°F | about 70% | Warmed 10°F; no water added |
| 70°F | about 49% | Warmed 20°F; the same vapor is now half-full |
| 80°F | about 35% | Warmed 30°F |
| 90°F | about 26% | Warmed 40°F; about a quarter-full |
A second calculation shows why RH alone misleads. Air at 90°F and 50 percent RH holds about 2.4 kilopascals of vapor pressure, while air at 50°F and 80 percent RH holds about 1.0.
The hot, 50 percent air carries roughly two and a half times as much water, though its RH is lower.
The same arithmetic explains winter: outdoor air at 20°F and 80 percent RH, warmed to 70°F with no humidifier and ignoring moisture added indoors, works out to about 12 percent RH.
That arithmetic is why heated homes can feel dry in cold weather 5.
Where you see relative humidity
On the drying log. NIH’s procedures call for daily temperature and relative humidity readings of both indoor and outdoor air 4, and for atmospheric readings including temperature, relative humidity and specific humidity 4. Compare them from day to day.
On a home thermostat or a hygrometer. A hygrometer is a small humidity meter sold in home stores. UNL Extension notes hygrometers measure relative humidity 3.
On dehumidifier settings. Many units show a target RH. Ask the restorer what target the job is using.
On inspection and claim records. A reading during the first visit shows how humid the building was. See water damage inspection and moisture mapping.
What RH is good for
- A quick health check. One reading in minutes shows whether a room is in EPA’s 30 to 50 percent range or above 60 percent 2.
- A drying gauge. Falling RH in a wet room shows that dehumidifiers are pulling out the moisture the materials give up 4.
- An early warning. A rising RH in a basement or closet shows a moisture source before mold or stains appear.
- A link to materials. Wood settles toward a moisture content set by RH and temperature 8.
What RH does not tell you
- Relative humidity is not an amount of water. RH depends on temperature. Two rooms at the same RH can hold very different amounts of vapor 6.
- It is a reading of the air, not the walls. Wet materials can sit behind a normal RH. Use a meter and a map for materials.
- It changes with the thermostat. A reading taken after the heat is turned up is lower though nothing dried.
- Dew point does the job better for condensation. Building Science Corporation says the most useful measure is the dewpoint temperature of the interior air 7.
- Cheap sensors drift. Check a hygrometer against a second one if a reading looks wrong.
How RH affects cost and energy
The relative humidity entry gives no prices. Equipment and energy costs vary by region and job, and no dollar figure here would meet our cost standard. The point is how RH drives the work.
A wet building with high RH needs more dehumidification to bring the air down so that materials can release their water.
NIH notes that moisture evaporating into the air has to be exchanged with less humid air or removed by dehumidifiers, and that air conditioning serving the area can run continuously to help 4.
More dehumidifier time and longer monitoring appear on the estimate as equipment days.
Keeping RH in range afterwards is cheap in effort: vents, fixing leaks and a small dehumidifier in a damp basement. Use the drying time tool to see what drives a job’s length and the cost guide for how monitoring feeds a total.
How restorers use RH during drying
The steps follow NIH’s drying procedures 4.
- Record air conditions at the start. Temperature and relative humidity, indoors and outdoors.
- Remove vapor with dehumidifiers and exchange air. Moisture evaporating into the air should be exchanged with less humid air or collected by dehumidification 4.
- Keep temperature up. NIH says drying-environment temperatures should be maintained to enhance evaporation and dehumidification effectiveness 4.
- Read again daily. Falling RH with falling material readings means drying is working.
- Compare to the goal. Air conditions and material readings together show when drying is done. See structural drying.
Keeping indoor RH in range
EPA says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent, to use air conditioners or dehumidifiers when needed, and to dry any condensation on windows, walls or pipes quickly 2.
UNL Extension’s home checklist adds practical items: vent bathroom and range fans to the outside, not into the attic; rarely use humidifiers; keep basement humidity between 30 and 50 percent; clean drip pans and dehumidifiers frequently 3.
Read a hygrometer at the same time each day for a week, and note the temperature. Rising readings in one room point to a source. See hidden water damage for what to check next.
Warning signs and when to call a professional
- RH that stays above 60 percent for days, especially in a basement, closet or bathroom 2.
- Condensation on windows, walls or pipes, which EPA says can be a sign of high humidity 2.
- A musty smell in a room with no visible water, which NIH ties to mold and moisture 4.
- A dehumidifier that fills fast every day with no change in RH, which suggests a continuing source.
- Anyone with asthma or a weakened immune system in a home with high humidity; NIH notes people with chronic respiratory conditions or immune disorders need to be very careful around mold 4.
Safety and risk
Do not plug in a dehumidifier or fan in a room with standing water or wet outlets.
CDC says never to use an electric tool or appliance while standing in water, and to turn power off at the main breaker if circuits or equipment are wet 10.
NIH warns that residential box fans can pose shock hazards if used for structural drying 4. Call an electrician and a restoration professional.
What guidance and rules cover humidity
No home rule. No federal law sets a legal RH for homes. EPA’s 30 to 50 percent range is guidance, and EPA says no federal limits for mold exist 2.
Definition. The scientific definition of RH as a ratio of vapor pressure to saturation vapor pressure appears in the American Meteorological Society’s Bulletin 5.
Restoration practice. ANSI/IICRC S500 is the water damage restoration standard, and the fifth edition dates from 2021 9. It is a paid standard and is not reproduced here. NIH’s 2023 procedures are one institution’s adopted rules for daily atmospheric readings 4.
Contract and policy. An insurer may ask for readings as proof. Wording varies by insurer and state.
Relative humidity vs related terms
| Term | What it measures | Changes with temperature? | Read more |
|---|---|---|---|
| Relative humidity | Percent of the air’s capacity at its current temperature 1 | Yes, strongly | This page |
| Dew point | The temperature at which the air would saturate 1 | No, unless water is added or removed | Dew point page, linked above |
| Specific humidity | Moisture in the air by mass 4 | No | Used in restoration logs 4 |
| Moisture content | Water in a material as a percent of dry weight 8 | Slowly, as materials equalize with the air | Moisture content |
| Equilibrium moisture content | The moisture content wood reaches in air of a given RH and temperature 8 | Yes, with RH and temperature | See Wood Handbook Table 4-2 |
Common myths about relative humidity
- Myth A higher RH means more water in the air.
- Reality RH is relative to temperature. Air at 90°F and 50 percent RH holds more water than air at 50°F and 80 percent, by calculation 5.
- Myth Heating a wet room dries it by lowering RH alone.
- Reality Heat lowers RH without removing water. Dehumidifiers remove vapor, and NIH calls for temperature to be kept up to speed evaporation 4.
- Myth Under 60 percent RH means there is no moisture problem.
- Reality RH describes the air, not wet materials inside walls. Use a meter.
- Myth RH can never pass 100 percent.
- Reality The air saturates at 100 percent and water condenses. Surface condensation can occur where a surface is cooler than the air’s dew point 7.
- Myth A hygrometer reading is fixed for a house.
- Reality It changes with temperature, weather and activity.
How climate sets your RH problem
Climate changes which humidity problem you have. In cold regions, outdoor air that is cold has little moisture, and when it warms indoors the RH can fall to very low values.
Building Science Corporation notes that interior relative humidity tends to fall as outdoor air gets colder, because dry outdoor air dilutes indoor moisture 7.
In hot, humid regions, outdoor air carries much more vapor, and air conditioning can chill surfaces below the air’s dew point.
The Wood Handbook shows the consequence for materials: at 70°F, wood settles near 6.2 percent moisture content at 30 percent RH and near 16.0 percent at 80 percent 8. That is why a normal reading differs by region.
When RH matters to you, and what to do next
RH matters when a room has been wet, when a space smells damp, or when condensation shows on windows and pipes. Put a hygrometer in the room and read it at the same time each day. Aim for the 30 to 50 percent range, and act if it stays above 60 percent 2.
During a restoration job, ask what RH and temperature the crew is targeting and what the daily numbers are. The how restoration works guide shows the drying phase, and the first 24 hours guide lists safe early steps.
If a number looks wrong, check the thermometer too, since RH moves with temperature.
Questions about Relative Humidity
What is relative humidity?
Relative humidity is the water vapor in the air compared with the most the air can hold at its current temperature, written as a percent. The National Weather Service describes it that way, and 100 percent means the air is saturated 1. It changes as the temperature changes even if no water is added.
What is a good relative humidity indoors?
EPA says to keep indoor humidity below 60 percent, ideally between 30 and 50 percent 2. UNL Extension’s home checklist gives about the same range and says the same for basements 3. Aim for the 30 to 50 range and act if readings stay above 60.
Why does relative humidity change when the temperature changes?
Warm air can hold more vapor than cold air. If the vapor stays the same and the air warms, the percent falls. If the air cools, the percent rises, up to 100 percent. Building Science Corporation says air can store much more water vapor as temperature increases 6.
What humidity level causes mold?
No single level is a mold line. EPA says that when wet materials are dried within 24 to 48 hours, “in most cases” mold will not grow 2.
NIH says some mold growth occurs when room humidity stays above 60 percent with no water source, “usually due to inadequate ventilation” 4. High humidity plus a damp surface raises the risk.
How do you measure relative humidity?
Use a humidity meter or hygrometer. EPA says relative humidity can be measured with a moisture or humidity meter 2, and UNL Extension notes hygrometers are sold in home and hardware stores 3. Take readings at the same place and time each day, and note the temperature.
What humidity do restorers aim for during drying?
It depends on the job, and public sources used here do not give one number. NIH says to exchange vapor with less humid air or remove it with dehumidifiers, to keep temperatures up, and to record temperature and humidity daily 4. Ask your restorer for the target.
Is relative humidity the same as dew point?
No. Relative humidity is a percent of capacity and changes with temperature. Dew point is the temperature at which the air would be saturated. The National Weather Service says RH cannot be found from the dew point alone, because the air temperature must also be known 1.
Can a dehumidifier lower relative humidity quickly?
It removes water vapor from the air, which lowers RH 4. Speed depends on how much water the building still releases and on the size of the unit. A restorer picks equipment by the size of the wet area and the materials. Use the drying time tool for what drives timing.
Does high humidity mean I have water damage?
Not by itself. High humidity can come from weather, cooking, showers or a lack of ventilation. But RH that stays high in one room, with condensation or a musty smell, can point to a hidden leak 24. A moisture meter and an inspection can tell the two apart.
Sources
- National Weather Service (Louisville), Humidity and dew point explained, retrieved .
- U.S. EPA, A Brief Guide to Mold, Moisture and Your Home, retrieved .
- University of Nebraska-Lincoln Extension, G1832 Identifying Moisture and Mold Sources in Houses (Niemeyer, 2008), retrieved .
- National Institutes of Health, Moisture and Mold Remediation Standard Operating Procedures (2023), retrieved .
- Lawrence, M. G., The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air: A Simple Conversion and Applications, Bulletin of the American Meteorological Society 86(2), 2005, retrieved .
- Building Science Corporation, Heat, Air and Moisture Movement (Building Science Fundamentals, Lstiburek and Straube, 2012), retrieved .
- Building Science Corporation, BSD-163: Controlling Cold-Weather Condensation Using Insulation (Straube, 2011), retrieved .
- USDA Forest Products Laboratory, Wood Handbook Chapter 4: Moisture Relations and Physical Properties of Wood (GTR-FPL-282, 2021), retrieved .
- IICRC, ANSI/IICRC S500 Standard for Professional Water Damage Restoration (edition history and scope), retrieved .
- CDC, Guidelines for Cleaning Safely After a Disaster, 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: The definition and the statement that RH cannot be found from dew point alone come from the National Weather Service’s humidity page.
The vapor-pressure definition and the Magnus coefficients (17.625 and 243.04°C) come from Lawrence (2005) in the Bulletin of the American Meteorological Society, read from the PDF.
The warming table and the 90°F versus 50°F comparison are our own arithmetic with those coefficients, labelled as calculated and rounded; they agree roughly with Building Science Corporation’s slides. Indoor ranges come from EPA, UNL Extension and NIH.
The wood equilibrium values come from Table 4-2 of the Wood Handbook at 70°F. No dollar figures are given.
Suggest a correction. We fix errors and say what changed.