Quick answer
Dew point is the temperature to which air must be cooled to become saturated with water vapor, so that cooling it any further, or touching it to a surface at that temperature, makes liquid water begin to condense.
Unlike relative humidity, the dew point does not change when the air is heated or cooled, as long as no water is added or removed. It is a direct measure of how much moisture the air holds, which makes it the best way to judge condensation.
Quick facts
The key facts about Dew Point, with sources:
- Definition
- The temperature to which air must be cooled to be saturated 1
- At saturation
- Dew point equals air temperature and relative humidity is 100 percent 1
- Condensation rule
- Water condenses on a surface at or below the dew point of the air touching it 2
- Reference point
- 70°F air at 50 percent RH has a dew point of about 50°F (10°C) 2
- Quick estimate
- The dew point falls about 1°C for every 5 percent drop in RH below 100, for RH above about 50 percent 3
- Comfort scale
- Dew points in the 50s feel comfortable; 60s somewhat humid; 70s quite uncomfortable 1
Key takeaways
- Dew point is a temperature. It is the temperature at which the air you have would be saturated 1.
- Condensation forms on any surface at or below the dew point of the nearby air 2. That is how a cold pipe or window gets wet with no leak.
- Relative humidity cannot be found from the dew point alone; the air temperature must also be known 1.
- A handy rule: the dew point drops about 1°C (1.8°F) for each 5 percent drop in relative humidity below 100, for moist air above 50 percent 3.
- Building Science Corporation says the most useful measure of indoor moisture for condensation is the dew point temperature of the interior air 2.
What dew point is
Pour a cold drink on a warm day and the glass gets wet. The water did not leak through. It came from the air. The glass was colder than the air’s dew point, so vapor in the air touching the glass turned to liquid.
The National Weather Service defines the dew point as the temperature to which air must be cooled to be saturated 1. Saturated air holds all the water vapor it can at that temperature. Cool it more, or touch it to something cooler, and the extra vapor becomes liquid water.
This makes dew point different from relative humidity. RH is a percent of what the air can hold at its current temperature, so it rises and falls as the room warms and cools.
Dew point stays put until water is added to the air or taken out. See relative humidity for the percent measure. A dew point of 50°F means the air holds the same amount of vapor whether the room is 60°F or 80°F.
In water damage work, dew point explains two things: why wet walls and cold pipes sweat in humid air, and why a building with a high interior dew point is at risk of condensation inside its walls. NIH’s restoration procedures record temperature, relative humidity and specific humidity each day for the same reason 5.
How condensation happens at the dew point, step by step
Condensation needs no leak. It needs air with enough vapor and a surface cold enough.
- Air carries vapor. A room has a certain amount of moisture in its air. That moisture sets its dew point.
- A surface is cooler than the air. A cold-water pipe, a window in winter or a wall chilled by air conditioning sits below the room temperature.
- The air next to the surface cools. The thin layer of air touching the surface drops to the surface temperature.
- If that layer reaches the dew point, it saturates. The surface temperature must be at or below the dew point of the air for condensation to occur 2.
- Water forms on the surface. Droplets appear. EPA says condensation on windows, walls or pipes is a sign of high humidity and should be dried quickly 4.
- Warming the surface or drying the air stops it. Raising the surface above the dew point, or lowering the air’s moisture so the dew point falls, ends the condensation 2.
- Humid airVapor sets the dew point
- Cold surfacePipe, glass or chilled wall
- Air layer coolsNext to the surface
- Reaches dew pointLayer becomes saturated
- Droplets formLiquid water on the surface
Dew point levels and what they feel like
| Dew point range | How the air feels | Source |
|---|---|---|
| In the 50s (°F) | Comfortable to most people | 1 |
| In the 60s (°F) | Somewhat humid and uncomfortable | 1 |
| In the 70s (°F) | Quite uncomfortable and very humid | 1 |
| In the 80s (°F) | Very oppressive, but relatively rare | 1 |
| Equal to air temperature | Saturated air; relative humidity is 100 percent | 1 |
Example: finding the dew point and checking a cold surface
The dew point values here are calculated, not measured. They use the Magnus formula and the coefficients Lawrence (2005) cites, and Lawrence’s rule of thumb for comparison 3.
The 70°F, 50 percent point matches Building Science Corporation’s statement that such air has a dew point of about 50°F 2. Values are rounded and apply to the example only.
| Room air at 70°F with RH of | Dew point, calculated | Dew point, rule of thumb | Gap |
|---|---|---|---|
| 90% | 67°F | 66°F | about 0.5°F |
| 80% | 63.5°F | 63°F | about 0.7°F |
| 70% | 60°F | 59°F | about 0.6°F |
| 60% | 55.5°F | 55.6°F | under 0.1°F |
| 50% | 50.5°F | 52°F | about 1.5°F; the rule is meant for RH above 50% |
Now test a surface. In a 70°F room at 50 percent RH the dew point is about 50.5°F. A cold-water pipe at 45°F is below it, so water condenses on the pipe.
A wall surface at 60°F is above it, so it stays dry. At 55°F, the air touching the surface would reach about 85 percent RH, close to condensing but not yet there.
Add humidity to the room, such as a shower or wet materials, and the dew point rises, so the same 55°F surface can start to sweat.
Where you meet dew point in a house or a job
On weather reports. Many forecasts list dew point next to temperature, and the National Weather Service uses it to describe how humid the air feels 1.
On a thermo-hygrometer. Some indoor meters show dew point directly. Others show temperature and RH, from which dew point can be worked out with a chart or the rule above.
On a restoration log. NIH’s procedures record temperature, relative humidity and specific humidity for each visit 5. Specific humidity tracks the actual moisture in the air, as dew point does. See moisture mapping for the full log.
In a building science digest. Building Science Corporation says the most useful measure is the dew point temperature of the interior air, and states that avoiding condensation means keeping surfaces warmer than that dew point 2.
On a sweating pipe, window or wall. Wet surfaces without a leak are the visible sign of a surface below dew point. DOE lists sweating pipes among common signs of moisture problems 7.
What dew point does well
- Tells you the actual moisture in the air. It does not change just because the room warms or cools 1.
- Predicts condensation. If a surface is colder than the dew point, it will sweat 2.
- Separates leaks from humidity. A wet cold pipe in humid air may be condensation. A wet spot that stays wet at room temperature points to a leak.
- Guides drying. Lowering the dew point of the air lets materials give up more water. Dehumidification does that by removing vapor 5.
What dew point does not tell you
- It does not say how wet the walls are. It describes air. Wet materials need a meter and a map.
- It is not a single building number. Different rooms have different dew points, especially near showers or kitchens.
- It needs surface temperatures to be useful. You need to know how cold the surface is. A thermal camera shows surface temperature patterns but does not show moisture 9.
- The quick rule has limits. Lawrence’s rule of thumb is meant for moist air, with relative humidity above about 50 percent, and is an approximation 3.
- It does not say a cavity is wet. Condensation inside a wall depends on where air flows and which surfaces are cold 8.
How dew point affects cost and effort
HyreRestore publishes no prices for dew point work. Costs vary by region and job, and no dollar figure here would meet our cost standard. The link between dew point and cost is about what it takes to keep the air’s dew point below surface temperatures.
To stop condensation you can warm surfaces, lower indoor moisture or both.
Building Science Corporation’s slides name controlling interior RH as a powerful means of controlling condensation in cold-climate buildings 8, and its digest describes keeping sheathing warmer than the interior dew point through insulation 2.
Those measures cost money in materials and labor, but they address the cause.
In a wet building, restorers lower the dew point with dehumidifiers so water leaves materials instead of condensing on cool surfaces. That adds equipment days to the estimate. See the cost guide for how equipment time feeds a total.
Dew point has no lifespan or warranty. It is a number that changes with the weather and the room.
How restorers and inspectors use dew point
The dew point steps combine NIH’s procedures with Building Science Corporation’s condensation guidance 52.
- Record air conditions. Temperature and humidity of indoor and outdoor air at each visit 5.
- Work out the dew point. From temperature and RH, or read directly from an instrument.
- Find cold surfaces. Pipes, windows, exterior walls and anything chilled by air conditioning.
- Compare. Any surface at or below the dew point will collect condensation 2.
- Lower the dew point or warm the surface. Dehumidify, ventilate with drier air, or insulate 25.
- Re-read daily. A falling dew point shows the air is drying. See structural drying.
Keeping condensation out of your home
Keep indoor humidity in range and the dew point stays low. EPA advises keeping indoor humidity below 60 percent, ideally 30 to 50 percent, using air conditioners or dehumidifiers when needed, and drying any condensation on windows, walls or pipes quickly 4.
UNL Extension’s checklist adds: control condensation on outer walls, pipes and vents through correct insulation, keep moderate indoor temperatures in the heating season, and vent moisture to the outside 6. These steps keep surfaces warmer and the air drier.
After a water loss, check windows and cold pipes for sweating. If sweating continues after drying, the room’s dew point may still be high. The mold risk calculator weighs the factors that make that matter.
Warning signs of condensation trouble
- Droplets or wet streaks on windows, cold pipes or exterior walls, which EPA calls signs of high humidity 4.
- Mold or mildew near windows, in corners or on cold exterior walls.
- A musty smell in a closet or on an outside wall.
- Stains on ceilings under cold ducts or pipes with no roof leak.
- Peeling paint on walls where humid air meets cold surfaces; DOE lists peeling paint and sweating pipes among moisture signs 7.
Safety and risk
Do not assume wet spots are only condensation. A leak and condensation can look alike, and wet electrical equipment is dangerous.
CDC says to turn off power at the main breaker if circuits or equipment are wet, and never to use an electric tool or appliance while standing in water 12.
Mold cleanup calls for goggles, an N-95 respirator and gloves 12, and EPA does not recommend bleach or other biocides as a routine practice during mold cleanup 4. Call a professional if unsure.
Where the definitions and rules come from
Definition. The National Weather Service defines dew point as the temperature to which air must be cooled to be saturated 1. The scientific relationship to RH appears in Lawrence’s 2005 paper in the Bulletin of the American Meteorological Society 3.
Building practice. Condensation control is a design topic. Building Science Corporation’s digests guide designers, and local building codes set rules for insulation, vapor control and ventilation that vary by jurisdiction 2.
No home rule. No federal law sets a dew point for homes. EPA’s humidity range is guidance 4.
Restoration practice. ANSI/IICRC S500 is the water damage restoration standard, fifth edition 2021, and is paid; it is not reproduced here 11. NIH’s 2023 procedures record daily atmospheric readings as one institution’s rule 5.
Dew point vs related terms
| Term | What it is | How it differs from dew point |
|---|---|---|
| Dew point | Temperature at which air becomes saturated 1 | A temperature; fixed unless moisture is added or removed |
| Relative humidity | Percent of capacity at the air’s current temperature 1 | Changes with temperature; cannot give dew point without air temperature 1 |
| Specific humidity | Moisture in the air by mass 5 | Another direct measure of how much water the air holds |
| Condensation | Liquid water forming on a surface at or below the dew point 2 | The result of dew point, not a measure |
| Moisture content | Water in a material as a percent of dry weight 10 | About materials, not air; see moisture content |
Common myths about dew point
- Myth Dew point is just another name for humidity.
- Reality It is a temperature, and it stays steady while RH shifts with the thermostat 1.
- Myth A wet window means there is a leak.
- Reality It can be condensation: the surface is colder than the air’s dew point 24.
- Myth You can get RH from the dew point.
- Reality The National Weather Service says RH cannot be determined from the dew point alone; the air temperature must also be known 1.
- Myth Dew point only matters outdoors.
- Reality Indoor dew point decides whether pipes, windows and cold walls sweat 2.
- Myth The 1°C per 5 percent rule works at any humidity.
- Reality It is intended for moist air above 50 percent RH 3.
How climate sets typical dew points
Dew points vary widely by region and season. The National Weather Service’s Louisville office says typical Ohio Valley summer dew points run from the mid-60s to the mid-70s, which it describes as somewhat to quite uncomfortable 1.
In regions with those summers, cold pipes, ducts and chilled walls are more likely to sweat.
In cold regions in winter, the outdoor dew point is low, and Building Science Corporation notes that interior RH tends to fall as outdoor air gets colder 2, so condensation risk shifts to the coldest surfaces such as windows and exterior walls. Your local weather service publishes dew point forecasts.
When dew point matters to you, and what to do next
Dew point matters when you see water on cold surfaces with no leak, when a basement or closet smells damp, or when a drying job runs longer than expected. Check the room temperature and RH, work out the dew point, and compare with the surface temperature.
If condensation is the cause, lower the humidity or warm the surface. If a surface stays wet at room temperature, suspect a leak and read hidden water damage and water migration. During a restoration job, ask what the air conditions are each day.
The how restoration works guide explains the drying phase, the first 24 hours guide lists safe early steps, and the drying time tool shows why readings end a job. If mold is already present, see mold remediation and choosing a restoration company.
- If Wet only on cold pipes, windows or chilled walls, in humid air
- Then Likely condensation: surface below the dew point
- If Wet spot stays wet in dry, warm air
- Then Suspect a leak or wet material; get a meter reading
- If Wet after you lowered humidity and warmed the surface
- Then Not condensation alone; look for a source
- If Stain on a ceiling under a cold duct or pipe
- Then May be condensation; confirm with a meter and a dry reference
Questions about Dew Point
What is dew point?
Dew point is the temperature to which air must be cooled to be saturated, according to the National Weather Service 1. At that temperature, water vapor starts to condense into liquid on surfaces. It measures the actual moisture in the air and does not change when the room warms or cools.
What is the difference between dew point and humidity?
Relative humidity is a percent of what the air can hold at its current temperature, so it changes as the temperature changes. Dew point is a temperature that stays the same unless moisture is added or removed. The National Weather Service says RH cannot be found from dew point alone 1.
What dew point is comfortable?
The National Weather Service says dew points in the 50s feel comfortable to most people, the 60s somewhat humid and uncomfortable, and the 70s quite uncomfortable and very humid 1. That describes weather comfort, not building safety. Indoors, EPA guides humidity at 30 to 50 percent RH 4.
How do you calculate dew point?
The exact method needs a calculator. A quick estimate comes from Lawrence’s rule: the dew point falls about 1°C for every 5 percent drop in relative humidity below 100, for RH above about 50 percent 3. For example, 70°F air at 50 percent RH has a dew point of about 50°F 2.
Why do my windows or pipes sweat?
The surface is colder than the dew point of the room air. Condensation forms when a surface is at or below the dew point 2. EPA says condensation on windows, walls or pipes is a sign of high humidity and should be dried quickly 4. Lower the humidity or warm the surface.
Is condensation the same as a leak?
No, though they can look alike. Condensation forms on cold surfaces from humid air and eases when humidity drops or the surface warms. A leak keeps wetting a spot regardless. A moisture meter compared with a dry reference helps tell them apart 5. See hidden water damage for what to check.
How does dew point matter in water damage restoration?
A wet building raises the moisture in the air and so its dew point. Cold surfaces then sweat, and drying slows. Restorers record temperature, RH and specific humidity each visit 5 and use dehumidifiers to lower the air’s moisture. Ask what the daily air readings are.
What dew point causes mold?
No dew point is a mold line in the sources used here. Mold needs moisture on a surface.
EPA says that when wet materials dry within 24 to 48 hours, in most cases mold will not grow 4, and condensation that stays on surfaces can supply moisture. Lowering humidity and drying condensation limits the risk.
Can the dew point be higher than the air temperature?
No, for ordinary air. When the dew point equals the air temperature, the air is saturated and RH is 100 percent 1. Cool the air further and vapor condenses, which removes moisture and lowers the dew point toward the new temperature.
Sources
- National Weather Service (Louisville), Humidity and dew point explained, retrieved .
- Building Science Corporation, BSD-163: Controlling Cold-Weather Condensation Using Insulation (Straube, 2011), 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 .
- 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 .
- University of Nebraska-Lincoln Extension, G1832 Identifying Moisture and Mold Sources in Houses (Niemeyer, 2008), retrieved .
- U.S. Department of Energy, Building Science Education: Common Signs of Moisture Problems, retrieved .
- Building Science Corporation, Heat, Air and Moisture Movement (Building Science Fundamentals, Lstiburek and Straube, 2012), retrieved .
- J.S. Held, Perspectives: Infrared Testing, 2nd Edition (2022), 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, the saturation relationship, the statement that RH cannot be found from dew point alone and the comfort scale come from the National Weather Service’s humidity page.
The condensation rule and the 70°F, 50 percent RH example come from Building Science Corporation’s BSD-163 (Straube, 2011). The rule of thumb and the Magnus coefficients come from Lawrence (2005), read from the PDF.
The comparison table and the surface check are our own arithmetic with those coefficients, labelled as calculated and rounded. The mold-cleanup protective gear and the no-bleach statement come from CDC and EPA. No dollar figures are given.
Suggest a correction. We fix errors and say what changed.