Dew point and condensation on basement walls
Short answer
The dew point is the temperature at which the moisture in the air condenses into water. If a surface is colder than the dew point of the surrounding air, it will get wet. That’s condensation. In basements it usually happens in the summer months, when humid air meets cold concrete and cold-water lines.
On this page
Dew point calculator
Enter your hygrometer reading. Add a surface temperature to check whether a wall, pipe or joist will get wet.
Dew point
The EPA recommends keeping indoor humidity below 60%, ideally between 30% and 50%. See how to size and drain a basement dehumidifier.
This calculator doesn’t diagnose mold, leaks or anything structural. The result is only as good as your hygrometer, which typically has a margin of a few percent. Nothing you enter is stored or sent.
How the calculation works
The dew point comes from the Magnus approximation with the Alduchov and Eskridge coefficients (a = 17.625, b = 243.04 °C), as recommended by Lawrence (2005). With a surface temperature, the calculator works out the relative humidity right at that surface. At 100% the surface condenses; from 80% to 100% it flags the surface to watch. The 80% line was the simplified mold criterion in ASHRAE Standard 160 until a 2016 addendum replaced it with a mold growth model, so treat it as a warning, not a diagnosis. Results are rounded to the nearest degree, because home hygrometers aren’t more precise than that. It covers air and surfaces between 32°F and 110°F (0°C to 43°C); below freezing, water deposits as frost and needs a different formula.
Condensation or a leak?
I look at when it’s happening. If it happens after it rains, I look at exterior water first. If it shows up on a warm, humid day on a cold surface, I look at condensation. Condensation usually appears fairly evenly on cold-water lines, concrete and metal, and on some lower areas of the foundation wall. A leak usually leaves a path: a crack, a joint, a corner, efflorescence or a stain.
You can tape a piece of plastic to the wall. Make sure it’s taped tight on all sides, and leave it in place. If moisture shows up on the side facing the room, you are most likely condensing your indoor air. If it shows up behind the plastic, against the wall, you have moisture coming through the wall. I would not use this as my only way of diagnosing the problem.
| Clue | Points to condensation | Points to a leak |
|---|---|---|
| When it happens | Warm, humid days | After rain |
| What it looks like | Fairly even film or droplets | A path from a crack, joint or corner |
| Where | Cold-water pipes, concrete, metal, low parts of the wall | Cracks, joints, corners, often with efflorescence or stains |
| Plastic sheet test | Moisture on the room side | Moisture behind the plastic |
Where and when basements sweat
Summer, clearly. Humid air gets into the basement and comes in contact with cold surfaces. I see it a lot on cold-water lines, foundation walls, metal ductwork, concrete floors, toilet tanks, and behind furniture where the air doesn’t move.
What works
- Keep the windows closed in summer
- Control the humidity, usually with a dehumidifier
- Seal air leaks
- Insulate cold-water pipes
- Insulate cold surfaces properly where it makes sense
I don’t like the suggestion of opening your basement windows to let it “breathe.” On a 90°F, humid day in Virginia, you can make the problem a lot worse. Waterproofing paint will not solve a condensation problem, and it won’t solve a serious leak either. You may not see the water for a while, but that doesn’t mean you solved what’s going on behind it.
How I check it
I don’t always run the numbers, but I compare the indoor temperature and humidity with the surface temperature. I use an infrared thermometer, and sometimes a thermal camera to find cold spots, and then I verify with other instruments. If the surface is below the dew point, I have a pretty good explanation for why it’s getting wet.
How the dew point calculator works
- Formula: the Magnus approximation with the coefficients of Alduchov and Eskridge (a = 17.625, b = 243.04 °C), as recommended by Lawrence (2005).
- Inputs: air temperature and relative humidity from your hygrometer, plus the surface temperature if you measure it (an infrared thermometer works).
- Results: the dew point, rounded to the nearest degree, and, if you entered a surface temperature, the relative humidity right at that surface.
- Condensation: the surface is at or below the dew point (100% at the surface). It will get wet.
- Watch: 80% to 100% at the surface. It’s a warning, not a diagnosis: 80% averaged over 30 days was the simplified mold criterion in ASHRAE Standard 160 until a 2016 addendum added a more detailed mold growth model.
- No condensation: below 80% at the surface.
- Accuracy: the result is limited by your hygrometer much more than by the formula.
Sources
- Lawrence, M. G. (2005). “The Relationship between Relative Humidity and the Dewpoint Temperature in Moist Air.” Bulletin of the American Meteorological Society, 86(2), 225–233.
- ASHRAE, Addendum e to Standard 160-2009 (approved November 30, 2016): replaces the flat 80% 30-day surface RH criterion with a mold growth model
Diagnosis advice comes from Robert Stulpt’s own inspections in Virginia.