Humidity calculator
Dew point calculator — and will it condense?
Get the dew point from temperature and humidity, or the humidity from a dew point. Then check it against the surfaces under a house — ducts, pipes, framing and foundation walls — to see where water will form.
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Optional: typical outdoor air for your zip
Fills in the month's average outdoor temperature and dew point (NOAA 1991–2020 normals) — the air a vented crawl space takes in — and the modelled ground temperature for the wall and pipe rows.
Dew point
63°F
Air at 78°F and 60% RH starts condensing on any surface at or below 63°F.
Feels: Becoming sticky, muggy evenings
Will it condense under the house?
Surface temperatures are typical — measure yours with an infrared thermometer and type them in.
- °F
Condensing · 5°F below the dew point
- °F
Condensing · 3°F below the dew point
- °F
Mould-risk humidity · 84% RH at the surface · 5°F above the dew point · wood heads toward ≈17.6% moisture
- °F
Mould-risk humidity · 97% RH at the surface · 1°F above the dew point
Status lines: condensing at or below the dew point; mould-risk at 80% RH or more at the surface (ASHRAE 160); damp at 60–80%; OK under 60% (EPA). Wood moisture is the long-run equilibrium from the USDA Wood Handbook, not a meter reading.
Short answer
What the dew point tells you
Dew point is the temperature at which air becomes saturated and water starts to condense. Enter air temperature and relative humidity to get it, or temperature and dew point to get RH. Any surface colder than the dew point sweats: at 85°F and 70% RH the dew point is about 74°F, so a 60°F duct will drip.
Will it condense?
Reading the surface check
Generic calculators stop at the dew point. The question that matters in a house is how close each surface is to it.
For each surface the calculator shows the relative humidity the air reaches when it touches that surface, and how many degrees the surface sits above the dew point. At or below the dew point, water forms (frost below 32°F). At 80% or more, the surface is in the range ASHRAE Standard 160 uses as its mould-growth line when it lasts about a month. Under 60% matches EPA's indoor guidance.
Worked example. Summer crawl space air at 78°F and 60% RH looks unremarkable — but its dew point is about 63°F. A 58°F supply duct and a 60°F water line sit below it and sweat. The foundation wall at 66°F stays dry to the touch, yet the air against it is at about 90% RH. Only the 70°F framing is out of the mould range, at about 78%.
The presets are typical values, not your crawl space. An inexpensive infrared thermometer pointed at the duct, a pipe, a joist and the wall gives real numbers in a minute; our guide to testing a crawl space for moisture covers the rest.
- AC supply duct at 58°F: condensing
- Cold water pipe at 60°F: condensing
- Foundation wall at 66°F: 90% RH — mould line
- Floor joist at 70°F: 78% RH
| Surface | Preset | Why |
|---|---|---|
| AC supply duct (cooling season) | 58°F | Air leaving a running AC coil is commonly about 55–60°F; a bare or thinly insulated metal duct sits close to it. |
| Cold water pipe | 60°F | Cold-water lines run near the temperature of the water coming in, which follows ground temperature. |
| Floor joist / subfloor underside | 68°F | Framing under a cooled house sits a little below room temperature; North Carolina field studies measured vented crawl floors around 65–70°F in summer. |
| Foundation wall / soil | 64°F | Masonry and soil follow ground temperature — the coolest large surface in most crawl spaces in summer. |
Typical — measure yours. With a zip code, the wall and pipe rows use the modelled ground temperature instead.
Framing
What humid air does to joists and subfloor
Wood trades moisture with the air around it. Hold the air at one humidity long enough and the wood settles at a matching moisture content.
The USDA Forest Products Laboratory's Wood Handbook gives that equilibrium moisture content for any temperature and humidity. At 70°F, air held at 60% RH settles wood near 11%; at 80% RH, about 16%; at 90% RH, about 20.5%. Decay fungi are generally treated as active once wood stays above roughly 20% — see how wood rot starts.
So the number to watch is not the humidity in the middle of the crawl space but the humidity at the wood, and for weeks rather than hours. That is why the joist row in the calculator shows where the wood is heading. For the targets to aim for, see what crawl space humidity should be.
| Air RH | Wood MC at 60°F | Wood MC at 70°F | Wood MC at 80°F |
|---|---|---|---|
| 50% | 9.4% | 9.2% | 9.1% |
| 60% | 11.1% | 11% | 10.8% |
| 70% | 13.3% | 13.1% | 12.9% |
| 80% | 16.2% | 16% | 15.7% |
| 85% | 18.2% | 18% | 17.7% |
| 90% | 20.7% | 20.5% | 20.2% |
| 95% | 24.1% | 23.9% | 23.6% |
The difference
Dew point vs relative humidity
Relative humidity is how full the air is compared with what it could hold at its current temperature, so it changes whenever the air warms or cools. Dew point measures the water itself and stays put. That is why outdoor air at 85°F and 50% RH reaches about 82% RH once it cools to 70°F in a crawl space, without a drop of water being added — and why comparing dew points, not RH, tells you whether opening vents brings water in. Our vents tool shows that month by month for your zip.
Dew point chart
Dew point table (°F)
Rows are air temperature, columns relative humidity. Computed with the same formula as the calculator, rounded to the nearest degree.
| Air temp | 20% | 30% | 40% | 50% | 60% | 70% | 80% | 90% | 100% |
|---|---|---|---|---|---|---|---|---|---|
| 40°F | 2° | 11° | 18° | 23° | 27° | 31° | 34° | 37° | 40° |
| 45°F | 6° | 15° | 22° | 27° | 32° | 36° | 39° | 42° | 45° |
| 50°F | 10° | 20° | 27° | 32° | 37° | 41° | 44° | 47° | 50° |
| 55°F | 15° | 24° | 31° | 37° | 41° | 45° | 49° | 52° | 55° |
| 60°F | 19° | 28° | 36° | 41° | 46° | 50° | 54° | 57° | 60° |
| 65°F | 23° | 33° | 40° | 46° | 51° | 55° | 59° | 62° | 65° |
| 70°F | 27° | 37° | 45° | 51° | 55° | 60° | 64° | 67° | 70° |
| 75°F | 31° | 41° | 49° | 55° | 60° | 65° | 68° | 72° | 75° |
| 80°F | 35° | 46° | 54° | 60° | 65° | 69° | 73° | 77° | 80° |
| 85°F | 39° | 50° | 58° | 64° | 70° | 74° | 78° | 82° | 85° |
| 90°F | 44° | 54° | 62° | 69° | 74° | 79° | 83° | 87° | 90° |
| 95°F | 48° | 59° | 67° | 73° | 79° | 84° | 88° | 92° | 95° |
| 100°F | 52° | 63° | 71° | 78° | 84° | 88° | 93° | 97° | 100° |
| Dew point | How it feels |
|---|---|
| 55°F or lower | Dry and comfortable |
| 56–64°F | Becoming sticky, muggy evenings |
| 65°F or higher | Oppressive — lots of moisture in the air |
Method
The formula and how accurate it is
The calculator uses the Magnus approximation with the Alduchov & Eskridge (1996) coefficients: saturation vapour pressure es(T) = 6.1094·exp(17.625·T / (T + 243.04)) hPa, with T in °C. Dew point inverts it: Td = 243.04·γ / (17.625 − γ), where γ = ln(RH/100) + 17.625·T / (T + 243.04). RH from a dew point is the ratio es(Td) / es(T).
Alduchov and Eskridge report this form accurate to within about 0.35°C between −40°C and 50°C — far finer than any household hygrometer, which is typically good to a few percent RH. It is the same engine behind our crawl space vents tool. Wood moisture uses the Wood Handbook's Equation 4–5, which reproduces its published table to 0.1%.
Limits
What this calculator does not tell you
A moment, not a season
Crawl space conditions swing hour to hour. One reading is a snapshot; a logging hygrometer over a few weeks of humid weather is the real test.
Surface temperatures are estimates
Unless you measure them. A duct with good insulation, or one that only runs at night, behaves very differently from the preset.
Not the source
It shows where water condenses, not where the moisture comes from — bare soil, leaks, rain entry or open vents.
Not a mould test
Humidity at a surface says conditions are favourable. Whether growth is present needs a look, and colour cannot identify it.
Next steps
What to do with the result
Ducts or pipes condensing
Duct and pipe insulation, air sealing and humidity control — what each fixes.
Open80%+ at the framing
Size a dehumidifier for the space once water sources are dealt with.
OpenVented crawl space?
See which months open vents add moisture at your zip.
OpenHigh crawl space humidity
Where the moisture usually comes from, and what to fix first.
OpenCrawl space dehumidifiers
How they are installed and drained, and when one is the right tool.
OpenTarget humidity
The range to aim for and how to measure it without fooling yourself.
OpenFAQs
Dew point and humidity: common questions
What is a comfortable dew point?
The National Weather Service's summer guide: a dew point of 55°F or lower feels dry and comfortable, 55–65°F feels sticky with muggy evenings, and 65°F or higher is oppressive. Relative humidity alone can't tell you this, because the same RH feels very different at 60°F and at 90°F.
Is relative humidity or dew point a better measure of moisture?
Dew point. It tracks the actual amount of water vapour in the air and does not change when the air warms or cools. Relative humidity does: the same air reads a lower RH in a warm room and a higher RH in a cool crawl space. Use dew point to compare outside, inside and crawl space air.
What dew point causes condensation?
Any surface colder than the air's dew point. If the air has a 63°F dew point, a 58°F duct or a 60°F water pipe will collect water, while a 70°F joist will not — though the air touching it will be near 80% relative humidity.
Why do ducts sweat in a crawl space in summer?
Air leaving a running air conditioner is commonly about 55–60°F, so a bare or thinly insulated supply duct sits near that temperature. Humid summer crawl space air — often with a dew point in the 60s or 70s in the South — is above it, so water condenses on the duct and drips onto insulation and soil.
How humid does a crawl space have to be before the wood gets wet enough to rot?
Sustained, not momentary. The USDA Wood Handbook's equilibrium moisture table puts wood at about 16% moisture content when the air at its surface sits at 80% RH and about 20.5% at 90% RH (at 70°F). Decay fungi are generally active above roughly 20%, so weeks of air near 90% at the framing are the warning sign; a single humid afternoon is not.
Sources
Sources
- Alduchov, O.A. and Eskridge, R.E. (1996). Improved Magnus form approximation of saturation vapor pressure. Journal of Applied Meteorology 35: 601–609.
- Glass, S.V. and Zelinka, S.L. (2021). Chapter 4: Moisture relations and physical properties of wood. Wood Handbook, FPL-GTR-282, USDA Forest Products Laboratory — Equation 4–5 and Table 4–2
- National Weather Service (La Crosse) — Dew Point vs Humidity (summer comfort guide)
- Glass et al. (2017), Analysis of improved criteria for mold growth in ASHRAE Standard 160 — the 80% surface humidity criterion
- US EPA — A Brief Guide to Mold, Moisture and Your Home (keep indoor humidity below 60%)
- Energy Vanguard — Why do air conditioning vents sweat? (supply air about 55–60°F)
- Advanced Energy — Closed crawl spaces field study, North Carolina (summer crawl surface temperatures)
- NOAA NCEI — U.S. Climate Normals 1991–2020, hourly (the zip pre-fill)
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