The dew point is the temperature at which air can no longer hold its water as vapour. Cool any air to its dew point and the surplus condenses out as liquid — on grass, on a cold glass, on your window, on a crawl space joist.
It is the single most useful number in damp control, because unlike relative humidity it tells you something absolute. Relative humidity says how full the air is at its current temperature. Dew point says how much water is actually in it. Two rooms can both read 60% RH and hold completely different amounts of water; two rooms with the same dew point hold the same amount, full stop.
The rule you actually use
Any surface colder than the dew point of the air touching it will get wet. That is the whole thing.
So: measure the room’s temperature and relative humidity, read the dew point off the table below, and compare it against the coldest surfaces in the room. Window glass, the wall behind a wardrobe on an external corner, a cold-water pipe, an uninsulated floor above a crawl space. Whichever of those sits below the dew point is where the damp appears.
Dew point table
| Air temp | 30% RH | 40% RH | 50% RH | 60% RH | 70% RH | 80% RH | 90% RH |
|---|---|---|---|---|---|---|---|
| 50°F (10°C) | 19°F | 26°F | 32°F | 36°F | 41°F | 44°F | 47°F |
| 60°F (16°C) | 27°F | 35°F | 41°F | 46°F | 50°F | 54°F | 57°F |
| 68°F (20°C) | 35°F | 43°F | 49°F | 54°F | 58°F | 62°F | 65°F |
| 77°F (25°C) | 43°F | 51°F | 57°F | 62°F | 67°F | 70°F | 74°F |
| 86°F (30°C) | 51°F | 59°F | 65°F | 71°F | 75°F | 79°F | 83°F |
Two patterns are worth noticing. Reading across a row, the dew point climbs steeply with humidity — at 68°F, going from 40% to 70% RH raises it by 15°F, which is the difference between a dry window and a wet one. Reading down a column, warmer air at the same relative humidity has a much higher dew point, because warm air holds far more water to begin with.
Why warm air holds more water
The capacity is not linear. It roughly doubles for every 20°F of warming.
| Air temperature | Maximum water it can hold |
|---|---|
| 32°F (0°C) | 4.8 g/m³ |
| 50°F (10°C) | 9.4 g/m³ |
| 68°F (20°C) | 17.3 g/m³ |
| 86°F (30°C) | 30.3 g/m³ |
This is why a cold winter’s day with 90% outdoor humidity brings dry air into your house. Outdoor air at 32°F and 90% RH carries about 4.3 g/m³. Warm that same air to 68°F indoors and its capacity jumps to 17.3, so its relative humidity collapses to around 25%. Nothing was removed — the air simply got roomier.
Ventilating in winter works for exactly this reason. Ventilating in summer often does the opposite.
Three problems this explains
1. Opening crawl space vents in July makes it wetter
Summer outdoor air at 85°F and 75% RH has a dew point around 76°F. The ground holds crawl space surfaces near 68°F all year. Push that outdoor air in and every joist, duct and square foot of soil is below the dew point — so it condenses, and the space ends up wetter than before you opened the vents. Our crawl space guide covers what to do instead.
2. The window sweats but the wall does not
Glass is the coldest surface in the room, and the edge of a double-glazed unit is colder than its centre. At 68°F and 60% RH your dew point is 54°F: a plastered internal wall at 66°F is fine, the pane’s edge at 50°F is not. Same air, different surfaces — see black mold on window sills.
3. A dehumidifier fixes it and a fan does not
A fan moves air around; it does not remove water, so the dew point is unchanged and the cold surface still gets wet. A dehumidifier physically pulls water out of the air, which lowers the dew point directly. Dropping a 68°F room from 60% to 45% RH takes the dew point from 54°F down to about 46°F — an 8°F margin that clears most surfaces in a normal house.
Working out your own dew point
Most modern hygrometers display dew point directly; check the display modes before assuming yours does not. Failing that, the table above covers normal indoor conditions.
For a rough estimate between rows, a useful approximation near room temperature: every 5% drop in relative humidity lowers the dew point by about 1.5°F. It holds well enough between 40% and 80% RH to be worth remembering.
To measure surface temperature, an infrared thermometer costs very little and is the fastest way to find the cold spot in a room. Point it at the middle of the glass, the edge of the glass, the corner behind the wardrobe, the wall behind the bed — then compare each reading against your dew point. Anything below it is where the mold will be.
How comfortable a given dew point feels
| Dew point | How it feels |
|---|---|
| Below 50°F | Dry and pleasant |
| 50–60°F | Comfortable |
| 60–65°F | Noticeably sticky |
| 65–70°F | Uncomfortable, muggy |
| Above 70°F | Oppressive; sweat barely evaporates |
This is why a 77°F day at 50% RH (dew point 57°F) feels fine and a 77°F day at 80% RH (dew point 70°F) feels awful. Indoors, aim to keep the dew point below about 55°F in a heated home and you will avoid nearly all condensation problems.
Frequently asked questions
What is the difference between dew point and relative humidity?
Relative humidity is a percentage of the air’s capacity at its current temperature, so it changes when the temperature changes even if no water is added or removed. Dew point is a temperature, and it only changes when the actual water content changes. That makes dew point the better number for comparing two rooms or predicting condensation.
What is a good indoor dew point?
Below about 55°F in a heated home. At a normal room temperature of 68°F that corresponds to roughly 40–60% relative humidity. Below 50°F is drier still and safe for almost any surface, including single glazing on a cold night.
Can the dew point be higher than the air temperature?
No. At most it equals the air temperature, which is 100% relative humidity — the air is saturated and fog or condensation forms. It cannot exceed it.
Does lowering the temperature lower the dew point?
Not by itself. Cooling air raises its relative humidity while leaving the dew point unchanged — until you reach the dew point, at which stage water condenses out and the dew point does then fall, because water has physically left the air. That is exactly how a refrigerant dehumidifier and an air conditioner both work.
How do I calculate dew point from temperature and humidity?
The standard method is the Magnus formula, which the table above uses. For everyday purposes the table is accurate to within a degree, and the shortcut of 1.5°F per 5% of relative humidity gets you between rows.