Dew Point and Relative Humidity Calculator for Records Rooms

The dew point is the temperature at which the air in a room would be saturated and start leaving water on anything that cold. Relative humidity tells you how close the air is to that point at its current temperature. The calculator above takes your hygrometer's two readings and returns the dew point, a plain reading against common storage guidance, and the relative humidity the same air reaches against something colder, such as a foundation wall or a slab floor. That last number is the one I wish somebody had shown me before my first summer in a basement vault.

How to use it

1. Read the temperature and relative humidity at shelf height, near the records, after the instrument has sat in place for an hour. Pick °F or °C.
2. Enter both. You get the dew point in both scales and a reading: under 30 percent is dry, 30 to 50 percent is within the common target, 50 to 60 percent is damp, and over 60 percent is mold risk.
3. If you can, enter the temperature of the coldest surface the records touch or sit near, read with an infrared thermometer or a probe taped to the wall for an hour. The calculator returns the relative humidity in the thin layer of air against that surface.

Dew point vs relative humidity

Relative humidity is relative to temperature. Cool a room without adding a drop of water and its relative humidity goes up; warm it and the number falls. The dew point does not move when you heat or cool air, only when you add or remove moisture. That makes it the honest number for comparing two rooms, morning and afternoon, or the vault and the parking lot.

Paper responds to relative humidity: paper, board, leather and photographic gelatin take up or give off moisture to match the air touching them. The dew point tells you how much water is in the room; the relative humidity at the record tells you what the record is living in.

Why cold walls and basements matter for records

A basement wall sits against ground that stays cool all summer. An outside wall upstairs goes cold every winter. The air right next to either one cools to the surface temperature, and its relative humidity climbs. A carton pushed flat against a foundation wall lives in that layer, not in the middle of the room where the logger is.

That is why mold in records rooms tends to start behind shelving, in corners and on the bottoms of boxes stored on a slab, while the instrument on the desk reads a comfortable 50 percent. Pull shelving a few inches off exterior walls and keep the lowest shelf off the floor; the storage equipment guide for permanent records covers shelving.

Worked example

The calculator opens with these values: a room at 68 °F (20 °C) and 50 percent relative humidity.

• Dew point: 48.7 °F (9.3 °C).
• Reading: within the common 30 to 50 percent target, at its upper edge.
• The same air against a 55 °F outside wall reaches about 79 percent relative humidity.
• Against a 50 °F wall it reaches about 95 percent, which is condensation risk.

Same room, same air, nothing added. Drying the room to 40 percent lowers the dew point by about six degrees, to roughly 43 °F, and the air against that 55 °F wall falls to about 63 percent. That is the whole case for a dehumidifier in one line.

How it works

The most water vapor air can hold rises steeply with temperature, roughly doubling for every 20 °F across ordinary room temperatures. The Magnus formula describes that curve: saturation vapor pressure in hectopascals equals 6.112 times e raised to the power of 17.62 times the temperature divided by 243.12 plus the temperature, with temperature in degrees Celsius.

To find the dew point, the calculator takes the natural logarithm of the relative humidity (as a fraction), adds 17.62 times the temperature divided by 243.12 plus the temperature, and solves the same expression backward for the temperature at which the air would be saturated. For the surface reading it holds the dew point fixed and asks what relative humidity that moisture represents at the surface temperature.

The constants, 17.62 and 243.12 °C, are the ones published by Sonntag in 1990 and given in the World Meteorological Organization's Guide to Meteorological Instruments and Methods of Observation. At indoor temperatures the formula is far more precise than any hygrometer you will put on a shelf.

Limits

• Your instrument is the weak link. Consumer hygrometers usually state an accuracy of ±2 to 5 percent RH, and they drift with age. At 68 °F, the difference between 45 and 55 percent moves the dew point from about 46 °F to about 51 °F.
• Infrared thermometers misread shiny metal; aim at painted block, plaster or concrete.
• The calculation assumes the air at the wall carries the same moisture as the room. A wall that wicks water adds its own, and then reality is worse than the result.
• The 30 to 50 percent band is common general guidance for mixed paper collections, and the 60 percent line is a conservative warning, not a standard for every material. Photographs, film and magnetic media have their own recommendations; the Northeast Document Conservation Center publishes guidance by material.

For setting up loggers and reading their graphs, see Environmental monitoring for a records vault. If the wall has already won, read Water in the records room: the first 48 hours.

Frequently asked questions

What is the dew point, in plain terms?

It is the temperature at which the air you have would be completely saturated. Any surface at or below that temperature collects water, the way a cold glass does on a summer porch.

Is dew point or relative humidity more useful for a records room?

Use both. The dew point tells you how much moisture is in the room and stays put when the temperature changes. Relative humidity at the records tells you what the paper will absorb.

How far below the coldest surface should the dew point be?

Keep it at least 12 °F (about 7 °C) below the coldest surface the records are near. That holds the air touching that surface to roughly 65 percent relative humidity or less.

Why does the basement feel damp when the hygrometer says 50 percent?

The hygrometer reads the middle of the room. Walls and floors are cooler, so the air against them runs much damper. Enter the wall temperature as the surface value to see by how much.