Dew Point and Condensation Calculator

What the dew point means

Air always carries some water vapour, and warm air can carry more than cold air. The dew point is the temperature at which the air, cooled without adding or removing moisture, becomes saturated. Any surface colder than that pulls water out of the air: the pipe sweats, the duct drips, the window mists up.

Relative humidity alone does not tell you this. Air at 26 °C and 60% RH has a dew point of about 17.6 °C. A chilled water pipe at 7 °C sits ten degrees below it and will drip steadily. The same pipe in a 20 °C room at 50% RH, dew point about 9.3 °C, still sweats, but far less. In a dry winter plant room it may stay completely dry.

How it is calculated

The calculator uses the Magnus formula for saturation vapour pressure over water, which is accurate to well under 0.5 °C in the range that matters for buildings and plants:

From the vapour pressure it also derives absolute humidity (grams of water per cubic metre of air), humidity ratio (grams per kilogram of dry air, the quantity used on psychrometric charts), wet-bulb temperature (Stull's approximation) and specific enthalpy.

Why condensation on pipes is worth stopping

A sweating chilled water line is more than a puddle on the floor. Water soaks into insulation that is not vapour-tight, and wet insulation conducts heat many times better than dry, so the line gets colder on the outside and sweats even more. Under the insulation, carbon steel corrodes quietly for years. Over ceilings, the drips stain finishes and feed mould. In plant rooms, they make floors slippery around electrical equipment.

The fix is insulation with a continuous vapour barrier, thick enough that its outer surface stays above the dew point of the worst air the pipe will see. Size it for the summer peak, not the average: a coastal plant room on a humid afternoon can reach a dew point above 20 °C. You can find the surface temperature of an insulated pipe with the pipe heat loss calculator (enter the chilled water and room temperatures) and check it here.

Practical margins

Keep the surface at least 1 to 2 °C above the dew point. Insulation joints, hangers and valves are colder than the straight run, humidity rises with weather and with people in the room, and sensors drift. If the margin is below 1 °C, expect occasional sweating.

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Frequently asked questions

What is the difference between dew point and relative humidity?

Relative humidity is how close the air is to saturation at its current temperature. Dew point is an absolute measure of how much moisture the air holds, expressed as the temperature at which it would saturate. When the air warms up during the day, relative humidity falls but the dew point stays the same, which is why dew point is the better number for condensation.

Is the calculation valid below 0 °C?

It uses vapour pressure over liquid water. Below freezing, the frost point over ice is slightly higher than the dew point shown here, by up to about 1 to 2 °C at -20 °C. For freezer rooms and outdoor winter conditions, treat the result as approximate.

Why does air pressure matter?

It does not change the dew point, which depends only on the amount of water vapour. It does change the humidity ratio and enthalpy, which are expressed per kilogram of dry air. At high altitude, enter the local pressure for accurate values.

How thick should chilled water pipe insulation be?

Thick enough that the outer surface stays above the dew point of the worst expected air, with a margin. For 6 to 7 °C chilled water in conditioned indoor spaces, 19 to 32 mm of elastomeric foam is common; humid plant rooms and outdoor runs need more. Calculate the surface temperature for your conditions rather than relying on a rule of thumb.