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Guide

Dew point at home: the number that follows the moisture

Relative humidity changes when air temperature changes, even if the amount of water vapor stays similar. Dew point is useful because it tracks the vapor level more directly. If a bedroom cools overnight, RH can climb while dew point moves much less.

The practical question

Do not ask only “Is 60% RH bad?” Ask: what is the dew point, and how cold are the coldest surfaces? A room can have moderate RH and still condense on a very cold window edge. Another room can show higher RH but remain safely above dew point because every surface is warmer.

Example: 20°C at 60% RH has a dew point of roughly 12°C. A surface around 12°C is at the condensation threshold for those air conditions.
Quick calculation

Dew-point and surface-margin check

Enter room temperature and RH. Add a measured cold-surface temperature if you have one; otherwise the result still tells you the room dew point.

°C
% RH
°C — glass, wall, pipe, frame

Dew point describes the air. Condensation still requires a surface cold enough to reach that dew point. Use the full Lab for estimated surfaces, room-RH ceilings and cold-snap breakpoints.

Reference examples from the same formula

Room temperature40% RH50% RH60% RH70% RH
18°C4.2°C7.4°C10.1°C12.4°C
20°C6.0°C9.3°C12.0°C14.4°C
22°C7.8°C11.1°C13.9°C16.3°C
24°C9.6°C12.9°C15.8°C18.2°C

Values are rounded Magnus-formula outputs, not comfort targets. A 14°C dew point means something different if your coldest surface is 18°C versus 12°C.

Use trends, not one snapshot

Morning bedroom readings, post-shower bathroom readings and a cool basement can each represent different moisture sources. Measure at similar times for several days. If the same room repeatedly has the highest dew point or the smallest surface-temperature margin, that is more informative than one isolated spike.

What lowering RH changes

At the same room temperature, lowering RH lowers dew point. That increases the temperature margin between room vapor and a cold surface. The Lab's what-if control is designed to show this change directly instead of giving a generic humidity target.

A simple two-room test

  1. Measure temperature and RH in the room that worries you.
  2. Take the same readings in a comparison room within a few minutes.
  3. Compare dew point first, then compare the coldest surface in each room.

If RH is different but dew point is similar, temperature explains more of the percentage difference. If dew point is also higher, the room contains more water vapor.

For general indoor moisture control, see the EPA and WHO sources used by this site.

Run the numbers for your room

Use the whole-home tool to compare this room with the rest of the house and test a lower RH or different room temperature.

Open Home Moisture Lab