Relative Humidity vs. Dew Point

The dew point is determined solely by the actual amount of water vapor in the air (the vapor pressure), not by temperature. So, for a given air mass, the dew point remains the same regardless of whether the air warms up or cools down.

• Relative humidity (RH): depends on both temperature and dew point.
• Dew point: depends only on the absolute moisture content.

Ventilated Attic vs Ambient Air

In a ventilated attic:

• The attic air is drawn from the outside (ambient air), so the moisture content is nearly the same as outdoors.
• As the attic heats up above ambient (because of solar heating of the roof), the dew point stays the same as the outside air.
• What changes is the relative humidity, which drops as temperature rises.

For example:

• Outside: 90 °F, 60% RH → dew point ~74 °F.
• Attic: 105 °F (same outside air, just heated up) → dew point still ~74 °F, but RH would be less than 40%.