Condensation occurs when water vapor in the air contacts a surface cold enough to cause that vapor to convert into liquid water, a physical process that can happen visibly on windows during cold weather or, more concerning, invisibly within wall or attic cavities where it can lead to hidden moisture damage over time.
This process happens whenever air reaches its dew point temperature, the specific temperature at which the air can no longer hold its current moisture content in vapor form, causing that excess moisture to condense onto whatever surface triggered the temperature drop.
Within building assemblies, condensation often occurs at thermal bridges or other localized cold spots, where the surface temperature drops below the surrounding air’s dew point even though the broader assembly might otherwise remain above this threshold.
What Causes Condensation?
- Warm, moisture-laden air contacting a cold surface
- Inadequate insulation creating cold surface temperatures
- Thermal bridges creating localized cold spots
- High indoor humidity levels increasing dew point risk
Why Is Condensation Important?
Hidden condensation within wall or attic assemblies can lead to mold growth, wood decay, and insulation performance degradation over time, making this a genuinely serious building durability concern beyond the more visible, easily addressed condensation that appears on window glass. This hidden risk is why building science places significant emphasis on proper vapor control and insulation design to prevent condensation within concealed assemblies.
Managing indoor humidity levels, ensuring adequate insulation to keep interior-facing surfaces above the dew point, and proper vapor control strategy all work together to prevent the condensation-related damage that inadequate attention to any one of these factors could otherwise allow.
Condensation vs. Dew Point
Condensation is the actual physical process of vapor converting to liquid water. Dew point is the specific temperature threshold at which this conversion occurs for a given air moisture content, the trigger point condensation depends on.