17 May

What is the Dew Point?

The temperature at which the water vapor contained in a volume of air at a given atmospheric pressure reaches saturation and condenses to form dew. The dew point varies depending on how much water vapor the air contains, with humid air having a higher dew point than dry air.  If the dew point temperature is reached within the wall, condensation occurs, wetting the walls resulting in insulation ineffectiveness and deterioration of moisture sensitive materials.

Because conditions on the interior of a building are typically different from the exterior conditions, temperature and saturated and partial vapor pressure gradients across the wall from the interior to the exterior will occur.  The dew points within a wall happen most often where the temperature drop is the greatest and thus occurs across materials that have good insulating properties.

The proper placement of the air/vapor/water resistive barrier and the exterior insulation is critical in keeping the wall assembly above the dew point. 

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