www.epa.gov/iaq/moisture
• Water vapor is adsorbed onto surfaces. Water as a
gas moves around very freely. Water adsorbed onto
a solid surface is far less free to move around than
water vapor. In this state, it takes more energy to
break the water free than if it is a liquid or a gas.
Water molecules clinging to a solid surface are less
available for chemical or biological activity than is
liquid water.
area, the greater the evaporation rate. It takes more
energy to evaporate water from porous materials
than from impermeable materials because the water
molecules are more tightly bound by capillary forces
and it is difficult to blow dry, ventilating air through
many porous building materials.
• Water vapor condenses on a surface, becoming
liquid. If surface temperatures are below the dew
point of the air next to them, water molecules in the
surrounding air will condense on the cool surfaces.
Cold water pipes, air conditioning ducts and cold
roof decks experience condensation, just like a cold
drink sweats in the humid summer air.
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