Characteristics of H2O

A molecule of water contains two relatively small hydrogen atoms and one relatively large oxygen atom composing the compound with the chemical name H2O. Water is the only common substance that we encounter in all three of the states of matter: solid, liquid, and gas.
Unlike many other substances, the solid (ice) is less dense than the liquid. Liquid water expands when it freezes and if freezing happens while the liquid water is in or near a building material, the ice can damage the material by its expansion and movement.
Water vapor molecules, floating around in the air, are about one-third of the size of other air molecules: nitrogen and oxygen. You can block both liquid water and air, while letting water vapor through using special materials like Tyvek and Gortex, which have pores big enough to pass water vapor but too small to pass air molecules or the clumped liquid water molecules. If a material, like polyethylene sheeting blocks water vapor, it also blocks air and liquid water because of this same size consideration.

Water and Materials

We classify materials as either porous or non-porous to water and water vapor. Porous materials include wood products, insulation, and masonry materials. Non-porous materials, which are impervious to moisture, include glass, hard plastic, steel, and aluminum.
Adsorbsion is when porous materials attract and store water molecules on the surfaces of their pores. The water-vapor molecules cling to walls of the pores. In drier conditions, the pores desorb the water vapor, clinging to their surfaces, and the water vapor exits the material. Many materials such as wood or brick expand and contract with the adsorbsion and desorbsion of water vapor.
If the porous material continues adsorbing water vapor, the pores eventually run out of surface area to hold the vapor molecules on the surface of the pores. Then the water molecules start to clump into a liquid, filling the pores with liquid water.
That’s when the trouble begins. With the wet material, the insects and mold have the liquid water that they need to colonize. The material’s thermal resistance plummets and its temperature drops, which reduces the dew-point. Water freezing to ice can damage the wet material.