Besides preventing water infiltration, waterproofing systems prevent structural damage to building components. In northern climates, watertightness prevents spalling of concrete, masonry, or stone due to freeze–thaw cycles. Watertightness also prevents rusting and deterioration of structural or reinforcing steel encased in exterior concrete or behind facade materials.

Waterproofing also prevents the passage of pollutants that cause steel deterioration and concrete spalling, such as chloride ions (salts, including road salts used for deicing) into structural components. This is especially true in horizontal exposed areas such as balcony decks and parking garages. Prevention of acid rain contamination (sulfites mixed with water to form sulfuric acid) and carbon acids (vehicle exhaust—carbon dioxide that forms carbonic acid when mixed with water) is also an important consideration when choosing proper waterproofing applications.

Building envelopes also provide energy savings and environmental control by acting as weather barriers against wind, cold, and heat. Additionally, envelopes must be resistant to wind loading and wind infiltration. These forces, in combination with water, can multiply the magnitude of damage to a structure and its interior contents. Direct wind load pressure can force water deeper into a structure through cracks or crevices where water might not normally penetrate. It also creates vertical upward movement of water (hydrostatic pressure) over windowsills and through vents and louvers. Air pressure differentials due to wind conditions may cause water that is present to be sucked into a structure because of the negative pressure in interior areas.

This situation occurs when outside air pressure is greater than interior air pressure. It also occurs through a churning effect, where cool air is pulled into lower portions of a building, replacing warmer air that rises and escapes through higher areas. To prevent this forced water infiltration and associated energy loss, a building envelope must be resistant and weather-tight against wind as well.

Finally, and possibly most important, health issues of building occupants are now directly related to the success of a properly design and constructed building envelope. All types of mold require the presence of moisture for formation and growth. This moisture is almost always the result of leakage attributable to improperly designed and/or constructed building envelopes. Since mold can cause numerous health problems, this may be the most important issue necessitating a proper understanding of the building envelope and the 90%/1% and 99% principles presented throughout this part.

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