Air and Moisture Control in Buildings
Air and moisture control in buildings is a fundamental function for any commercial project involving conditioned space. Unfortunately moisture problems are common in many buildings and people often consider them inevitable. Moisture accumulation is the scourge of buildings throughout the United States, from tropical Hawaii to arctic Alaska and from the hot, humid Gulf Coast to the hot, dry Sonoran Desert. Between 1994 and 1998, the U.S. Environmental Protection Agency (EPA) Building Assessment Survey and Evaluation(BASE) gathered information about the indoor air quality of over 100 public and private office buildings in the 10 US climatic regions. The study found that 85 percent of the buildings had been damaged by water at some time and 45 percent had leaks at the time the data were collected. It is clearly confirmed that moisture causes problems for building owners, maintenance personnel and occupants.
Many common moisture problems can be traced to poorly designed construction or maintenance. The American Society of Heating, Refrigerating and Air Conditioning Engineers (ASHRAE) notes that more often than not, the more serious problems are caused by decisions made by members of a number of different professions. However, such problems can be avoided with techniques that are based on a solid understanding of how water behaves in buildings.
Ventilation as a control is intended to remove or dilute pollutants and to control the humidity and thermal environment in buildings. It is essential to ventilate so pollutants and humidity concentrations are diluted to acceptable levels for the health and comfort of the occupants, and must be sufficient to maintain the building’s integrity. A number of reviews by Seppänen, Fisk, Mendell, Wargocki et al. and Sundell, Levin, show an association between ventilation and health, although the specific exposure and response relationships differ with each individual study.
Exact values for ventilation are difficult to identify. A limit on values have not been set for the pollutants as it is seldom possible to determine the necessary ventilation rates as well as the associated risks on the basis of the concentrations of pollutants. The selection of ventilation rates is based on research epidemiology, odor perception, laboratory and field experiments, irritation, productivity, as well as occupant preferences and experience and building design.
There are multiple options available to builders to provide ventilation using natural and mechanical methods. These can improve health, however they can also have adverse effects, according to Fisk Seppänen, if not properly installed, designed, maintained, and operated because ventilation may allow the entry of harmful substances that degrade the indoor environment.
Ventilation affects moisture and air flow through the building envelope and may lead to moisture problems that degrade the structure. Ventilation changes the pressure differences throughout the building and can cause or prevent the infiltration of pollutants from adjacent spaces. While ventilation is used to control humidity under certain circumstances, it may result in very high or very low humidity. Humidity is another critical issue when considering limiting the use of outdoor air ventilation.
In non-residential buildings and in very hot climates, the ventilation is sometimes integrated with air-conditioning, which then has an impact on the operation of these systems. The addition of humidifiers to ventilation systems or as stand-alone units can introduce excess humidity or microorganisms that grow on components in humid locations, such as drip pans in air-conditioning units or humidifiers.
Principles for Design
To control moisture for good building life and clean indoor air quality:
- Prevent extreme indoor humidity and water vapor migration by air flow and diffusion in order to limit condensation on and moisture absorption into cool materials and surfaces.
- Control liquid water.
- Select moisture-resistant materials for unavoidably wet locations.
Additionally, using a high-performance product like DELTA®-VENT SA, a self-adhesive water-resistive and air barrier can improve the performance of the building enclosure with increased air-tightness. DELTA®-VENT SA is vapor permeable, allowing moisture within the building enclosure to escape, preventing excess moisture buildup.
A good elementary understanding of these principles will help builders be prepared to control moisture and prevent the vast majority of moisture problems that are common in buildings.