Building Envelope – Documented Causes and Effect of Unintended Moisture Intrusion



With increasing rain, rising temperatures and humidity in the forecast for the United States, many aspects of building design, construction, maintenance and operation may be impacted by the increased levels of moisture. These aspects can also affect the health and comfort of the people in the building.

Moisture intrusion is most prevalent during a region’s “rainy” season. The effects of moisture intrusion are typically magnified during high humidity spans. In 2013 and 2014, the United States experienced increased levels of rainfall of 2 inches above the 20th century average. In fact, the U.S. Army Corps of Engineers is so concerned about the effects of moisture and air intrusion that since 2009 they have required Building Air Barrier Pressurization Testing on all new buildings and those undergoing major renovations.

Controlling moisture flow in a building has significant impacts on occupant health and safety, comfort, building system durability and energy efficiency. Most all unintended moisture flow into the building is through the building envelope. In technical terms the building envelope is defined as “the physical separators between the conditioned and unconditioned environment of a building including resistance to air, water, heat, light and noise”. In simple terms the building envelope is the roof, bottom floor slab and exterior walls as well as window systems.

Four moisture transport vehicles create avenues and allow moisture flow through the building envelope; bulk water movement, capillary action, air transported moisture and diffused air flow. Bulk moisture movement, also known as liquid (rain, snow, ground water) flow, has the potential to be immediate and the most damaging. Three conditions are required to allow bulk moisture flow into a building; a source of water, an opening or crack in the building envelope to allow direct action and a driving force such as gravity or pressure. Capillary action refers to the ability of water to travel up against the pull of gravity through a porous material. Although not as serious as bulk water movement, capillary forces are both destructive and secretive, since they often work and accumulate in commonly unobserved areas potentially causing significant damage over a period of time to a building without the occupant’s knowledge. Air-transported moisture (water vapor hitchhiking on air) can leak into, or out of buildings. Both uncontrolled pressure sources (wind or stack effect) and controlled sources (fans and air handlers) can move significant amounts of moist air past a building’s envelope through openings and improperly flashed transitions. In high humidity regions many experts believe air-transported moisture is the leading cause of moisture related problems in buildings. A process known as diffused air flow is small amounts of moisture in the form of water vapor that passes directly through a building’s envelope. The amount of diffused air flow (vapor) that occurs in a building is determined by two things: the driving force that pushes it and the permeability of the building material the vapor is passing through.

Moisture is not often thought of in terms of occupant health and safety. Yet the majority of indoor air quality professionals consider moisture to be the primary “agent or pollutant” that can have a significant impact on the occupants health. Moisture is the key ingredient for mold and bacteria growth. Not only are many of these fungi odorous, unsightly, and a cause of wood and building material deterioration, but they can also cause asthma and allergic reactions in many individuals. There are even well-documented cases linking certain airborne fungi to more serious health problems, including cancer, birth defects and immune system suppression. Instances of “sick building syndrome” are often related to mold growth in buildings, with unintended moisture intrusion being the major catalyst.

Excess moisture, particularly in the air, also provides a favorable environment to promote asthma and allergy problems. Since moisture, in the form of relative humidity, plays such a key role in how we perceive comfort, it is a primary driving force in determining how to operate building systems.

Moisture can attack a building’s durability on many fronts, from wet wall cavities and crawlspaces to leaking roofs. Moisture-rich air can even become trapped in building structural assemblies, leading to mold growth and deterioration. Building insulations are also adversely effected by moisture, absorption of only 20% moisture can cause up to 55% loss of their insulation value.

To assist our clients PSI provides specific testing, inspection and consulting services for new construction and major renovations to verify that Building Envelope systems have been installed correctly to prevent unintended consequences of moisture intrusion. We also utilize these same services for investigation of existing buildings. For more information, contact one of PSI’s Facilities Services offices or the Facilities Services Principal-In-Charge at (407) 304-5560 ext. 287.