Moisture & The Effect on The Indoor Environment

Sources of Moisture Infiltration

Water can enter a building from numerous external and internal sources, each with its own unique mechanism of infiltration.

Building Envelope Breaches (Liquid Water Entering the Envelope)

The building envelope (the physical separation between the conditioned interior and the unconditioned exterior of a building) is the primary defense against moisture. When this barrier is compromised, water infiltration becomes a significant risk.

• Roof and Wall Penetrations: Damage to roofing materials, loose or missing shingles, or improperly installed flashing (a thin material, often metal, used to direct water away from an object on the roof) can allow rainwater to penetrate. Similarly, failed sealants and caulking around windows, doors, and exterior joints can create entry points for wind-driven rain.

• Below-Grade Infiltration: In areas with poor soil drainage, water buildup around a foundation creates hydrostatic pressure (water pressure pushing in on a foundation), forcing water through cracks. Additionally, capillary action (the movement of water through a porous material like concrete) allows moisture to be drawn from the soil into the foundation and up into interior walls.

• Window and Door Openings: Failed seals and gaskets around windows and doors can allow rain to seep past the frame. Improper installation without a proper draining system, such as a sloped sill or weep holes (small openings designed to let water out), can trap moisture, leading to infiltration.

• Foundation and Slab Breaches: As a building settles, cracks can form in the foundation walls or concrete slab, creating direct conduits for water. A failed perimeter drainage system can also lead to water pooling, exacerbating these issues.

Vapor Pressure Differentials (Water Vapor Entering the Envelope)

Moisture also infiltrates as water vapor, governed by vapor pressure, which is the force exerted by water vapor in the air. This vapor will always move from an area of high pressure to an area of low pressure, a phenomenon known as a vapor pressure differential (the difference in the amount of water vapor in the air between two spaces).

• Hot, Humid Seasons: The high vapor pressure of outdoor air drives moisture into the cooler, drier conditioned space. If this humid air reaches a surface inside the wall cavity that is below the dew point (the temperature at which the air becomes saturated and water vapor condenses into liquid water), it turns into liquid water, causing condensation damage.

• Cold, Dry Seasons: Indoor activities create high vapor pressure inside the building. This drives moisture outward. As the water vapor passes through the cold exterior wall materials, it can condense, creating a hidden moisture problem within the walls.

• It is important to note that both large and small gaps in the building envelope act as pathways for this moisture movement. 

  • Large gaps, or a significant amount of air leakage (uncontrolled movement of air through gaps in the building envelope), can lead to a rapid intrusion of humid air, potentially overwhelming a space's ability to maintain a stable environment. 

  • Small, seemingly insignificant gaps can have a profound effect over time, allowing a slow but continuous stream of moisture to enter a wall cavity, where it can accumulate and lead to long-term degradation and microbial growth. 

Plumbing and HVAC (Liquid Water from Internal Sources of Water)

Internal moisture sources can be as destructive as external ones, often causing hidden damage over time.

• Plumbing Leaks: These can be catastrophic failures (a sudden, large-scale event) like a burst pipe or slow leaks (a small, continuous leak) from aging pipes or faulty seals. Slow leaks can go undetected for weeks, saturating materials behind walls or under floors.

• HVAC System Malfunctions: An HVAC system can become a moisture source if a clogged condensate line or overflowing drain pan spills water into the surrounding ceiling or walls. Condensation (the process of water vapor turning into liquid water) will also form on poorly insulated ducts, dripping and causing damage, given that a source of humidity is present.

• Appliance Failures: Appliances like dishwashers, washing machines, or water heaters can fail, introducing moisture into hidden spaces through leaking tanks or disconnected drain hoses.

Occupant-Generated Moisture (Water Vapor from Internal Sources)

The occupants of a building are a significant source of moisture, as everyday activities generate water vapor that can raise the overall relative humidity (the amount of moisture in the air compared to the maximum amount the air can hold at that temperature).

• Human Respiration and Perspiration: A single person releases a significant amount of water vapor each day through breathing and sweating.

• Cooking, Showering, and Bathing: These activities produce large amounts of steam. Without adequate ventilation from a range hood or bathroom exhaust fan (a fan that pulls air out of a room), this moisture can spread and condense on cold surfaces.

• Other Sources: Indoor plants release moisture through transpiration (the process of plants releasing water vapor into the air), and the improper use of dehumidifiers can raise humidity to damaging levels.

The Impact on Building Materials and Health

Once moisture is present, its effects on the indoor environment can be severe and far-reaching.

• Microbial Growth: Sustained moisture leads to microbial amplification (the rapid growth of microorganisms), particularly mold. Porous materials like drywall and wood become substrates for mold growth, which releases spores and other particles that can impact occupant health.

• Material Degradation: Moisture acts as a catalyst for destructive processes. It accelerates the oxidation (the chemical reaction of a substance with oxygen) of ferrous metals, leading to rust and corrosion. For wood and other cellulose-based materials, a moisture content above 20% makes them susceptible to wood-destroying fungi, leading to rot and the irreversible decomposition of the building component.

• Odor and Aesthetics: Excess moisture can also cause unpleasant, musty odors and cosmetic damage such as peeling paint, bubbling plaster, or warping floors, contributing to a poor indoor environment and devaluing the property.

• Thermal Diffusion: Heat flow is always from a warmer area to a colder area. This is why we insulate walls to resist the movement of heat away from the conditioned indoor space during winter.

  • Read here: Warmer air, moves towards cooler air or surfaces.

• Molecular Diffusion: Moisture flow is always from a wetter, higher concentration area to a drier, lower concentration area.

  • Read here: Moisture moves towards dryer air & surfaces.

• Air Pressure: Airflow is always from a higher-pressure zone to a lower-pressure zone.

  • Read here: Air with higher pressure, moves towards air with lower pressure.

• Gravity: This constant force pulls everything, including water, downward.

  • Read here: Liquid will move horizontal, until a path is exposed to allow it to go vertical down.

When these principles are in conflict, they create the complex dynamics we see in buildings. For instance, the pull of gravity on liquid water is a powerful, unidirectional force, but the vapor pressure differential (the difference in the amount of water vapor in the air between two spaces) can drive moisture through an enclosure even against the pull of gravity. As a rule of thumb, it's critical to remember that "more to less always beats warm to cold." While heat and moisture are both drawn to colder temperatures, the sheer volume or concentration of moisture will always dictate the primary direction of its movement.

Practical Applications and Observations

These foundational principles can be demonstrated with simple, homemade experiments and are directly observable in the field.

• The Wet Towel Analogy: If you place a wet towel next to a dry towel, the dry towel will become wet. The moisture moves from a state of higher concentration (the wet towel) to a state of lower concentration (the dry towel) regardless of their relative temperatures.

• Capillary Action: Moisture can move great distances through a material even without bulk water flow. This capillary action (the movement of a liquid through a narrow space) is driven by surface tension and is more pronounced in materials with smaller pore sizes. To prevent this, a capillary break (a physical barrier that stops the movement of liquid water) is installed in material systems to disrupt the continuous path of water. For example, the ground is, for all practical purposes, always wet. A building's foundation must be designed with a capillary break to prevent moisture from wicking up into the structure from the soil.

• Condensation: When a building has a cold wall or surface, it will be the location that collects water from the air. This happens because the air adjacent to that surface cools to its dew point (the temperature at which the air becomes saturated and water vapor condenses into liquid water), causing the water vapor to turn into a liquid.

• Makeup Air: The ventilation of a building, such as through an exhaust fan, creates a negative pressure inside. To balance this, makeup air (air drawn into a building to replace air being exhausted) is pulled in from outside. This air often comes from undesirable locations like gaps around windows and doors, through dirty crawlspaces, or from attics. This infiltration can bring in contaminants and uncontrolled temperature and moisture levels, creating complex problems with both comfort and air quality.

The Role of Testing and Calculations

Testing and complex calculations are not the starting point of our work. Rather, they are tools used to confirm a hypothesis that has already been formed based on scientific principles. We must first understand the "why" behind a problem—for example, that moisture moves from a wetter to a drier area. Once we have a hypothesis, we can use testing to support our summary and make it clear to stakeholders.

The analogy is that of a doctor: they don't run every test on a patient from the outset. They first consider the patient's symptoms and their knowledge of the human body to form a preliminary diagnosis. They then order specific tests to confirm that diagnosis. Similarly, in building science, we use our foundational knowledge to identify the most likely cause of a problem and then use targeted testing to provide irrefutable evidence.

This approach ensures that we don't engage in what has been described as an "expensive tax on stupid people, unwilling to accept undisputed science." By leveraging our understanding of physics, we can avoid unnecessary and costly testing, providing clear, concise, and scientifically sound solutions.

Science is not about proving something, but about being unable to currently disprove it. The evolution of scientific understanding, from Newton's discovery of gravity to Einstein's theory of relativity and beyond, shows that each successive advancement adds detail and nuance without negating the core truths of the previous work. Our work as building scientists is to apply this ever-deepening understanding to the real-world challenges of building performance.