The Effect of Pressurization on The Indoor Environment

As an IAQ professional, it's vital to understand pressurization, which is the difference in air pressure between the inside and outside of a building. Think of it like a balloon: if you squeeze it, the inside pressure is higher. If you suck air out, the inside pressure is lower. The pressure inside a building can be positive, negative, or neutral, and this has a significant effect on air quality and building health.

Negative Pressure

Negative pressure occurs when the air pressure inside the building is lower than the pressure outside.

• How it happens: It's caused by more air leaving the building than is coming in.

• The effect: Since air always moves from areas of high pressure to low pressure, a negative pressure building acts like a vacuum. It pulls in unfiltered, unconditioned air from any cracks or openings in the building envelope (the walls, roof, windows, and foundation that separate the indoor and outdoor environments). This can bring in dust, pollen, allergens, and moisture, which can degrade indoor air quality and, over time, lead to mold growth and damage to building materials.

Positive Pressure

Positive pressure is the opposite: the air pressure inside is higher than the pressure outside.

• How it happens: This happens when more air is being supplied to the building than is being exhausted.

• The effect: The building acts like a pump, pushing conditioned indoor air out through any gaps in the building envelope. While this can help keep some pollutants out, it's a major cause of energy waste. A more serious issue is when warm, humid indoor air is forced into a cool wall cavity; the moisture can condense into liquid water, creating a perfect environment for mold growth inside the walls.

Neutral Pressure

Neutral pressure is the ideal balance where the indoor and outdoor pressures are equal.

• How it happens: The amount of air being exhausted from the building is perfectly balanced with the amount of fresh air being brought in.

• The effect: This balance prevents unwanted air from being pulled in or pushed out. It maintains the intended indoor air quality, protects the building envelope against moisture damage, and ensures the HVAC system operates efficiently without wasting energy.

Common Sources of Pressure Imbalances

Pressurization issues often come from mechanical systems that are not working as intended or were installed incorrectly.

• Leaking HVAC Ductwork: Leaks in return ducts (the ducts that pull air back to the HVAC unit) can draw air from unconditioned areas like attics or crawl spaces, creating negative pressure in the living space. Leaks in supply ducts (the ducts that push conditioned air into rooms) can push air into those same unconditioned areas, causing a positive pressure inside the building.

• Ventilation Systems: Any system that moves air in or out of a building can cause a pressure change if not properly balanced.

  • Residential Systems: A powerful kitchen or bathroom exhaust fan can create significant negative pressure if there isn't enough makeup air (fresh air brought in to replace the air being exhausted) to compensate. This can even cause dangerous fumes from combustion appliances like furnaces or water heaters to be pulled back into the house.

  • Commercial Systems: Large exhaust fans in places like restrooms or kitchens in a commercial building can create a negative pressure if they are not balanced with a dedicated makeup air unit.

• Ducting Problems: Undersized (too small) or oversized (too large) ductwork, as well as damaged or poorly sealed ducts, can restrict or over-deliver airflow to certain areas, causing pressure differences in different parts of a building.

The Second Law of Thermodynamics is a fundamental principle of physics that helps explain why moisture moves the way it does. The law states that in any energy transfer, systems tend to move from a state of order to a state of disorder. In simpler terms, everything in the universe seeks a state of balance or equilibrium.

When it comes to moisture, this law manifests in two key ways:

• Moisture moves from a high-pressure state to a low-pressure state.

• Moisture moves from a high-energy state (warm) to a low-energy state (cold).

• Moisture moves from an area of high concentration (wet) to a low concentration (dry).

The movement of moisture from a wet area to a dry area is an example of diffusion—the movement of molecules from an area of high concentration to an area of low concentration. Similarly, moisture moves from an area of high pressure to one of low pressure.

From High Pressure to Low Pressure

Moisture in the air is in a gaseous state called water vapor. This vapor exerts its own pressure, called vapor pressure. According to the Second Law, water vapor will always try to move from an area where its pressure is high to an area where its pressure is low. This is the primary driving force behind moisture movement in a building. For example, if it's hot and humid outside, the vapor pressure is high. If a building is cooled and dehumidified, the vapor pressure inside is low. Moisture will naturally try to move from the outside of the building, through the walls, and into the inside to balance the pressure.

From Hot to Cold

The Second Law also explains why moisture moves from hot to cold. Heat is a form of energy. Water molecules in warm air have more energy and are more active than those in cold air. When warm, moisture-laden air comes into contact with a cold surface, the water molecules lose energy. This causes them to slow down and bond with each other, which leads to condensation.

An everyday example of this is a cold can of soda on a hot day. The warm, humid air surrounding the can comes into contact with the cold surface. The water vapor in the air cools, condenses, and forms liquid droplets on the outside of the can. This same process can happen inside a building's walls if warm, humid air reaches a cold surface.