Condensation

In building science, the dew point is the temperature at which air becomes saturated with moisture and can no longer hold all of its water vapor. When air cools to this point, the water vapor condenses from a gas into a liquid, forming dew, fog, or, in the case of buildings, liquid water on surfaces. Understanding dew point is essential for managing moisture, preventing mold, and maintaining a durable structure.

How Dew Point Affects Buildings

A building's thermal envelope is constantly at risk of moisture-related issues because of the dew point. Warm, humid air from the inside of a home naturally seeks to move toward colder, drier air on the outside. As this moisture-laden air travels through a wall assembly, it encounters cooler and cooler surfaces. If the temperature of any surface inside the wall cavity drops to or below the air's dew point, condensation will occur.

This is a major issue because it causes moisture to form inside the walls where it cannot be seen or easily dried out. This can lead to:

Mold and Mildew Growth: Mold spores, which are always present in the air, only need two things to grow: moisture and a food source. Condensation provides the necessary moisture on surfaces like wood, drywall, and paper, which serve as the food source. This can lead to poor indoor air quality and structural damage.
Structural Damage: Long-term exposure to moisture can cause wood framing and sheathing to rot and decay, compromising the structural integrity of the home.
Reduced R-Value: When insulation gets wet, its R-value plummets. Wet insulation loses its ability to trap air, making it an ineffective thermal barrier.

Preventing Dew Point Problems

Building scientists use several strategies to prevent condensation and manage dew point within a building's envelope.

Air Sealing: The most effective way to prevent condensation is to stop humid air from getting into the wall cavity in the first place. A continuous air barrier that seals all cracks and gaps in the building envelope prevents the movement of moisture-laden air.
Vapor Retarders: In some climates, a vapor retarder is installed on the warm side of the insulation. This barrier, often a plastic sheet or kraft paper on insulation batts, slows the movement of water vapor into the wall cavity. However, its placement is critical and depends on the climate; in hot, humid climates, it may be placed on the exterior side to prevent moisture from moving inward.
Continuous Insulation: Applying a layer of continuous insulation (like rigid foam sheathing) to the exterior of the framing keeps the temperature of the wall cavity warm. By preventing the sheathing from getting cold, it raises its surface temperature above the dew point, ensuring that any moisture that makes it into the cavity remains in vapor form and can dry out.
Ventilation: Controlling indoor humidity levels is also key. A whole-house dehumidifier or a ventilation system like an ERV (Energy Recovery Ventilator) can help lower the dew point of the air inside the home, reducing the risk of condensation on all cold surfaces, including windows and ductwork.

In summary, the dew point is a foundational concept in building science. By understanding where and why condensation can occur, builders can design and construct buildings that are durable, energy-efficient, and free from the damaging effects of moisture.

Condensation forms on any surface that is cooler than the dew point of the surrounding air and often appears in less obvious places than just walls and windows. This is especially true in areas where cold surfaces meet warm, humid air.

Here are some other common locations for condensation, in addition to the condensation forming in wall cavities & windows.

HVAC System Components

The cooling process in HVAC systems actively creates cold surfaces, making condensation a natural occurrence, though excessive condensation can indicate a problem.

Cold HVAC Registers/Vents: When the metal or plastic register is significantly cooled by the conditioned air flowing through it, and it comes into contact with warm, humid room air, moisture will condense directly on the register's surface. This is often called a "sweating" vent.
Condensate Drain Lines and Pans: The main purpose of the air handler's evaporator coil is to cool the air, which also dehumidifies it. The water that is removed from the air, is cooled off as it collects on the evaporator coil & drains into a drip pan, before being channeled out through the condensate drain line. Condensation can also form on the exterior of the cold metal or typical plastic drain line itself if it's uninsulated and running through a warm, humid space (like an attic). This is also extremely true of pipe that's partially covered in blown insulation.
Ductwork: Exposed or poorly insulated air ducts carrying cold air through hot, humid areas (like attics or crawlspaces) will inevitably "sweat," leading to condensation dripping onto ceilings or damaging building materials. This is especially true of ductwork at register boots & the connection at the main plenums. Often installers will not ensure that the insulation covers the register boot, or doesn't go all the way against the metal take off connector.

Refrigeration and Cold Appliances

Surfaces that maintain a sharp temperature difference from the room air are also prime spots for condensation.

Refrigerator/Freezer Weather Stripping (Gaskets): These seals keep the cold air inside and the warm air out. If the seal is compromised (dirty, damaged, or not closing correctly), the warm, humid air from the room leaks in and immediately hits the super-cold surface of the freezer or refrigerator cabinet, causing condensation or even frost to form on the gasket or the nearby cabinet. Many refrigerators have a small heater near the seals to combat this external condensation.

Other Surfaces

Pipes and Plumbing: Cold water pipes (especially during summer) or refrigerant lines for air conditioning that run through humid spaces (basements, closets, or walls) often develop condensation on their exterior, which can lead to staining, mold, or water damage.
Mirrors and Tiling: These materials are generally colder than the air in a room, making them common places for condensation to form, especially after a hot shower when the air's humidity level spikes dramatically.
Interior of Closets or Cabinets: Condensation can occur on the back walls of closets or inside cabinets built against uninsulated exterior walls. The stagnant air in these confined spaces prevents the surface from warming up, and the moisture content is often higher, creating an ideal spot for condensation and mold growth.

Excessive condensation in any of these locations is usually a sign of high indoor humidity, and/or poor insulation.

Condensation from the register boot, or the register itself causing the drywall around the register diffuser to become wet, which leads to mold growth.

In many situations, the register cover itself will sweat as warm humid air in the room, condenses on the register diffuser leading to mold growing on the debris on the diffuser.

The cold condensate water traveling through uninsulated PVC pipe to the exterior in the house can lead to condensation forming on the outside of the pipe, dripping onto surfaces which leaves them wet, typically leading to mold growth.

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