The Effect of Seasonal Changes on The Indoor Environment

Winter: The Heating Season

In the winter, the air outdoors is typically very cold. This means there is not a lot of energy available for evaporation (the process of water changing from a liquid to a gas). Because of this, the air is naturally very dry.

Low indoor humidity levels can lead to a number of issues:

• Health Risks: Dry air can irritate the nose and throat, leading to frequent nosebleeds. It also dries out the mucous membranes in the respiratory system, making us more susceptible to infections and viruses.

• Static Buildup: The lack of moisture in the air allows for static electricity to easily build up on surfaces and people.

• Comfort: The air can feel cold and uncomfortable, even at higher temperatures, because the lack of moisture causes a cooling effect on the skin.

To combat these problems, buildings often use humidifiers to add moisture back into the air and maintain a comfortable and healthy relative humidity level.

Summer: The Cooling Season

During the summer, the outdoor environment is warm, and there is a lot of energy for evaporation. This creates a large amount of water vapor in the air, leading to high outdoor humidity.

This humid outdoor air creates a humidity load (an excess of water vapor) on the building envelope (the walls, roof, doors, and windows that separate the inside from the outside). The building's interior, which is being cooled and dried by air conditioning, has a lower vapor pressure than the outside. To achieve equilibrium, moisture from the humid outdoor air will try to move through the building envelope and into the indoor space.

When indoor relative humidity levels exceed 60%, several problems can arise:

• Building Component Issues: Building materials like wood flooring, doors, and furniture can absorb this excess moisture. This can cause the materials to swell, warp, and become damaged.

• Mold Growth: Mold requires moisture to grow. High indoor humidity provides the perfect conditions for mold spores to land on surfaces and begin to multiply, which can cause both health issues and damage to the property.

To manage the humidity load and prevent these issues, air conditioning systems and dehumidifiers are used to remove moisture from the indoor air.

The Science Behind Moisture Weight Gain

Building materials like wood, drywall, concrete, and insulation are hygroscopic, meaning they have an ability to absorb and release moisture from the air. Think of it like a sponge: it can hold onto water without being visibly wet.

Here’s how a building "gains weight" between seasons:

• Winter: Cold outdoor air is very dry. When this air is heated inside a building, its capacity to hold moisture increases, but the amount of moisture it holds is still very low. This dry air acts like a vacuum, pulling moisture out of the building materials and carrying it away through air leaks or ventilation. This process, called vapor diffusion, causes the building materials to "dry out," reducing the overall moisture content and, consequently, the building's weight.

• Summer: In contrast, hot, humid summer air is packed with water molecules. When this air enters a building, the hygroscopic materials absorb the moisture until they reach a state of equilibrium with the surrounding air. This process of absorption is what causes the significant weight gain.

Why This Matters for Building Health

The constant cycle of wetting and drying is a major concern for building scientists because it can lead to long-term damage.

• Structural Integrity: Repeated expansion and contraction from moisture changes can weaken materials like wood, leading to warping, cracking, and eventual rot.

• Mold and Mildew: Elevated moisture levels within walls and ceilings provide the perfect environment for mold and mildew to grow, which can cause health issues for occupants.

• Energy Efficiency: A moisture-laden wall assembly is a less effective insulator. The water molecules in the building materials make it easier for heat to transfer through the wall, increasing the burden on air conditioning systems.

This moisture absorption is a natural process, but it underscores the importance of proper moisture management. Builders and designers use a combination of air barriers, vapor retarders, and dehumidification systems to control the amount of moisture that gets into and stays within the building's structure.