As the cooler months approach, many will find themselves spending more time indoors. In fact, according to the Environmental Protection Agency, Americans spend approximately 90 percent of their time indoors, where concentrations of some pollutants can be many times higher than those outdoors. These pollutants can be mold, dust, lead and chemicals from paints, which can lead to health problems and a decrease in the overall comfort of building occupants. What is perhaps less commonly known is that humidity levels and temperatures indoors contribute to increased pollutant concentrations during this period. This is why it is important for architects and builders to design, construct and remodel buildings in a way that promotes healthier indoor air quality.

Build for Sustainability

Efforts to enhance energy efficiency have led to significant advancements in minimizing air leakage in buildings while ensuring optimal indoor comfort. Older buildings can exhibit air leakage rates that may exceed several times their own volume per hour during the heating season. This unintended infiltration of outdoor air—often misleadingly termed “fresh air”—has been effectively addressed by contemporary building codes.

Fresh air is currently integrated from the outdoors directly into mechanical ventilation systems, where it is mixed with return air. This controlled approach compensates for any minimal air leakage from the building, ensuring that the required volume of fresh air for occupant health and comfort is provided. Energy-efficient buildings, by design, often demonstrate improved air quality due to enhanced air tightness and well-regulated ventilation.

However, it is crucial to recognize that these ventilation standards are not aimed at diluting contaminated air; they require careful consideration of the materials and substances introduced into indoor environments. This thoughtful approach ensures that the air circulating within buildings supports the health and well-being of occupants.

HVAC Systems and Humidity Levels

The HVAC systems in our buildings condition the temperature and humidity of the indoor air, as well as provide particle filtration (but importantly, usually not gases). The engineering of these systems is based on the building design, size, type, use, occupancy and local climate. The demands placed upon the HVAC system are, to a large degree, controlled by the building envelope. This is why these systems must be designed and operated in concert if they are to be energy-efficient. When done well, the results for the occupant can be as healthy as and more comfortable than the outdoors.

Drywall and Mold Resistance

Mold can grow on nearly any surface if it has access to food, oxygen, an appropriate temperature and water. We provide favorable temperatures in our buildings for our own comfort, which also happens to suit mold. We also ensure that oxygen is present, a necessity we share with mold. Considering building materials, it becomes clear that we sometimes introduce the “food” that mold needs into our products. To prevent mold, it’s essential to limit its access to water or nutrients, similar to dealing with an unwelcome guest. Fortunately, leading construction solutions now incorporate performance enhancements that help prevent mold growth, even in high-humidity environments. For example, gypsum boards designed for extreme abuse and extreme impact may feature enhanced moisture- and mold-resistant face and back paper, or they may even be paperless.

In buildings with existing moisture issues, it’s important to locate and repair the moisture source first. Once the walls are dry, replacing affected drywall with a mold- and moisture-resistant product is ideal. Moisture issues aren’t limited to exterior walls; they can occur anywhere, including a building’s core. Whether building new or remodeling, using drywall with enhanced mold and moisture resistance near any water source—like drain or water pipes—is always wise. It’s not a question of if, but when, water will become an issue.

Insulation, Vapor Retarders and Moisture Management

Beyond gypsum, insulation also plays a crucial role in keeping wall interiors dry by reducing moisture retention. Insulation types vary widely, and most control heat gain or loss through layers of trapped gas, similar to how layers of clothing retain warmth. While different insulation materials may offer comparable thermal performance, the products vary significantly in how they influence moisture flow in walls. Each material has a distinct water vapor permeability, or resistance to vapor flow. For instance, fiberglass insulation typically allows vapor flow and promotes breathability, whereas cellular foam plastics offer minimal breathability.

In colder climates, vapor retarders are often added to breathable insulation materials to reduce the risk of condensation during winter, when indoor vapor tends to move outward. However, in areas with high exterior humidity, traditional vapor retarders can trap moisture within the wall cavity during warmer months—a period of high mold growth potential. Today, advanced vapor retarders respond to elevated humidity within the cavity by altering their properties to permit “drying” to the interior. Once dry and colder weather returns, the vapor retarder reverts to its original function, preventing winter moisture accumulation. This approach enables walls to resist moisture while allowing breathability when needed.

Looking Ahead

Building designs have evolved over time in response to environmental shifts and new concerns. When planning for long-term indoor air quality, architects and builders should also prioritize durability and performance, as these factors are linked to a key concept: safe moisture storage limit. Staying below this limit helps prevent mold, rot, rust and other forms of material degradation. Identifying the ideal thermal, moisture resistance or airflow properties for each layer of an assembly may be complex, but experts and predictive software, like WUFI, can provide guidance. Once these properties are identified for a particular design, it’s important to choose materials with low or no harmful emissions for the interior environment. Sustainability and performance programs, such as LEED and WELL, emphasize both environmental and occupant health, with specific areas focused on moisture management and material emissions. These programs recognize that the materials we choose to build with remain in our environments—and affect us—for a long time.